respectively-Tuscan, base twenty-eight minutes, capital fourteen minutes; Doric, base thirty-two, and capital sixteen minutes; Ionic base thirty-six, and capital eighteen minutes; and Corinthian, base forty, and capital twenty minutes.

Greek mouldings vary from the Roman, and are remarkable for being almost invariably drawn by the hand, and not formed, as in the Roman examples, of parts of circles struck with the compasses.

The flutings of columns vary in the depth and form of their curves; some, as in the Doric orders, are flat segments, without fillets between them, others are deep segments and semicircles, and others are semielliptical, and sometimes more than semi-elliptical, on the plan, as in the Jupiter Stator. Some columns of antiquity are decorated with spiral flutes, and some with leaves, as in the Temple of Clitumnus, and in a fragment in the British Museum.

The method of drawing the entasis of the columns employed in Roman architecture is described by Chambers, p. ii. of the Preface. It is done by means of a sliding rule, called the rule of Nicomedes.

Some account of the entasis of columns, by Mr. Jenkins, is given in the 4th vol. of Stuart's Athens,' with comparisons of the entasis of several columns, and more fully and elaborately in Mr. Penrose's treatise on the Parthenon, published by the Dilettanti Society. The Greek entasis is much more subtle than the Roman.

We may here observe, generally, that the principle of a base is support, which is admirably shown in the Attic base, where the two tori are proportioned and arranged, with the graceful sweep of the cavetto or hollow moulding between them, to sustain the shaft. The hollow-moulding gives additional height to the base, and the profile is in no part within the perpendicular line of the shaft, which would give it a weak appearance. The annexed variety of bases, from Greek examples (see preceding page), present some of the beauties and some of the defects even of Greek architecture. The base of the Apollo Didymæus shows weakness, and the torus of the Minerva Polias, at Priene, appears too heavy for the delicate astragals and cavettos beneath. Some columns, instead of being fluted the whole height of the shaft are, for about one-third from the base, made polygonal, each side being the width of the flute. This is particularly the case at Pompeii, where the Doric columns are often very slender. In the internal order of the Pantheon, the flutings are filled with cabling about one-third of their height. Cabling is a carved band projecting out of the fluting.

PROPORTIONS OF SOME OF THE COLUMNS EMPLOYED IN ANCIENT EDIFICES, WITH THE ENTABLATURE.

Vitruvius makes the height of the Doric (Roman) seven times the diameter; the Ionic, 8; and the Corinthian, two-thirds higher than the Ionic, or nine diameters and one-sixth. The proportions of Palladio and Vignola differ; the latter are, we believe, preferred by the French school. See the Architecture of Palladio and Vignola. The proportions of the columns given above are from Stuart's 'Athens;' Taylor and Cressy's 'Rome;' 'Ionian Antiquities;' 'Unedited Antiquities of Attica,' &c.

In studying these works the student should be guided by the figures denoting the dimensions rather than by the outlines.

COLUMN. This term is applied to any formation of troops in which the divisions are placed behind one another exposing a narrow front to the enemy. Thus a column of battalions is formed by a number of successive battalions being deployed close behind one another; while a battalion is said to be in column when the companies of which it is composed are deployed in rear either of one of the flank companies, or in rear of the two centre companies or two centre subdivisions. battalion is said to be in open column when the distance between the companies is such as to admit of their wheeling into line; and to be in close column when the front rank of one company is within a few paces of the rear rank of the one before it, while the half distance column is intermediate between the two.

A

The formation of the Greek and Roman armies was almost wholly in column, both for attack and defence. And it was only on the introduction of fire-arms and field artillery that, from the ravages committed in dense masses, a more extended formation was adopted. Gustavus Adolphus was one of the first to employ a more open formation.

