well as chromatic aberration, and at the same time to make the adjacent surfaces fit, very suitable forms were obtained with the data furnished by Mr. Harcourt's glasses.

After encountering great difficulties from striæ, Mr. Harcourt at last succeeded in preparing disks of terborate of lead and of a titanic glass which are fairly homogeneous, and with which it is intended to attempt the construction of an actual objective which shall give images free from secondary colour, or nearly so.

This notice has extended to a greater length than I had intended, but it still gives only a meagre account of a research extending over so many years. It is my intention to draw up a full account for presentation to the scientific world in some other form. I have already said that the grant made to Mr. Harcourt for these researches in 1836 has long since been expended, as was stated in his Report of 1844; but it was his wish, in recognition of that grant, that the first mention of the results he obtained should be made to the British Association; and I doubt not that the members will receive with satisfaction this mark of consideration, which they will connect with the memory of one to whom the Association as a body is so deeply indebted.

On one Cause of Transparency. By G. JOHNSTONE STONEY, M.A., F.R.S.

The motion of the æther which constitutes light is known to be subject to four restrictions:-First, it is periodic; secondly, it is transversal; thirdly, it is (at all events temporarily) polarized; and, fourthly, its periodic time lies between the limits which correspond to the extent of the visible spectrum. By temporary polarization is meant the persistence of the same kind of wave over a long series of waves before waves of another kind succeed, that persistence which the phenomena of diffraction have made known to us*.

And the many respects in which radiant heat and light have been found to be identical enable us to say that the first three of the foregoing restrictions apply to radiant heat. We also know (see Philosophical Magazine' for April and for July 1871) that the lines in the spectra of gases arise from periodic motions in the molecules of the gas, each such motion giving rise to one or more lines corresponding to terms of an harmonic series. And we know that under certain conditions these lines dilate and run into one another, so as in many cases to produce regions of continuous absorption. All these phenomena may safely be attributed to periodic motions in the molecules of the gas, the dilatation of the lines being due to perturbations which affect the periodic times. After the periodic time has been disturbed (probably on the occasion of the collisions between molecules) it seems to settle down gradually towards its normal amount, thus imparting breadth to the corresponding spectral lines.

The question now naturally presents itself What results from motions in the molecules which are not periodic, or which are in any other way unfitted to produce radiant heat? And here the phenomena of acoustics come to our aid. When a bell is struck, more or less regularly, periodic motions are both produced. The more regularly periodic motions produce the tone of the bell which is heard at a distance, while the less regular motions, though they are often very intense, produce a clang heard only in the vicinity of the bell; in other words, the energy is expended in the neighbourhood of the bell. Similarly, if the molecules of a body are engaged in irregular motions, such motions, though they may occasion a violent agitation of the others, are mechanically incapable of producing such an undulation as constitutes radiant heat. The disturbance is necessarily local; in other words, as much energy is restored by the moving æther to the molecules as is imparted by the motion of the molecules to the æther. This absence of radiation is one of the properties of a transparent body; and the other thermal (or optical) properties of transparent bodies may be presumed to depend also on these partially irregular motions. Thus Fizeau has proved by experiment that a flow of water of about

*Rays of common light have been found to interfere, of which one was retarded 15 millims., or about 30,000 wave-lengths, behind the other, showing what a long series of nearly similar waves usually succeed one another in unpolarized light before waves of another type come in.

seven metres per second produced a very sensible effect on the velocity with which light was propagated in the direction of the motion; in other words, when the molecular motions had a preponderance in one direction, this was found to alter the refractive index in that direction. This shows that the molecular motions do affect the refractive index; and it is perhaps not too much to presume that the phenomena of the irrationality of the spectra produced by prisms of different materials of double refraction and polarization in crystals of other than the cubical system, and of circular polarization in solids and liquids, will be found to result from modifications of the irregular motions either of or within the molecules. Other facts appear to confirm this presumption: where from the form of a crystal we have reason to suppose that the irregular molecular motions are not symmetrically distributed in different directions, there we uniformly find the phenomena of double refraction; and in those solids where they are symmetrically disposed the refraction becomes double if they are exposed to strain, i. e. as soon as an unsymmetrical distribution of the molecular motions is artificially induced.

