THE THEORY OF THREE PRIMARY

COLOURS.

THIS subject was recently very ably treated by Professor J. C. Maxwell, at the Royal Institution. The Duke of Northumberland was in the chair. Our power of vision, said the Professor, depends entirely on our being able to distinguish the intensity and quality of colours. The forms of visible objects are indicated to us only by differences in colour or brightness between them and surrounding objects. To classify and arrange these colours, to ascertain the physical conditions on which the differences of coloured rays depend, and to trace, as far as we are able, the physiological process by which these different rays excite in us various sensations of colour, we must avail ourselves of the united experience of painters, opticians, and physiologists.

Painters have studied the relations of colours, in order to imitate them by means of pigments. As there are only a limited number of coloured substances adapted for painting, while the number of tints in nature is infinite, painters are obliged to produce the tints they require by mixing their pigments in proper proportions. This leads them to regard these tints as actually compounded of other colours, corresponding to the pure pigments in the mixture. It is found that, by using three pigments only, we can produce all colours lying within certain limits of intensity and purity. For instance, if we take carmine (red), chrome yellow, and ultramarine (blue), we get, by mixing the carmine and the chrome, all varieties of orange, passing through scarlet to crimson on the one side, and to yellow on the other; by mixing chrome and ultramarine, we got all hues of green; and by mixing ultramarine with carmine, we get all hues of purple, from violet to mauve and crimson. Now, these are all the strong colours that we ever see or can imagine: all others are like these, only less puro in tint. Our three colours can be mixed so as to form a neutral grey; and if this grey be mixed with any of the hues produced by mixing two colours only, all the tints of that hue will be exhibited, from the pure colour to neutral grey. If we could assume that the colour of a mixture of different kinds of paint is a true mixture of the colours of the pigments, and in the same proportion, then an analysis of colour might be made with the same ease as a chemical analysis of a mixture of substances.

Physical optics does not lead us to any theory of three primary colours, but leaves us in possesson of an infinite number of pure rays, with an infinitely more infinite number of compound beams of light, each containing any proportions of any number of the pure rays.

These beams of light, passing through the transparent parts of the eye, fall on a sensitive membrane, and we become aware of various colours. We know that the colour we see depends on the nature of the light; but the opticians say there are an infinite number of kinds of light; while the painters, and all who pay attention to what they see, tell us that they can account for all actual colours by supposing them mixtures of three primary colours.

In examining the results obtained by physiologists, we find difficulties in accounting for the perception of colour. Some have supposed that the different kinds of light are distinguished by the time of their vibration. There are about 447 billions of vibrations of red light in a second; and 577 billions of vibrations of green light in the same time. It is certainly not by any mental process of which we are conscious that we distinguish between these infinitesimal portions of time, and it is difficult to conceive any mechanism by which the vibrations could be counted so that we should become conscious of the results, especially when many rays of different periods of vibration act on the same part of the eye at once. Besides, all the evidence we have on the nature of nervous action goes to prove that, whatever be the nature of the agent which excites a nerve, the sensation will differ only in being more or less acute. By acting on a nerve in various ways, we may produce the faintest sensation or the most violent pain; but if the intensity of the sensation is the same, its quality must be the same. Now, we may perceive by our eyes a faint red light, which may be made stronger and stronger, till our eyes are dazzled. We may then perform the same experiment with a green light, or a blue light. We shall thus see that our sensation of colour may differ in other ways, besides in being stronger or fainter. The sensation of colour, therefore, cannot be due to one nerve only.

The theory of Dr. Thomas Young, in the opinion of Professor Maxwell, is the only theory which completely reconciles these difficulties in accounting for the perception of colour.

In studying the mixture of colours, we must avoid these sources of error, either by mixing the rays of light themselves, or by combining the impressions of colours within the eye by the rotation of coloured papers on a disc.

Next, the results obtained by opticians claim attention. White light, according to Newton, consists of a great number of different kinds of coloured light which can be separated by a prism. Newton divided these into seven classes, but we now recognise many thousand distinct kinds of light in the spectrum, none of which can be shown to be a compound of more elementary rays. If we accept the theory that light is an undula tion, then, as there are undulations of every different period, from the one end of the spectrum to the other, there are an infinite number of possible kinds of light, no one of which can be regarded as compounded of any others.