In the wars of the French Revolution, at the close of the 18th century, the dense column was universally employed, almost of necessity, as the raw levies were not sufficiently trained to be easily handled in any other formation, and the loss of life entailed was not considered by their generals, to whom success, however dearly purchased, was the only obligation. From these victories and those of the first Napoleon, who also employed very dense columns in attack, it resulted that close columns came much into repute, and their advantages as compared with other formations are much contested points among military writers, especially between English and foreign authors; the former principally upholding the formation in line, which had been so successfully applied by the Duke of Wellington in the Peninsula and at Waterloo against the columns of the French. Still we must not blind ourselves to the fact, that a formation which can be applied with troops of such morale as the English may not be the best with others. Jomini, when discussing this question in his Précis de l'art de la Guerre remarks," Dès lors quelle serait l'armée Européene (si l'on excepte les Anglais), que l'on pût se hasarder à deployer en lignes sur deux rangs." And as a three rank formation makes a column too dense, and it would be next to impossible to change from a three to a two rank formation, according as a deployment or column was required, he recommends the constant employment of small columns of attack on a two rank formation.

From the great advantage that the formation in column gives in the movement and handling of troops, and the impossibility of conducting extended movements in line, even if there were any object in attempting it, the column will always in some shape or other be

employed in all the preliminary dispositions of a battle, and it is only when within range that the question arises as to formation. For the offensive, the objects to be kept in view in the formation of troops, are mobility combined with solidity and impulse; while for the defensive, solidity should be united with the greatest possible amount of fire. Now the column, if not too deep, combines the former qualities in the highest possible degree, and in an attack firing is not required; indeed, if the men once commence to fire, the attack will in all probability fail, for the impulse and impetuosity of the charge being lost, if received by a steady fire from the enemy, they soon get into confusion and retire. At the same time it must be remembered, that the deeper the forma tion the greater will be the carnage, especially if artillery be employed in the defence of the position attacked; and therefore, if the troops are sufficiently steady and well disciplined to be managed under fire in line they will have a great advantage. The deployment from the column into line for attack, should not however take place at too great a distance from the point attacked, as, especially in moving over rough ground, troops in line soon lose their formation; whilst on the other hand, if the deployment is too long deferred, and the column while deploying is received with a heavy fire, it is likely to be thrown into disorder, and if then promptly charged, (as at Waterloo), will probably be routed.

Jomini, while condemning the formation of troops in large masses, as at Waterloo, argues in favour of battalion columns, formed on the two centre companies, with two companies as skirmishers, thus making the depth only six ranks, as a good general formation for attack. For he says, that although at Talavera, Busaco, Fuente d'Onor, Albuera, and Waterloo, heavy columns, (in one case twelve battalions deployed behind one another, or a depth of thirty-six ranks,) had been annihilated by the fire and attack of the English infantry in line, on front and flank, yet that this is no proof against small battalion columns, which have not been sufficiently tested. And for defence, he considers the first line should be deployed, and the second in battalion columns. The success of the line or column will, however, depend upon its proper application by the general, according to the modifying circumstances of ground, whether broken or even, of discipline and of morale. And no general rule can be laid down, though the Duke of Wellington appears to have almost always deployed his columns before attacking. COLURES (ai Kóλovpoi, colúri). The term originally applied to any great circles of the sphere passing through the poles, but came at last to mean only the circles which also pass through the equinoxes and the solstices, which are distinguished as the equinoctial and solstitial colures. These terms are now of very little use, as the fact of a star being upon either circle is attended with no remarkable phenomenon. Astronomers would describe a star on the equinoctial colure as having either no right ascension, or twelve hours of right ascension, according as it is on the vernal or autumnal half of the circle; and a star on the solstitial colure as having either six hours or eighteen hours of right ascension, according as it is on the summer or winter side of the heavens. If we say that the sun is on the equinoctial colure at the quarter-days of March and September, and on the solstitial colure at those of June and December, we rather elucidate the term colure than derive information from it.

The solstitial colure passes through the poles of the ecliptic also, and might be called an ecliptic colure; but the other circle, which passes through the equinoxes and the poles of the ecliptic, has no distinct name, and would be best described as the circle from which celestial longitude is reckoned.