On the whole we appear justified in drawing the probable inference that all the phenomena of transparency are intimately associated with the molecular motions which want that kind of regularity which would fit them to be the source of luminous undulations. What is certain is, first, that certain periodic molecular motions do produce the phenomena of opacity in gases; and secondly, that irregular molecular motions are incapable of producing the effect of opacity, since they cannot radiate. By irregular motions, where the phrase occurs in this communication, are to be understood motions which are not approximately periodic, or which from any other cause cannot set up in the æther such an undulation as that which constitutes radiant heat.

On the advantage of referring the positions of Lines in the Spectrum to a Scale of Wave-numbers. By G. JOHNSTONE STONEY, M.A., F.R.S.

At the last Meeting of the British Association Mr. Stoney made a communication, from which it seemed to appear that each periodic motion in the molecules of a gas will in general (.e. unless the motion be a simple pendulous one, or else mechanically small) give rise to several lines in the spectrum of the gas, and that the lines which thus result from one motion have periods that are harmonics of the periodic time of the parent motion. Since that time he has been engaged, in conjunction with Dr. Emerson Reynolds, of Dublin, in testing this theory; and in this inquiry it has been found convenient to refer the positions of all lines in the spectrum to a scale of reciprocals of the wave-lengths. This scale has the great convenience, for the purposes of the investigation, that a system of lines with periodic times that are harmonics of one periodic time are equidistant upon it; and it has the further convenience, which recommends it for general use, that it resembles the spectrum as seen in the spectroscope much more closely than the scale of direct wave-lengths used by Ångström in his classic map.

The position marked 2000 upon this scale occurs about the middle of the spectrum, and corresponds to Angström's wave-length 5000. The numbers which Angström uses are tenth-metres, i. e. the lengths obtained by dividing the metre into 1010 parts; and from this it follows that each number on the new scale signifies the number of light-waves in a millimetre: thus 2000 upon a map drawn to this scale marks the position of the ray whose wave-length is of a millimetre. The new scale may therefore be appropriately called a scale of wavenumbers. If, then, k be the wave-number of a fundamental motion in the æther, its wave-length will be-th of a millimetre, and its harmonics will have the wavek 1 1 2k 3k

་

lengths &c.; in other words, they occupy the positions 2k, 3k, &c. upon the new map. Hence it is easy to see that a system of lines which are equally spaced along the map at intervals of k divisions are harmonics of a fundamental motion whose wave-number is k, whose wave-length is th of a millimetre, and

whose periodic time is

T

where is the periodic time of an undulation in the æther consisting of waves one millimetre long. If we use Foucault's determination of the velocity of light, viz. 298,000,000 metres per second, the value of this constant is

T = 3.3557 twelfth-seconds,

meaning by a twelfth-second a second of time divided by 1012, which, in other words, is the millionth part of the millionth of a second of time."

Thus the proposed numbers give the same information as a list of direct wavelengths, and in a more commodious form for theoretical purposes; while at the same time the map of the spectrum drawn to this scale is to be preferred for use in the laboratory, because it represents the spectrum formed by a prism with comparatively little distortion. This will be apparent from the following Table of the wave-numbers of the principal lines of the solar spectrum :—

On the Wave-lengths of the Spectra of the Hydrocarbons.
By Professor WILLIAM SWAN, LL.D., F.R.S.E.

The author stated that in 1856 he had communicated to the Royal Society of Edinburgh a paper, published in vol. xxi. of their Transactions, entitled "On the Prismatic Spectra of the Flames of Compounds of Carbon and Hydrogen." In his observations on these substances he made use of an arrangement (employed by him still earlier in 1847) identical with that which, since the publication of Kirchhoff and Bunsen's researches in Spectrum-analysis, is familiarly known as a "Spectroscope," namely, an observing telescope, a prism, and a collimator, receiving the light to be examined through a narrow slit at its principal focus.