If we could excite one of these sets of nerves without acting on the others, we should have the pure sensation corresponding to that set of nerves. This would be truly a primary colour, whether the nerve were excited by pure or by compound light, or even by the action of pressure or disease. If such experiments could be made, we should be able to see the primary colours separately, and to describe their appearance by reference to the scale of colours in the spectrum.

But we have no direct consciousness of the contrivances of our own bodies, and we never feel any sensation which is not infinitely complex, so that we can never know directly how many sensations are combined when we see a colour. Still less can we isolate one or more sensations by artificial means; so that in general, when a ray enters the eye, though it should be one of the pure rays of the spectrum, it may excite more than one of the three sets of nerves, and thus produce a compound sensation.

The terms simple and compound, therefore, as applied to colour-sensation, have by no means the same meaning as they have when applied to a ray of light.

Some of the consequences of Young's theory are as follows:

1st. There are three primary colours. 2nd. Every colour is either a primary colour, or a mixture of primary colours.

3rd. Four colours may always be arranged in one of two ways. Either one of them is a mixture of the other three, or a mixture of two of them can be found, identical with a mixture of the other two.

4th. These results may be stated in the form of colour-equations, giving the numerical value of the amount of each colour entering into any mixture, and such equations may be obtained with a degree of accuracy, showing that the colour-judgment of the eye may be rendered very perfect.

By testing in this way more than 100 different pigments and mixtures, it has been found that the results agree with the theory of three primaries in every case. All the colours of the spectrum give the same result.

The experiments with pigments do not indicate what colours are to be considered as primary; but experiments on the prismatic spectrum show that all the colours of the spectrum, and, therefore, all the colours in nature, are equivalent to mixtures of three colours of the spectrum itself-namely, red, green (near the line E), and blue (near the line G). Yellow was found to be a mixture of red and green.

THE LATE PROFESSOR HODGKINSON,

F.R.S.

PROFESSOR EATON HODGKINSON died at Eaglesfield, near Manchester, on the 18th inst. He was a high authority as a Mechanical Engineer, and for thirty years past he has rendered good service to science and human progress by his extensive experiments and valuable contributions to several of written, we believe, by Dr. Joule, is told in the folour scientific societies. The history of his life, lowing extracts from the "English Cyclopædia:"

Professor Hodgkinson, Professor of the Mechanics of Engineering in University College, London, is an European authority upon the properties of iron, cast or wrought, with regard to its application in architecture and engineering. He was born at Anderton, near Northwich, in Cheshire, on the 26th of February, 1789. Before the period of Mr. Hodgkinson's researches, the chief authority on the subject of iron beams was Tredgold, who reasoned on the supposition that, when subject to cross-strain, a body resisted the force of compression along the top, and that of extension along the bottom, equally; and who therefore devised a sectional form like the letter I. Mr. Hodgkinson however showed that cast-iron and all crystalline bodies resist a crushing force far more effectually than they do a force tending to tear them asunder, and has thus established the fact that the form of the letter T inverted (L), with a bottom flange about six times as large as the top one, constitutes the most economical disposition of the material— the gain of strength being two-fifths or upwards. The earliest application of the discovery in a railway bridge, was about 1830, at Water-street, Manchester, for the Manchester and Liverpool line, by the late George Stephenson.

Mr. Hodgkinson's researches have also seriously invalidated the assumption of Tredgold, Moseley, Navier, and many others, that all "rigid" bodies are elastic up to a certain degree of strain, at least; for, cast-iron, and some other bodies, as stone, he has found are never absolutely elastic-their defects of elasticity varying nearly as the squares of the weights laid on, or of the changes of form produced. With reference to the strength of pillars, the profound researches of Euler had been of little value to practical men. Euler's theory depended upon the force necessary to produce incipient bending in a pillar loaded at the top; but failing to discover regularity in that force, Mr. Hodgkinson sought for that necessary to break the pillar. This proved to be regular. His experiments in this inquiry (which were 227 in number) established some remarkable facts, such as the diminution of strength by adding to the height of the pillar above a certain point-though with the same load, and the same vertical pressure; that a pillar with two rounded ends is only one-third of the strength of a pillar