COLZA, OIL OF. The seeds of the Brassica campestris oleifera yield by expression about 39 per cent. of a yellow oil, which possesses a specific gravity of 9136, and solidifies at 21° Fahr. It is a mixture of the glycerine compounds of two acids, namely Brassic acid (C,H,,O,). The other acid resembles oleic acid, but does not seem to be identical with it. Colza oil is used both for lamps and in cookery. COMA, a Greek word (kwμa), signifying profound sleep; a morbid condition of the brain, attended with loss of sensation and voluntary motion, the patient lying as if in deep sleep.

13

It can scarcely be considered a primary or idiopathic disease; it is rather symptomatic of that condition of the brain which, when in sufficient intensity, produces apoplexy. If it be regarded as a positive disease, it must be considered as a milder form of apoplexy. It exists in different degrees of intensity; several of which degrees, as they are attended with some variety in the symptoms, and are dependent on some modification of the pathological condition of the brain, so they have acquired distinct names. When there is a state of mental and physical torpor, indicated by an almost constant tendency to sleep, and great inaptitude for muscular exertion; when the patient is sensible only as long as he is strongly excited, and as soon as the external stimulus is withdrawn, lapses into a state of forgetfulness, the affection is called Lethargy. When no distinct consciousness returns, however the patient may be roused or stimulated, though there still remain some indication of feeling on the application of mechanical irritation, as on being pricked or pinched, the affection is called Carus. But when the insensibility is so great that the patient indicates neither sensation nor feeling, whatever mechanical stimulus be applied, this state is often called, by way of eminence, Coma. This comatose state invariably accompanies apoplexy; and, as has been stated, coma, when intense, passes into apoplexy.

The abolition of sensation and voluntary motion (animal functions), which constitutes coma, is always attended with a greater or less disturbance of the organic functions. The circulating system is disordered; the pulse at one time is slow and full, and at another quick and small. The respiration is laborious, and is commonly preternaturally slow. The power of generating animal heat is almost always diminished, the skin being cold and clammy; though there are cases in which the temperature is elevated somewhat above the natural standard. The countenance is usually pale and sunk; the pupils dilated, but in the worst cases contracted; the position of the body is supine; in the worst cases there is a constant tendency to sink down in the bed; the limbs are motionless; and the evacuations, if not wholly retained, which is usually the case, are passed without consciousness. In coma there is an exhaustion or suppression of the sensorial powers, in other words, an abolition of the cerebral functions. This state of the nervous is always attended with a morbid condition of the vascular system. There is either a congestion of the capillary blood-vessels, occasioning obstructed circulation of the blood through the brain, or there is too rapid and violent a flow of blood through the cerebral vessels; or an inflammatory condition of the blood-vessels; or an extravasation of blood, or an effusion of serum into the cerebral substance. In addition to its disordered motion, there is also sometimes a depraved quality of the blood. There is reason to believe that to some morbid change in the constitution of the blood, the coma incident to bad types and advanced states of fever is mainly owing. The morbid condition of the brain, on which coma depends, may be induced by any of the causes which have been enumerated as constituting the predisposing and exciting causes of apoplexy. [APOPLEXY.] COMA BERENICES (Constellation), the hair of Berenice, placed among the stars by the astronomer, Conon, in memory of Berenice, the wife of Ptolemy Euergetes. (B.c. 246.) The legend is, that she had dedicated this hair to Venus, in case of her husband's safe return from Asia, and that it disappeared from the temple in which it was placed, and was never seen again till found in the starry heavens, where it now is, close to the tail of the Lion, and passing the meridian about an hour before Arcturus. [BERENICE.] Geminus attributes the constellation to Callimachus, who mentions it, as do Catullus and Pliny. Ptolemy does not place the stars now belonging to this constellation by themselves, but in the tail of the Lion; and Hyginus makes no separate mention of it. It was constantly mentioned by writers on the sphere, but not figured or catalogued separately, as far as we can find, till the time of Tycho Brahé.

