about the centre of the indigo portion of the spectrum, whilst bromide of silver was affected by the green, blue, and some of the indigo rays.

It is very likely that a continuance of these experiments would lead to the construction of a photometer capable of measuring the luminous rays; for although bromide of silver behind quinine is not affected by the red or yellow rays, still it is by the green and blue; and as the proportion of red, yellow, green, and blue rays is always invariable in white light (or the light would not be white but coloured), a method of measuring the intensity of one set of the components of white light would give all the information we want; just as, in an analysis of a definite chemical compound, the chemist is satisfied with an estimation of one or two constituents only, and calculates the others.

Method based upon the foregoing considerations would supply us with what may be termed an absolute photometer, the indication of which would be always the same for the same amount of illumination, requiring no standard light for comparison; and pending the development of experiments which the writer is prosecuting in this direction, he has been led to devise a new and, as he believes, a valuable form of relative photo

meter.

A relative photometer is one in which the observer has only to determine the relative illuminating powers of two sources of light, one of which is kept as uniform as possible, the other being the light whose intensity is to be determined. It is therefore evident that the great thing to be aimed at is an absolutely uniform source of light. In the ordinary process of photometry the standard used is a candle, defined by Act of Parliament as a "sperm candle of six to the pound, burning at the rate of 120 grains per hour." This is the standard from which estimates of the value of illuminating gas are deduced, hence the terms 12-candle gas,' 14-candle gas," &c. In his work on Gas Manipulation, Mr. Sugg gives a very good account of the difficulties which stand in the way of obtaining uniform results with the Act of Parliament candle. A true spermcandle is made from a mixture of refined sperm with a small proportion of wax, to give it a certain toughness, the pure sperm itself being extremely brittle. The wick is of the best cotton, made up into three cords and plaited. The number of strands in each of the three cords composing the wick of a six-to-the-pound candle is seventeen, although Mr. Sugg says there does not appear to be any fixed rule, some candles having more and others less, according to the quality of the sperm. Sperm-candles are made to burn at the rate of one inch per hour, and the cup should be clean, smooth, and dry. The wick should be curved slightly at the top, the red tip just showing through the flame, and consuming away without requiring snuffing. To obtain these results, the tightness of the plaiting and size of the wick require careful attention; and as the quality of the sperm differs in richness or hardness, so must the plaiting and number of strands. A variety of modifying circumstances thus tend to affect the illuminating power of a standard sperm candle. These difficulties, however, are small, compared with those which have resulted from the substitution of paraffin, &c., for part of the sperm; and Mr. Sugg points out that candles can be made with such combinations of stearin, wax, or sperm, and paraffin, as to possess all the characteristics of sperm candles, and yet be superior to them in illuminating power; while, on the other hand, candles made from he same materials otherwise combined are inferior.

NEWS

When, in addition to this, it is found that candles containing paraffin require wicks more tightly plaited and with fewer strands than those suitable for the true sperm candle, our readers will be enabled to judge of the almost unsurmountable difficulties which beset the present system of photometry.

But assuming that the true parliamentary sperm candle is obtained, made from the proper materials and burning at the specified rate, its illuminating power will be found to vary with the temperature of the place where it has been kept, the time which has elapsed since it was made, and the temperature of the room wherein the experiment is tried.

The Rev. W. R. Bowditch, in his work on The Analysis, Purification, &c., of Coal Gas, enters at some length into the question of test-candles, and emphatically condemns them as light-measures; one experiment quoted by this author showed that the same gas was reported to be 1463 or 17:36 candle gas, according to the way the experiment was conducted.

The present writer has taken some pains to devise a source of light which should be at the same time fairly uniform in its results, would not vary by keeping, and would be capable of accurate imitation at any time and in any part of the world by mere description. The absence of these conditions seems to be one of the greatest objections to the sperm candle. It would be impossible for an observer on the continent, ten or twenty years hence, from a written description of the sperm candle now employed, to make a standard which would bring his photometric results into relation with those obtained here. Without presuming to say positively that he has satisfactorily solved all difficulties, the writer believes that he has advanced some distance in the right direction, and pointed out the road for further improvement.