The observations published in 1856 consist of carefully observed minimum deviations of fourteen dark lines of the sun spectrum, and of twelve bright lines of the hydrocarbon spectra, which bright lines were found to be identical in fifteen different hydrocarbons examined. No absolute coincidences between the lines in the solar and terrestrial spectra were observed, except that, long before discovered by Fraunhofer, between the double sun-line D and the double yellow line of ordinary flames, now, wherever it may be seen, referred to sodium.

The yellow line was generally present in the hydrocarbon spectra; but, from a careful quantitative experiment, it was ascertained that the 2,500,000th part of a troy grain of sodium rendered its presence in a flame sensible: and the conclusion was then distinctly stated, it is believed for the first time, that whenever or whereever the double yellow line appears it is due to the presence of minute traces of sodium.

In this state the observations of 1856 had remained until lately, when the author was requested by his friend Professor Piazzi Smyth to compute the wave-lengths of some of the hydrocarbon lines. As no exact coincidence existed between these and the lines of the solar spectrum, it was necessary to have recourse to some process of interpolation; and that which suggested itself to the author was founded upon Lagrange's well-known Interpolation theorem. In order to verify as far as possible the results, the computation of the wave-lengths of the hydrocarbon lines was repeated by interpolating between different groups of sun lines; and the discrepancies between the numbers so obtained in no case extended beyond the place of units in Ångström's scale of wave-lengths, where unity expresses the ten millionth part of a millimetre. The subject was brought before the Association in order

to elicit an opinion whether the results likely to be obtained would be of sufficient importance to warrant a more elaborate discussion of the entire series of observations with a view to future publication.

Poste Photographique. By the ABBÉ MOIGNO.

An Account of a New Photographic Dry Process. By R. SUTTON.

HEAT.

Description of Experiments made in the Physical Laboratory of the University of Glasgow to determine the Surface Conductivity for Heat of a Copper Ball. By DONALD M FARLANE.

The experiments described in this paper were made under the direction of Sir W. Thomson during the summers of 1865 and 1871. A hot copper ball, having a thermoelectric junction at its centre, was suspended in the interior of a closed space kept at a constant temperature of about 16° Cent., the other junction was kept at the temperature of the envelope, the circuit was completed through a mirror galvanometer, and the deflections noted at intervals of one minute as the ball gradually cooled.

The method of reducing the observations was explained at length. The difference of the Napierian logarithms of the differences of temperatures of the junctions, indicated by the deflections, divided by the intervals of time, gives the rate of cooling; and this, multiplied by a factor depending on the capacity for heat of the ball and on the extent of its surface, gives the quantity of heat emitted in gramme water units in the unit of time per square centimetre, per 1° of difference of temperatures. Formula were given which express the results of the experiments very closely, and a table calculated by them exhibits the rates of emission for every 5° of difference throughout the range.

The first and second series had a range of from 5° to 25° only, which was too small to give decided results; but the third and fourth series, made with a polished copper surface and a blackened surface respectively, gave variations in the emissive power from 000178 at 50 diff. of temperature to 000226 at 60° diff. for the polished surface, and from 000252 at 5° diff. to 000328 at 60° diff. for the blackened surface; and the emissive powers of the two surfaces exhibit throughout a nearly constant ratio to each other of about 694.

On a Respirator for Use in Extinction of Fires.
By WILLIAM LADD, F.R.A.S.