The constellation Coma Berenices will be found shut up in the triangle formed by the three bright stars, Arcturus, 8 Leonis, and o Canum Venaticorum. It contains no stars of conspicuous magnitude. COMB MANUFACTURE. Combs are generally made of a thin plate of wood, horn, tortoise-shell, ivory, bone, or metal, which may be either flat or curved, having one or two of its edges indented with narrow slits, which divide the substance of the comb into long, fine pointed teeth. Combs employed in the woollen manufacture are de scribed in the article relating to that subject. Curry-combs, used in dressing horses, consist of a number of iron plates notched on one edge to form saw-like teeth,and attached by the other edge to an iron back, in parallel lines, so as to form an instrument the action of which is between that of a scraper, a comb, and a brush.

In the mode of cutting and shaping the plates of which combs are formed there is little call for remark; but in the act of comb-cutting much ingenuity has been called into exercise. The hand-method is by means of a double saw, consisting of two separate fine saws, placed parallel with each other, and adjusted to such a distance as to embrace a tooth of the required fineness between them. These two saws are so arranged that while one cuts into the comb to the full depth required, the other cuts only about half that depth; by this contrivance the uniformity of the comb is secured; because, while the deeper saw is completing the first cut, the shallower one is forming the commencement of the second; and when, on the completion of the first cut, the deep saw is put into the second cut to complete it, the shallower one immediately commences a third. The cuts thus formed are subse quently enlarged and rendered smoother by means of a very thin wedge-shaped file, which also points the teeth. With such accuracy may these operations be performed, that delicate ivory combs, with from fifty to sixty teeth in an inch, are produced in this way. Though this method of comb-cutting is still practised, a superior and much quicker mode by means of circular saws and revolving cutters for pointing the teeth has been long in use.

By the above-described modes of comb-cutting all the material of the interstices between the teeth, is lost or destroyed; but by the operation known as the parting of combs such loss or waste of material may be avoided in the manufacture of combs of tortoiseshell, horn, or any other tough material; two combs being, by this process, made out of one piece, the teeth of one being cut, by the pressure of chisel-like instruments, out of the interstices of the other. Mr. Lyne's machine, for this purpose, is worked by the alternating action of a lever handle. The piece of tortoiseshell or horn is secured upon a traversing carriage, which is capable of motion in a direction perpendicular to that of the teeth of the comb. This carriage is made hollow, to receive an iron heater, the heat of which softens the horn, and thereby renders it more easy to cut. Between each movement of the carriage the cutter,

which is a chisel equal in breadth to the length of the intended teeth of the comb, is brought down with sufficient force to cut completely through the horn or tortoiseshell. As the teeth are not required to be perfectly parallel, but in a slight degree wedge-shaped, the successive cuts of the chisel must incline a little obliquely, first in one direction, and then in the other, for which provision is made by an arrangement for alternately changing the position of the cutter during its descent. A pair of very narrow chisels mounted at right angles with, and at the ends of, the long cutter, serve, by their alternate descent, to connect each pair of cuts at their converging extremities, so as to detach the ends of the teeth. In Mr. Rogers's machine for the same purpose, everything is effected by the continuous turning of a winch, which, by means of a crank, works a double cutter, the two chisels of which are capable of adjustment according to the size of the teeth, while the screw for moving the bed or carriage is turned by means of a cogwheel upon its axis, working into another on the axis of the winch. The extent of motion between each operation of the cutter is regulated by varying the relative sizes of these cog-wheels.

Among recent novelties in comb-making is Mr. Griffith's curious patent, in 1852, for "galvanic combs," of which the teeth are alternately of copper and zinc, while the handle is hollowed into a chamber for containing a roll of flannel moistened in acid solution. The inventor seems to expect a beneficial galvanic action by combing the hair with this apparatus. A more promising novelty is the application of the very tough substance called vulcanised india-rubber [CAOUTCHOUC MANUFACTURES] as a material for combs.

COMBAT, SINGLE. [DUEL.]