Before deciding upon a standard light, experiments were made to ascertain whether the electric current could be made available. Through a coil of platinum wire, so as to render it brightly incandescent, a powerful galvanic current was passed; and its strength was kept as constant as possible by a thick wire galvanometer and rheostat. To prevent the cooling action of aircurrents, the incandescent coil was surrounded with glass; and it was hoped that by employing the same kind of battery and by varying the resistance so as to keep the galvanometer needle at the same deflection, uniform results could be obtained. In practice, however, it was found that many things interfered with the uniformity of the results, and the light being much feebler than it was advisable to work with, this plan was deemed not sufficiently promising, and it was abandoned. The method ultimately decided upon is the following:-Alcohol of sp. gr. 0.805, and pure benzol boiling at 81° C., are mixed together in the proportion of 5 volumes of the former and I of the latter. This burning fluid can be accurately imitated from description at any future time and in any country, and if a lamp could be devised equally simple and invariable, the light which it would yield would, it is presumed, be invariable. This difficulty the writer has attempted to overcome in the following manner.

A glass lamp is taken of about 2 ounces capacity, the aperture in the neck being o'25 inch diameter; another aperture at the side allows the liquid fuel to be introduced, and by a well-known laboratory device, the level of the fluid in the lamp can be kept uniform. The wick-holder consists of a platinum tube 181 inches long and o'125 inch internal diameter. The bottom of

this is closed with a flat plug of platinum, apertures being left in the sides to allow free access of spirit. A small platinum cup 5 inch diameter and I inch deep is soldered round the outside of the tube 0.5 inch from the top, answering the threefold purpose of keeping the wick-holder at a proper height in the lamp, preventing evaporation of the liquid, and keeping out dust. The wick consists of 52 pieces of hard-drawn platinum wire, each o'01 inch in diameter and 2 inches long, perfectly straight and tightly pushed down into the platinum holder until only o'I inch projects above the tube. The height of the burning fluid in the lamp must be sufficient to cover the bottom of the wick holder; it answers best to keep it always at the uniform distance of 175 inches from the top of the platinum wick; a slight variation of level, however, has not been found to influence the light to an extent appreciable by our present means of photometry. The lamp having the reservoir of spirit thus arranged, the platinum wires parallel and their projecting ends level, a light is applied, and the flame instantly appears, forming a perfectly-shaped cone 125 inches in height, the point of maximum brilliancy being o'56 inch from the top of the wick. The extremity of the flame is perfectly sharp, without any tendency to smoke; without flicker or movement of any kind, it burns, when protected from currents of air, at a uniform rate of 136 grains of liquid per hour. The temperature should be about 60° F., although moderate variations on either side exert no perceptible influence. Bearing in mind Dr. Frankland's observations on the direct increase in the light of a candle with the atmospheric pressure, accurate observations ought only to be taken at one height of the barometer. To avoid the inconvenience and delay which this would occasion, a table of corrections should be constructed for each o'1 inch variation of barometric pressure.

There is no doubt that this flame is very much more uniform than that of the sperm candle sold for photometric purposes. Tested against a candle, considerable variations in relative illuminating power have been observed; but on placing two of these lamps in opposition, no such variations have been detected. The same candle has been used, and the experiments have been repeated at wide intervals, using all usual precautions to ensure uniformity. The results are thus shown to be due to variations in the candle and not in the lamp. It is expected that whoever may be inclined to adopt the kind of lamp here suggested will find not only that its uniformity may be relied upon, but that, by following accurately the description and dimensions here laid down, each observer will possess a lamp of equivalent and convertible photometric value, so that results may not only be strictly comparable between themselves, but, within slight limits of accuracy, comparable with those obtained by other experimentalists. The dimensions of wick, &c., here laid down are not intended to fix the standard. Persons engaged in photometry as an important branch of their regular occupation will be better able to fix these data than the writer, by whom photometry is only occasionally pursued as a means of scientific research. Already many improvements suggest themselves, and several causes of variation in the light have been noticed. Future experiments may point out how these sources of error are to be overcome; but at present there is no necessity to refine our source of standard light to a greater degree of accuracy than the photometric instrument admits of

The instrument for measuring the relative intensities of the standard and other lights, next demands attention. The contrivances in ordinary use are well known. Most of them depend on a well-known law in optics, namely, that the amount of light which falls upon a given surface varies inversely as the square of the distance between the source of light and the object illuminated. The simplest observation which can be taken is made by placing two sources of light (say a candle and gas-lamp) opposite a white screen a few feet off, and placing a stick in front of them, so that two shadows of the stick may fall on the screen. The strongest light will cast the strongest shadow; and by moving this light away from the stick, keeping the shadows side by side, a position will at last be found at which the two shadows appear of equal strength. By measuring the distance of each light from the screen, and squaring it, the product will give the relative intensities of the two sources of light.