with the ends flat; and that increase of strength results from the thickening of the column in the middle. From these experiments Mr. Hodgkinson deduced formula for solid and hollow pillars, which have been adopted in England and on the Continent, and have been expanded into tables for ready reference by architects. His researches last referred to were communicated to the Royal Society, and printed in the "Philosophical Transactions," in 1840, under the title "Experimental Researches on the Strength of Pillars of Cast-Iron and of other Materials," and for his efforts he had the honour of receiving the Royal Gold Medal, and was elected a fellow of the society. These and his earlier researches on the strength of materials were at the expense of Mr. Fairbairn, of Manchester, whose own investigations he greatly assisted; and some were aided by grants from the British Association for the Advancement of Science, and with his later experiments yet to be referred to, have probably involved an expenditure of £10,000. In the researches for the association he was in some instances named contemporaneously with Mr. Fairbairn for the same subjects, as in determining the relative values of hot and cold blast iron. ("Reports of the British Association for the Advancement of Science," vol. vi.)

When Mr. Stephenson conceived the idea of constructing the Britannia Bridge in the form of a wrought-iron tube, he applied, as Mr. Hodgkinson states, first to Mr. Fairbairn, and then through Mr. Fairbairn to Mr. Hodgkinson himself, in order that the necessary data might be got together for so novel an application of material. Mr. Hodgkinson had been consulted privately from near the origin of the scheme; but in 1845 he assisted in experiments at Mr. Fairbairn's works at Millwall, London; and subsequently he was engaged in the important duties of experiment and calculation, from which resulted the determination of the proportions and structure of that which is perhaps the most remarkable effort in engineering science of modern times. For his co-operation in this work he received a first-class medal at the Paris Exhibition

in 1855.

In August, 1847-on the issue of a royal commission to enquire into the application of iron to railway structures, consequent upon the accident at the Dee Bridge, Chester-Mr. Hodgkinson was named a member; and, in the form of appendices to their report of July, 1849, are 180 pages giving the results of experiments made by him for the commission and for the Britannia Bridge. For the "remarkable series" for the commission he deservedly received thanks for the "zeal and intelligence" with which the experiments were carried

out.

The records of these numerous and valuable investigations are to be found interspersed through the "Transactions" of the British Association for the Advancement of Science, and of the Literary and Philosophical Society of Manchester, of which last society Mr. Hodgkinson was for some time president, and in other publications which have been referred to; but the nature of his discoveries may also be gathered from the edition of "Tredgold

the Chain-bridge at Broughton, Manchester, with an account of its Failure" (two papers); and one on the subject of the strength and form of ironbeams. The fourth report of the British Association contains the result of an extensive series of experiments "On the Collision of ImperfectlyElastic Bodies," and the fifth report a paper "On Impact upon Beams."

Mr. Hodgkinson in 1856 was engaged in pursuing various researches, at the expense of the Royal Society and of Mr. Robert Stephenson. He was elected an honorary member of the chief societies connected with architecture and engineering, and his discoveries excited the highest interest on the continent.

tion at issue. Indeed, Mr. Hughes considers the ventilation of all mines begins at the wrong end, and that the miner at the extreme end of the working should have the first of the pure air; since, if an ample supply of air be provided at the points farthest from the shaft its efforts to regain the surface will effectually purify every other portion of the workings. The advantages which are claimed for the invention are that no air-doors will be required, which will be very numerous; amongst other things, it is affirmed highly desirable, inasmuch as many accidents take place owing to air-doors not being attended to; that there need be no building in of old workings, and thereby making gasholders to blow up sooner or later, as by the new system the air if let out beyond must pass through these and keep them safe; that no down. cast shaft is required, which would admit of mines being opened at a much less cost; that there would be no difficulty in providing a register which should show the precise quantity of air which had passed into the mine in any given time, whether a day, a week, or a year previously-the Government Inspector being thus enabled to ascertain whether at any time since his previous inspection the ventilation has been permitted to become defective; and that air may be sent down dry and cool; and when from any exceptional circumstances the mine has become fouled with any gas other than that usually to be guarded against, it may be charged so as to neutralize it. The inventor provides for sending down the air dry and cool, because he finds the greater part of the explosions take place in damp foggy weather.