:

COMBINATION LAWS. The laws known by this name were repealed in 1824. Till then any combination of any two or more masters, or of any two or more workmen, to lower or raise wages, or to increase or diminish the number of hours of work, or quantity of work to be done, was punishable at common law as a misdemeanour and there were also thirty-five statutes in existence, most of them applying to particular trades, prohibiting combinations of workmen against masters. The Act passed in 1824 (5 Geo. IV. c. 95) repealed all the statute and common law against combinations of masters and of workmen; provided a summary mode of conviction, and a punishment not exceeding two months' imprisonment for violent interference with workmen or masters, and for combinations for violent interference; and contained a proviso with regard to combinations for violent interference, that no law in force with regard to them should be altered or affected by the Act. But all the common law against combinations being repealed by the Act, this proviso was considered as of no force; and the Act also went beyond the intentions of the framers in legalising combinations unattended with violence for the purpose of controlling masters in the mode of carrying on their trades and manufactures, as well as peaceable combinations to procure advance of wages or reduction of hours of work. The Act was passed after an inquiry into the subject by a committee presided over by Mr. Hume, which reported to the House the following among other resolutions :-

"That the masters have often united and combined to lower the rates of their workmen's wages, as well as to resist a demand for an increase, and to regulate their hours of working, and sometimes to discharge their workmen who would not consent to the conditions offered to them; which have been followed by suspension of work, riotous proceedings, and acts of violence.

That prosecutions have frequently been carried on under the statute and the common law against the workmen, and many of them have suffered different periods of imprisonment for combining and conspiring to raise their wages, or to resist their reduction, and to regulate their hours of working.

That several instances have been stated to the committee of prosecutions against masters for combining to lower wages, and to regulate the hours of working; but no instance has been adduced of any master having been punished for that offence.

"That it is the opinion of this committee that masters and workmen should be free from such restrictions as regard the rate of wages and the hours of working, and be left at perfect liberty to make such agreements as they may mutually think proper.

"That therefore the statute laws which interfere in these particulars between masters and workmen should be repealed; and also that the common law, under which a peaceable meeting of masters or workmen may be prosecuted as a conspiracy, should be altered."

Immediately after the passing of this Act a number of widely organised and formidable combinations arose in various trades and manufactures for the purpose of controlling the masters as to the way in which they should conduct their business; and the extent to which the Act had repealed the common law being doubtful, and the Act | having clearly gone beyond the resolutions on which it was grounded in legalising combinations, Mr. Huskisson, then President of the Board of Trade, moved early in the session of 1825 for a committee to consider the effects of the Act 5 Geo. IV. c. 95; and a committee was appointed with Mr. (afterwards Lord) Wallace, then Vice-President of the Board of Trade, for its chairman. This committee recommended the repeal of the Act of the previous session, and the enactment of another; and in consequence of their recommendation the 6 Geo. IV. c. 129, was passed, which is the Act now in force relative to combinations.

This Act repealed the 5 Geo. IV. c. 95, and all the statutes which that Act had repealed. It relieved from all prosecution and punish ment persons meeting solely to consult upon rate of wages or hours of work, or entering into any agreement, verbal or written, on these points. And it provided a punishment of not more than three months imprisonment, with or without hard labour, for any one using violence, or threats to make a workman leave his hiring, or return work unfinished, or refuse to accept work, or belong to any club, or contribute to a common fund, or pay any fine for not belonging to a club, or contributing to a common fund, or refusing to conform to any rules made for advance of wages or lessening of the hours of work, or regulations of the mode of carrying on any business, and for any one using violence to make any master alter his mode of carrying on his business. By the Act 6 Geo. IV. c. 129, therefore combinations of masters and workmen to settle as to rate of wages and hours of labour are made legal and freed from all punishment; but the common law remains as it was as to combinations for otherwise controlling masters. Although, therefore, workmen may conspire together not themselves to work under certain wages, they must carry out their object by lawful means, and cannot intimidate or prevent masters from employing, or workmen from taking employment, at any wages those other workmen may agree for. (Reg. v. Rowlands, 2 Den. C. C. 364.)