This instrument combines the advantages of the charcoal and the cotton-wool respirators. The respirator is intended to be fitted on the heads of firemen, and it will enable a fireman to enter into the midst of any smoke, however dense. There is sufficient protection for the eyes, by means of glasses. The results of an experiment with the respirator have been stated by Prof. Tyndall. In a small cellar-like chamber, furnaces containing resinous pine-wood were placed, and the wood being lighted, a dense smoke was generated. In this room, Prof. Tyndall and his assistant, using these respirators, remained for more than half an hour, when the smoke was so dense and pungent that a single inhalation through the unprotected mouth and nostrils would have been perfectly unendurable. The instrument has been tested by Capt. Shaw, chief officer of the Metropolitan Fire Brigade, who has taken very great interest in perfecting it, by attaching to it suitable hoods.

On the Temperature-equilibrium of an Enclosure in which there is a Body in Visible Motion. By Prof. BALFOUR STEWART, F.R.S.

It is now several years since Professor Tait and the author of this paper came jointly to entertain the belief that there is some transmutation of energy, the exact nature of which is unknown, when large bodies approach or recede from one another. It is desirable to vindicate an idea of this nature, both from the theoretical and the practical point of view-that is to say, we ought, if possible, to exhibit it as a probable deduction from those laws of nature with which we are already acquainted; and, on the other hand, it ought to be supported by observations and experiments of a new kind. In our case the experiments and observations have been of a difficult nature, and are yet in progress; it is therefore premature to bring them before the notice of the Association. A theoretical vindication of the idea has been obtained by Professor Tait, and more recently one has occurred to the author of these remarks, which he now ventures to bring forward. Men of science are now sufficiently well acquainted with Prevost's theory of exchanges, and its recent extension. We know that in an enclosure, the walls of which are kept at a constant temperature, every substance will ultimately attain the very same temperature as these walls, and we know also that this temperature-equilibrium can only be brought about by the absorption of every particle being exactly equal to its radiation, an equality which must separately hold for every individual kind of heat which the enclosure radiates. This theoretical conclusion is supported by numerous experiments, and one of its most important applications has been the analysis of the heavenly bodies by means of the spectroscope. Let us now suppose that in such an enclosure we have a body in visible motion, its temperature, however, being precisely the same as that of the walls of the enclosure. Had the body been at rest, we know from the theory of exchanges that there would have been a perfect equilibrium of temperature between the enclosure and the body; but there is reason to believe that this state of temperature-equilibrium is broken by the motion of the body. For we know both from theory and experiment that if a body, such for instance as a star, be either rapidly approaching the eye of an observer or receding from it, the rays from the body which strike the eye will no longer be precisely the same as would have struck it had the body been at the same temperature and at rest-just as the whistle of a railway engine rapidly approaching an observer will have to him a different note from that which it would have had if the engine had been at rest. The body at motion in the enclosure is not therefore giving the enclosure those precise rays which it would have given it had it been at the same temperature and at rest; on the other hand, the rays which are leaving the enclosure are unaltered. The enclosure is therefore receiving one set of rays and giving out another, the consequence of which will be a want of temperature-equilibrium in the enclosure, in other words, all the various particles of the enclosure will not be of the same temperature. Now, what is the consequence of this? The consequence will be that we can use these particles of different temperature so as to transmute part of their heat into the energy of visible motion, just as we do in a steam-engine; and if it is allowable to suppose that during this process the moving body has retained all its energy of motion, the result will be an increase of the amount of visible energy within the enclosure, all the particles of which were originally of the same temperature. But Sir W. Thomson has shown us that this is impossible; in other words, we cannot imagine an increase of the visible energy of such an enclosure unless we acknowledge the possibility of a perpetual motion. It is not, therefore, allowable to suppose that in such an enclosure the moving body continues to retain all its energy of motion, and consequently such a body will have its energy of motion gradually stopped. Evidently in this argument the use of the enclosure has been to enable us to deduce our proof from the known laws of heat and energy, and we may alter the shape of the body without affecting the result; in other words, we should expect some loss of visible energy in the case of cosmical bodies approaching or receding from one another.

On a new Steam-gauge. By Prof. CH. V. ZENGER.

is intended to avoid the defects of common air-gauges, which have