it

As

THE LATE MR. BRAIDWOOD.-The melancholy death of Mr. Braidwood has excited great and universal sympathy. Mr. Braidwood was the son of a respectable carpenter in Edinburgh. He was born there about the year 1800. In 1824 he joined the police establishment there, and undertook the organization of a regular fire brigade. Before his plans were far advanced, and before he had either his force or his engines in working order, occurred the great conflagration of 1824, the most memorable and extensive fire in the annals of Edinburgh, and in which a great part of the High-street, including the steeple of the Tron Church, was burnt down. At this fire Mr. Braidwood first exhibited those qualities of cool determination, great daring, and skilful management, which is well known that more miners are killed by afterhe has so often put to good purpose in the fires of the damp than by fire-damp, Mr. Hughes anticipates that, metropolis. A pamphlet, which he published in 1832, inasmuch as after an explosion the men not dead on the causes and means of extinguishing fires, first might go to the outlets of the pipes, and get the fresh gave him more than local celebrity, and led to his air, there would be a diminution of 75 per cent. in the removal to London. Many stories are still told of the number of deaths from colliery explosions, and the intrepidity and presence of mind he displayed during apparatus would not get out of order by an explosion. the eight years he was fire-master at Edinburgh. On The tendency of Mr. Hughes's system of ventilation and then another, through the midst of a fire from a one occasion he carried first one barrel of gunpowder, is to disperse, and not collect the gas, and by its use the gas in the coal would be driven in rather than cellar through an ironmonger's shop, thus preventing, drawn out. The air would always be fresh, as it at the utmost personal risk, an explosion which might would go direct from the surface to the man by whom have caused great destruction of life and property. it was to be breathed, and the air channels could be On another occasion, above thirty years ago, he readily extended as the workings progressed. The narrowly escaped the sad end which ultimately befell holders may be made self-acting, and to work by him, being struck by some of the fragments of a falling steam and water. In case of fire in the mine the airroof, in consequence of which he was so much injured pipes could be used as water-pipes, and the fire thus that he had to be removed from the scene. Mr. speedily extinguished. The atmosphere would be Braidwood may be said to have left to his native city rendered much more healthy for the miners to work the legacy of a well-appointed and well-organized in, and a supply of fresh air would be always at his brigade, his plans having been fully matured before command, which could be applied at any required he left, and since that time well carried out. Since point by means of flexible tubes. The inventor conhis removal to London he has occupied for nearly fidently believes that by the means he proposes the thirty years the position of superintendent of ventilation could, with ease and economy, be made so the London Fire Brigade, and had become so perfect that naked lights might be used with safety; well-known and popular that no words of ours can and that the merits of the invention would be particudeepen the impression made by the high culogiums larly manifest in sinking shafts or wells, tunnelling, or of the daily press. The fact that her Majesty sent twice for clearing the atmosphere after blasting.-Mining in favour of his worth and well-earned reputation. to make inquiries regarding his fate, speaks volumes Journal. has published the annual reports of the London For many years past the MECHANICS' MAGAZINE RAILWAYS IN FRANCE.-During the late debate in fires." These valuable records should serve, in the Legislative Body, on the bill relative to the consome measure, as a lasting monument of the invalu-struction of new railways, M. Auguste Chevalier comable services rendered by Mr. Braidwood to the plained of the slow rate of speed common in France. He stated that he knew lines where the speed was only metropolis, to other cities, and to industry and science generally. We hope, however, our insurance comfive or six leagues an hour; on the Poitiers and Rochelle line the rate is reported to be seven leagues an hour; panies, the merchant princes and residents of London, on the Marseilles line and the Bordeaux line the and the public will not be satisfied with the history of Mr. Braidwood's services recorded in this and other journals. None of the famous soldiers or seamen deserved more highly the statues raised in our public places to commemorate their deeds, than does the plain, faithful, courageous Mr. Braidwood, a true Christian hero, killed in the discharge of his duty.