By 9 Geo. IV. c. 31, assaults in pursuance of a combination to raise the rate of wages are made punishable by imprisonment and hard labour.

COMBINATIONS AND PERMUTATIONS. By the word com. bination is usually meant any selection which can be made out of a number of different objects without reference to the order in which they are placed; while by a permutation is meant a combination in which different orders of position are to be considered as constituting a specific difference. Thus abcd, acbd, da cb, are all the same combination of four out of the alphabet, but different permutations of four. The investigation of questions relating to combinations, &c., is the principal mathematical part of the theory of probabilities, and was first considered in detail, with reference to that science, by James Bernouilli and Montmort (see Library of Useful Knowledge: Probability'); but the common rules had previously found their way into arithmetical treatises. The enormous number of different arrangements of which objects are susceptible, even when their number is not large, drew early attention to the subject. We shall give some of the most simple rules, and a help to calculation for high numbers.

I. The number of permutations having a in each, which can be made out of X things, is the product of x terms of the series, X, X-1, X2, X-3, &c.

Thus, out of 10 things, there are 10 × 9, or 90 permutations of two; 10 x 9 x 8, or 720 permutations of three; 10 x 9 x 8 x 7, or 5040 permutations of four; and so on. Finally, the number of different arrangements which the whole ten will admit of, say the number of changes which can be rung on ten bells, is

10. 9. 8. 7. 6. 5. 4. 3. 2. 1. or 3,628,800.

II. When the whole number of things, X, contains a which are ber of arrangements of the whole is not the product of X, X-1, &c., alike of one sort, b which are alike of another sort, &c., the total numdown to 1, but that product divided by the product of 1, 2, 3. up to a, then by that of 1, 2, 3. . up to b, &c. This result can be most easily formed by striking out common factors from the numerators and denominators.

...

being required, no simple rule can be given, but each case must be III. In the last case, the number of permutations of x out of X formed out of solved by itself. For instance, how many permutations of three can be

a a a b b c.

(1.) All being different, 3. 2. 1. or 6. (2.) Where a is repeated twice, we have 6. (3.) Where a is repeated three times, one only. (4.) Where b is repeated twice, we have 6. In all, 19. ÍV. The number of combinations of different, is

things out of X, all prod. of x terms of X, X 1, X 2, &c. divided by prod. of x terms of 1, 2, 3, &c.

Thus out of 10 things, the number of combinations of four is 10. 9. 8. 7. divided by 1. 2. 3. 4, or 210. The best way of arriving at this result is by destroying common factors, which shows it to be 5. 3. 2. 7. Observe also that we may shorten this process, when x is greater than the half of X, by finding out, not how many selections can be taken, but how many remainders can be left. Thus the number of combina tions of 25 out of 30, is the same as the number of combinations of 5, for 25 can only be taken in as many ways as 5 can be left. V. The number of combinations of x things out of X, any repetition being allowed, is

prod. of x terms of X, X + 1, X + 2, &c.
divided by

prod. of X terms of 1, 2, 3, &c.

VI. The number of ways in which n places may be filled up from x

letters, allowing any letter to be repeated in all or or any of the places, is a", or the product of x, x, x, . .. (n factors in all). This is the number of permutations of n out of x, allowing repetition.

VII. The number of ways in which a different letters can be distributed into boxes, all possible modes of distribution being equally allowable, is a".

VIII. The total number of combinations of all sorts out of x things, from one at a time up to all together, both inclusive, is 2*, or 2. 2. 2. ( factors in all) diminished by 1. Thus out of 4 things, there are 2. - 1, or 15 different sections: they are

...

-

abcd, bed, a cd, abd, abc, ab, ac, ad, bc, bd, cd,
a, b, c, d.

Among the curiosities of this subject, it will suffice to mention the following: The number of all possible arrangements of letters, repeated or not, and capable of being pronounced or not, up to words of 24 letters, is of the following order of magnitude: Take a million of millions; repeat it a million of million times: the result is between 1391 and 1392 millions of such numbers. As an instance of the manner in which the dropping of consonants and confusion of vowels may permit possible alterations of spelling, M. de Mairan computed that the word Hainaut might be spelt in 2304 different ways, so as to be pronounced in the same way by as many different Frenchmen, or very nearly so.