In practice, this plan is not sufficiently accurate to be used except for the roughest approximations; and from time to time several ingenious contrivances, all founded upon the same law, have been introduced by scientific men, by which a much greater accuracy is obtained: thus in Ritchie's photometer the lights are reflected on to a piece of oiled paper in a box, and their distances are varied until the two halves of the paper are equally illuminated. In Bunsen's photometer, which is the one now generally used, the lights shine on opposite sides of a disc of white paper, part of which has been smeared with melted spermaceti to make it more transparent. When illuminated by a front light the greased portion of the paper will look dark; but if the observer goes to the other side of the paper, the greased part looks the lighter. If, therefore, lights of unequal intensity are placed on opposite sides of a piece of paper so prepared, a difference will be observed; but by moving one backwards or forwards, so as to equalise the intensity, the whole surface of the paper will appear uniformly illuminated on both sides. This photometer has been modified by many observers. By some the disc of paper is moved, the lights remaining stationary; by others the whole is enclosed in a box, and various contrivances are adopted to increase the sensitiveness of the eye and to facilitate calculation: but in all these the sensitiveness is not greatly augmented, as the eye cannot judge of very minute differences of illumination approximating to equality.

In 1833, Arago described a photometer in which the phenomena of polarised light were employed. This instrument is fully described, with drawings, in the tenth volume of the Euvres Complètes de François Arago; but the description, although voluminous, is far from clear. The principle of its construction is founded on "the law of the square of the cosines," according to which polarised rays pass from the ordinary to the extraordinary image. The knowledge of this law, he says, will not only prove theoretically important, but will further lead to the solution of a great number of very important astronomical questions. Suppose, for example, that it is wished to compare the luminous intensity of that portion of the moon directly illuminated by the solar rays with that of the part which receives only light reflected from the earth, called the partie cendrée. Were the law in question known, the way to proceed would be as follows:-After having polarised the moon's light, pass it through a doubly refracting crystal, so disposed that the rays, not being able to bifurcate, may entirely undergo ordinary refraction. A lens

DC is pushed into the end в, one white surface, D B, may be illuminated (as in Fig. 2) by the candle, and the other surface, B c, by the lamp. If the eye-piece A is removed, the observer, looking down the tube GB, will see at the end a luminous white disc divided vertically into two parts, one half being illuminated by the candle E, and the other half by the lamp F. By moving the candle E, for instance, along the scale, the illumination of

FIG. 2.

the half D B can be varied at will, the illumination of the other half remaining stationary.

The eye-piece A (shown enlarged at Fig. 3) will be understood by reference to Fig. 1, the same letters representing similar parts. At L is a lens to collect the rays from D B C (Fig. 2), and throw the image into the proper part of the tube. At M is another lens, so adjusted as to give a sharp image of the two discs into

E

which I is divided by the prism K. adaptation of Arago's polarimeter; series of thin plates of glass capable of moving round the axis of the tube, and furnished with a pointer and graduated arc (shown at A, G, Fig. 2). By means of this pile it is possible to partially polarise the rays coming from the illuminated discs in one or the other direction, and thus bring to the neutral state the partially polarised beam c d (Fig. 1), so as to get the images gr free from colour. It is so adjusted that when at the zero point it produces an equal effect on both discs.

The action of the instrument is as follows. The standard lamp being placed on one of the supporting pillars which slide along the graduated stem (Fig. 2), it is adjusted to the proper height, and moved along the bar to a convenient distance, depending on the intensity of the light

K

The part N is an it consists of a

FIG. 3.

to be measured; the whole length being a little over four feet, each light can be placed at a dis

tance of twenty-four inches from the disc. The flame is then sheltered from currents of air by black screens placed round, and the light to be compared is fixed in a similar way on the other side of the instrument. The whole should be placed in a dark room, or surrounded with non-reflecting screens; and the eye must also be protected from direct rays from the two lights. On looking through the eye-piece two bright discs will be seen, probably of different colours. Supposing E represents the standard flame, and F the light to be compared with it, the latter must now be slid along the scale until the two discs of light, seen through the eye-piece, are about equal in tint. Equality of illumination is easily obtained; for, as the eye is observing two adjacent discs of light, which pass rapidly from red-green to green-red, through a neutral point of no colour, there is no difficulty in hitting this point with great precision. It has been found most convenient not to attempt to get absolute equality in this manner, but to move the flame to the nearest inch on one side or the other of equality. The final adjustment is now effected at the eye-end, by turning the polarimeter one way or the cther up to 45°, until the images are seen without any trace of colour. This will be found more accurate than the plan of relying entirely on the alteration of the distance of the flame along the scale; and by a series of experimental adjustments, the value of every angle through which the bundle of plates is rotated can be ascertained once for all, when the future calculations will present no difficulty. Squaring the number of inches between the flames and the centre will give their approximate ratios; and the number of degrees the eye-piece rotates will give the number to be added or subtracted in order to obtain the necessary accuracy.