speed is about eleven leagues an hour; on the Lyons line eleven and a-half leagues an hour; on the Havre

which he edited, adding a supplementary volume. This edition bears date 1842-46; and subsequent to that, the experiments for the Britannia and Conway bridges demonstrated the remarkable fact of the opposite character of wrought to cast-iron, as to the capability to resist the relative forces of

tension and compression, and showed the value of the cellular top in a tubular beam. Amongst his writings, one of his earliest, "On the Transverse Strain and Strength of Materials," will be found in the fourth volume of the "Memoirs of the Manchester Society" (second series, 1822), in which he put forth his views in opposition to those which were general amongst scientific men as to the situation of the neutral line in a bent body. In the fifth volume of the same memoirs (1831) are five papers by him, namely, "On the Forms of the Catenary in Suspension Bridges," of which an abstract was given by the Rev. Dr. Whewell in his "Analytical Statics" (Cambridge, 1833), and an amplification by the Rev. Canon Moseley in his "Mechanical Principles of Engineering," &c.; "On

The opinions respecting the most effective means of NEW METHOD OF VENTILATING COLLIERIES.ventilating collieries are so numerous and conflicting, that doubtless many have concluded that it matters little what system is employed provided it be carried out in its integerity; it cannot, however, be doubted that there is ample room for improvement in by far too many collieries, aud, therefore, any proposition brought forward as an improvement upon existing means of accomplishing that very desirable object will be regarded with attention by all practical men, if it be only to endeavour to obtain a hint worthy of their own adoption. A proposal, which will, no doubt, give rise to much discussion, has just been made, by the specification of an invention emanating from Mr. R. H. Hughes, of Hatton Garden, which, if favourably received, cannot fail to revolutionize all the ideas we at present entertain regarding the ques

a-half leagues an hour. In England seventeen leagues, an hour, and on the Northern Railway, fourteen and and on the Great Western twenty-four leagues an hour are attained. If the speed on the railways of France were only fifteen leagues an hour, there would be a saving of three hours and thirty-two minutes between would be a saving of two hours and twenty minutes. Paris and Bordeaux; in the journey to Lyons there Between Paris and Marseilles there would be a saving

of five hours and thirty-eight minutes, and two hours would be saved on the journey to Strasbourg. The cost to attain this increased speed is said to be small; for instance, from Paris to Marseilles the cost would be only £3 4s. 2d., or 1 per cent. upon the ordinary receipts of a train of nine carriages-£400. The high rentals charged by the companies to the sellers of newspapers and refreshments, and the practice of giving free passes to persons able to pay first-class fares, were also condemned.

PRODUCTS FROM THE COMBUSTION OF GUNPOWDER UNDER DIFFERENT

PRESSURES.

BY B. F. CRAIG.

THE perusal of the paper of Bunsen and Schiskoff on "The Chemical Theory of Gunpowder," whose publication about two years ago attracted much attention, (American Journal of Science and Arts [2], xxvi. 106), led me, as it no doubt did others, to make some experiments on the residues of exploded powder.

The investigations of the above-named chemists were made on the products of gunpowder which had been burned in small quantities, and under very slight pressure, and, in concluding their paper, they advance the supposition that, under a difference of circumstances, the products may be essentially different from those which were obtained by themselves.

Such is actually the case, for the solution obtained by washing a gun with well-boiled distilled water, immediately after its discharge, and filtering while hot, will be found to contain sulphuret of potassium, with very small quantities of other substances, while Bunsen and Schiskoff found their residues to consist mainly of sulphate and carbonate of potash. This difference is what was to be expected from our knowledge of chemical laws; and it is also to be observed that the products of combustion in the gun vary somewhat according as the explosion takes place under a greater or less pressure. Thus, in firing with a shot very heavy in proportion to the charge of powder, as in firing a mortar, the washings of the piece are found to be of a bright green colour, while the ordinary washings of a gun are colourless after the suspended carbon has been allowed to settle, or has been removed by filtration.