The most useful proposition in the higher part of the theory of combinations is the reduction of the formula 1. 2. 3... (x-1) x to a very close approximation, which can be easily calculated by logarithms. It affords at the same time a useful lesson to those who have not studied mathematics at all, or very little; we have seen ignorance comport itself with laughter more than once at the idea of the preceding product being found by employing the proportion which the circumference of a circle bears to its diameter. But let = 3.141593 be this proportion; €= 2·718282, the base of Napier's logarithms: then we have

1.2.3......(x-1) x = √√√ 2 π x (~~)* very nearly.

which is a little too small, but the error is only about the 12r-th part of the whole less than 1 per cent. even when x is so low as 10. The expression can easily be calculated by logarithms. Tables of the logarithms of this product will be found at the end of the article 'Theory of Probabilities,' in the Encyclopaedia Metropolitana.' For an instance of the computation, see the Library of Useful Knowledge: 'Examples of Arithmetic,' &c., p 45.

COMBINING PROPORTION. [ATOMIC THEORY.]
COMBINING VOLUME. [ATOMIC VOLUME.]

COMBUSTIBLE. In its more restricted and usual sense, this termi signifies a body which is capable of combining with oxygen, with the evolution of so much heat as to become luminous or incandescent. [COMBUSTION.]

COMBUSTION is a term usually restricted to describe the phenomenon that ensues when chemical action is sufficiently intense to produce light and heat. The burning of coal, wood, paper, candles, oil, or coal-gas are familiar illustrations of combustion. Less common, but more brilliant, instances of combustion are seen in the explosion of gunpowder, or fireworks, or in the burning of steel-wire, charcoal, or phosphorus in oxygen gas.

In the examples of combustion above alluded to, the action lies between the burning body on the one hand, and pure or diluted oxygen on the other; and inasmuch as our world is enveloped in an atmosphere of which the most important constituent is oxygen, it follows that all ordinary instances of combustion are owing to the rapid oxidisation of bodies at a high temperature. It would be wrong, however, to suppose that the word combustion expresses no other actions than those indicated. Many substances burn equally well in atmospheres from which oxygen is excluded altogether, and in some cases even burn more readily than they would under similar circumstances in pure oxygen. For instance, when the metals arsenic or antimony are finely powdered and thrown into an atmosphere of chlorine, they instantly ignite, and burn with evolution of light and heat; in fact, literally undergo combustion.

Combustibles, and supporters of combustion or non-combustibles, are terms used to designate two distinct classes of substances. Air, oxygen, chlorine, &c., are non-combustible, that is, in the common acceptation of the word; they do not burn, but they support the combustion of other substances, such as wood, coal, &c., which latter are called combustibles. The phraseology is, however, purely conventional, and only applicable so long as the circumstances under which it is applied remain the same. For instance, common coal-gas burns in atmospheric air, and under these circumstances the gas is called the combustible and the air the non-combustible or supporter of combustion. But change the conditions, fill a jar with coal-gas, introduce a jet of common air and ignite the latter, perfect combustion will then go on at the jet: the air may now with equal propriety be said to be the combustible and the gas the non-combustible, for the gas just as much supports the combustion of the jet of air as in the former case the air supported the combustion of the jet of gas; in fact, both are equally combustible, and both equally support the

combustion of each other. In like manner a jet of mercury vapour is combustible in an atmosphere of chlorine, and a jet of chlorine is combustible in an atmosphere of mercury. In these and other similar instances the chemical action between the two bodies at their line of contact with each other, is sufficiently intense to produce light and heat, and consequently it is at that line that the phenomenon of combustion ensues.