The delicacy of the instrument is very great. With two lamps, each about twenty-four inches from the

centre, it is easy to distinguish a movement of one of them to the extent of 1-10th of an inch to or fro; and by using the polarimeter, an accuracy considerably exceeding that can be attained.

The employment of a photometer of this kind enables us to compare lights of different colours with one another, and leads to the solution of a problem which, from the nature of their construction, would be beyond the powers of the instruments in general use. So long as the observer, by the eye alone, has to compare the relative intensities of two surfaces respectively illuminated by the lights under trial, it is evident that unless they are of the same tint it is impossible to obtain that absolute equality of illumination in the instrument which is requisite for a comparison. By the unaided eye one cannot tell which is the brighter half of a paper disc illuminated on one side with a reddish, and on the other with a yellowish, light; but by using the above-described photometer, the problem becomes practicable. For instance, on reference to Fig. 1, suppose the disc D were illuminated with light of a reddish colour, and the disc c with greenish light, the polarised discs d' d would be reddish and the discs c c' greenish, the central disc c d being of the tint formed by the union of the two shades. The analysing prism K, and the selenite disc I will detect free polarisation in the disc c d, if it be coloured, as readily as if it were white; the only difference being that the two discs of light gr cannot be brought to a uniform white colour when the lights from D and c are equal in intensity, but will assume a tint similar to that of c d. When the contrasts of colour between D and o are very strongwhen, for instance, one is a bright green and the other scarlet-there is some difficulty in estimating the exact point of neutrality; but this only diminishes the accuracy of the comparison, and does not render it impossible, as it would be according to other systems. No attempt has been made in these experiments to ascertain the exact value of the standard spirit-flame in terms of the parliamentary sperm candle. Difficulty was experienced in getting two lots of candles yielding light of equal intensities, and when their flames were compared between themselves and with the spiritflame, variations of as much as 10 per cent. were sometimes observed in the light they gave. Two standard spirit-flames, on the other hand, seldom showed a variation of 1 per cent., and had they been more carefully made they would not have varied o'i per cent.

This plan of photometry is capable of far more accuracy than the present instrument will give. It can scarcely be expected that the first instrument of the kind, roughly made by an amateur workman, should possess equal sensitiveness with one in which all the parts have been skilfully made with special adaptation to the end in view.-Quarterly Journal of Science.

. stop-cock tube, which should be made straight and somewhat longer than usual, and in front of the stopcock itself, a short vertical tube is attached and connected by means of india-rubber tubing with the washbottle, or other vessel through which gas is to be drawn. On partially opening the stop-cock, the deficiency of water is made up by a large quantity of air or gas, which is drawn in through the vertical tube above mentioned.—Am. Journ. Sci., May, 1868.

The figures representing zinc, copper, and sulphur are the mean results of several analyses; the other subThe rock, or stances were only once determined. gangue, consists in this case entirely of quartz and phur is mostly free, and often visible. In the deposit mica schist; it is, therefore, principally silica. The sulformed in the tunnel itself only a trace of arsenic was residue in its composition; and this again differs from found. The red flue dust differs considerably from this gets too hot-and from that in No. 1 chamber adthe deposits in the tunnel-especially when the latter joining.

ON SOME OF

THE CONSTITUENTS OF COAL GAS.*
A LECTURE BY DR. ODLING, F.R.S., F.C.S.,

BEFORE THE BRITISH ASSOCIATION OF GAS MANAGERS.

EVER since the French Exhibition of last year there has been much public talk and discussion about the decline of English scientific manufactures, especially in relation to those of the Continent. It has been as

BY PROFESSOR ALBERT R. LEEDS,

OF HAVERFORD COLLEGE, PA.

A PAIL Containing water is placed at the edge of the table, and to the stop-cock which is attached to the side of the pail near the bottom, a tube of two or three feet in length is connected, to carry off the water to a bucket placed on the floor below. When the bucket is filled, the stop-cock is turned off for a moment, and the water poured back into the pail. To the top of the

lighting took its origin, where the first locomotive ran, and where some of the earliest and greatest successes of the electric telegraph have been achieved-are not making such a degree of progress in scientific manufactures as continental nations are. Now, whether this be the fact or not-and for my own part I am by no means disposed to admit it as a fact-the circumstance of such an allegation being made is, I conceive, a warning to us that we must for the future devote our

* Reprinted, by permission, from the Journal of Gas Lighting.