The colour above spoken of is exactly the tint of a solution of a sesquisalt of chromium; it is not impaired by filtration of the liquid, but disappears in a few hours by exposure to the air, and immediately by the action of acids. The cause of this colour seemed to me at first obscure, but I found that by adding a very minute quantity of freshly precipitated sulphuret of iron to solution of protosulphuret of potassium, the same colour was produced, and a sensible trace of iron may always be found in gunpowder.

A greenish colour is frequently observed when sulphuret of iron is precipitated in the presence of an alkaline sulphuret, but as it disappears on standing, it has been commonly attributed to the presence of particles of sulphuret of iron held in suspension, an opinion which must be erroneous, since I have obtained a solution of iron in sulphuret of potassium of an intensely green colour, perfectly clear and transparent, and permanent for many days in a sealed tube.

Why this green compound should be formed under some circumstances, and not under others, in a mortar, and not in a cannon, is not insusceptible of explanation.

It has been shown that when powder is exploded under slight pressure the alkaline metal remains in large part as a sulphate, but under higher pressure, and corresponding temperature, it is reduced to the condition of a sulphuret. Now, by analogy, it would seem, that with the ordinary charges of a cannon or musket the iron is not reduced to the state of sulphuret, but if, as is often the case in a mortar, the projectile is fifty times the weight of the powder, or upwards, the gases are brought to a state of great tension before the shot moves, and in that state of things the carbon is completely converted into carbonic acid, and the iron, as well as the potassium, brought to the condition of sulphuret.-American Journal of Science and

Arts.

BULMER AND SHARP'S BRICK AND TILE MACHINE.

expensive tempering, but simply to be dug and supplied with water sufficient to reduce it by absorption to a plastic state. It is then passed through the pug mill, in which is fixed a series of screw knives, which press it downward through two orifices at the bottom, into a chamber from which, by means of a revolving arm, it is forced into dies, and from thence on to rollers on the cutting frame. Perforated or solid bricks being produced at pleasure, each semi-revolution of the mill forces a band of clay sufficient to make four to six bricks, which is produced at either side of the mill alternately, and thus allows the clay to be stationary whilst being cut, and so ensuring uniform size, with straight side and well-defined angles. The simple arrangement of the chamber at the bottom of the mill, and the arm revolving in it, constitute an important improvement.

These machines are constructed entirely of iron, and are driven by tooth gearing, protected by friction straps, the whole being extremely strong and simple. At Middlesbro' a machine has been worked for some time with perfect success, producing 25,000 bricks per day of ten hours. The validity of this patent was fully sustained, a short time since, in a severely contested trial of four days' duration.

The second engraving shows a machine to be worked by hand-power, by which 5,000 bricks per day are produced. This machine is so simple and efficient that well-moulded bricks of superior quality can be rapidly produced by labourers, skilled or unskilled, whether previously accustomed to the making of bricks or not. A combination of power and speed hitherto unknown in the manufacture of bricks and tiles is secured in this invention by the introduction of toothed oval wheels. The machine is mounted on wheels, and is so portable as to be

ALMOST every branch of manufacturing industry has been revolutionized by the application of machinery and steam, but the brick-maker, for the most part, continues to ply his craft in the same primitive manner as did the generations of anThe tiquity. The inventions hitherto in use have fre-readily moved from one place to another. quently been found expensive and complicated in with clay while at work, and the bricks are delivered motion is continuous, the machine being casily fed construction; and, owing in part to the intractable sufficiently stiff to allow of walling direct from the nature of the raw material, comparatively little machine, the necessity for flat ground being no benefit has resulted from the application of steam longer required. These machines are equally to this branch of industrial arts. machine of Messrs. Bulmer and Sharp, of Middles- applicable to the making of tiles, &c. bro'-on-Tees, now brought under review, and illustrated in the above engravings, is calculated to obviate these objections. It combines simplicity, efficiency, and economy. The clay requires no

The patent

THE Times newspaper of Friday last contained, according to the calculation of a correspondent, 38,166 lines of type, 345,104 words, and 1,589,040 letters.