Spontaneous combustion is combustion that is set up between two bodies at common temperatures, without any application of artificial heat. Thus, the burning of arsenic and of antimony in chlorine, previously referred to, are examples of spontaneous combustion. In kindling a lucifer match friction is necessary to produce a temperature at which the exposed phosphorus will ignite; but a piece of phosphorus, exposed to the direct rays of the sun on a warm day, will inflame spontaneously. Tow or cotton waste, moistened with oil and exposed to the air, frequently undergoes spontaneous combustion, on account of the attenuated state of the oil produced by the fibre being a condition favourable to the chemical action of the oxygen of the air upon it; on this account great care should be taken in factories where oily machinery is cleaned with cotton waste, to prevent the accumu lation of materials of that description.

The spontaneous combustion of the human body is a subject that properly belongs to the past century; nevertheless, as some ignorant people at the present day believe that excessive and long-continued drinking of ardent spirits sets up a condition of system under which spontaneous burning of the body may ensue, it may be as well to notice here that, before the causes of combustion were investigated, it was customary when any person was found burnt to death and the origin of the fire could not be discovered, to assume that combustion had occurred spontaneously. The hypothesis has, however, long since been found to be untenable, and, amongst intelligent persons, is now only held by a few who have not taken the trouble to acquaint themselves either with the laws of heat or the causes of the phenomenon of burning.

The term combustion is sometimes used to describe certain chemical actions in which heat is evolved but no light. Thus the heat of the body is sometimes spoken of as being caused by the combustion of the carbon of the blood with the oxygen of the air. Occasionally, the word is used to denote particular chemical actions where not only no light but even no sensible heat is evolved; thus, the decay of animal and vegetable matter is said to be due to slow combustion. It is, however, far more convenient to speak of such as phenomena of oxidation, and restrict the term combustion to the meaning given to it at the commencement of this article. COMEDY. [DRAMA.]

COMENAMIC ACID. [MECONIC ACID.]
COMENIC ACID. [MECONIC ACID.]
COMETARY BODIES. [COMETS.]

COMETS. This term has been applied to bodies of a nebulous aspect which occasionally appear in the heavens, accompanied in the more conspicuous cases by a long train of light called the tail. The principal part of a comet's structure, in contradistinction to the lastmentioned appendage, is denominated the head. The outline of the head is hazy and ill defined; hence the origin of the term comet (Kounτns, from κóun, hair). The head gradually increases in brightness towards the centre, where it assumes a planetary aspect. In some instances it exhibits a small bright central point, bearing a resemblance to a star. This point is called the nucleus. The head of a bright comet is usually shrouded in a paraboloidal envelope of light, the prolongation of which forms the tail. The tail is turned in the direction opposite to the region in which the sun is situate. Its outline is gently curved, being convex on the side towards which the comet is travelling. This remarkable appendage frequently extends over a considerable arc of the heavens, imparting a grand and mysterious aspect to the object with which it is connected. Nor is the length of the tail merely apparent; on the contrary, it not unfrequently extends to an enormous distance in space. Thus, for example, the tail of the great comet of 1843 attained a maximum length of 150 millions of miles; while the tail of the great comet of 1858, at the time of its greatest development, did not certainly fall short of 50 millions of miles in length.

The tail, however, must not be considered as forming an essential part of the structure of a comet. Multitudes of bodies of this class have been observed in modern times, which exhibited merely a round nebulous mass of light without the slightest vestige of a tail. Such bodies are generally visible only by the aid of the telescope; but even in some instances of comets of conspicuous magnitude, no trace of a tail has been discovered. For example, the comet of 1585, observed by Tycho Brahé, is said to have exhibited neither tail nor coma, but appeared perfectly round like a planet. ("Planè rotunda extitit; nec ullam caudam aut barbam in unam magis quam in aliam partem portendebat."*) Cassini relates a similar fact with respect to the comets of 1665 and 1682.

The appearance of a great comet in the heavens has in all ages strongly attracted the attention of mankind. During the earlier periods of history, bodies of this class were generally contemplated with superstitious dread as omens of divine displeasure, and were regarded

Epist. ad Landgrav. p. 13.