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and thus F can never assume negative values. Also

F=(2a2 +262 +2c* — 302+A2+B?+C?), . (29) this is the expression for the “ Dissipation Function " F originally given by Lord Rayleigh. The cause for this denomination is obvious. Suppose a, b, c, A, B, C to have their sign changed ; the sign of po will change but that of F will not ; hence the term po corresponds to a reversible, the term F to an irreversible phenomenon. Indeed the conversion of molar energy into molecular energy which goes on, of whatever kind the existing disturbance may be, is irreversible. We have thus before us an example of Dissipation of Energy in a purely dynamical system. 3. If we now suppose

every molecule

possesses an amount Mh of internal energy in addition to that which depends on its motion of translation, we shall have


[lp (ve2 + x2 + w? +82 + m +52) + p]=0. င်/

(30) Now it is easily seen that 8/8t and dīdt are equivalent, since h cannot be affected by external forces any more than by convection; therefore d

ip (E+ n° +8+ + P it

+pg?a +pr96+p8*c+pnA +PŠEB+p&C dt a

(p8* + pn? + pĚ5°) + dy

(ρηξ? + ρη2 +ρης?)

+ (ρξξ +ρζη? +ρζ3) = 0.

(31) Maxwell puts =}(B-1)p(+ m2 +$2); if this relation is adopted, equatiop (31). will be that numbered (94) in Maxwell's paper, with a difference of factor, however, in the terms relating to conduction of heat. The foregoing deduction will not be subject to the criticism which M. Poincaré has offered with respect to Maxwell's deduction (see Comptes Rendus, vol. cxvi. p. 1017; see also ibidem, p. 1165, where the internal energy of the molecules is taken into account in the same way as that here adopted).




XLVI. Experiments on the Radiation of Heated Gases.


[Plate VII.] THE renewed interest aroused in certain fundamental

questions in spectroscopy by the publication of the researches of Pringsheim, Paschen, and Sinithells has led me recently to undertake a series of observations on the visible

* Communicated by the Author.



radiations of certain elementary substances, particularly iodine and sodium, when their vapours are subjected to external heating. The important article published in the Philosophical Magazine (March 1894), by Prof. Smithells, may be said to have furnished the main incentive in this work, as it likewise determined the direction followed in the experiments.

I am further indebted to Prof. Smithells, to whom I submitted a statement of my results, for valuable advice and criticism, which has enabled me

to anticipate certain objections that might have been raised to my conclusions.

The experiments were undertaken primarily with a view to obtaining a closer personal acquaintance with what I may call the “spectroscopic behaviour" of heated gases. I had

" no idea that they would lead to results which could throw any new light on the questions discussed in Prof. Smithells paper. Nevertheless, after continuing the experiments in a somewhat desultory fashion for some six months, and with very simple appliances, certain facts have presented themselves in a prominent, way, which, so far as I can discover, have not been sufficiently noticed by previous workers, and which would appear to lend a strong support to the view there advocated, namely, that the luminosity of gases in flames may be directly due to high temperature, and that external heating is in itself sufficient to make a gas emit visible rays.

The fact that iodine vapour can be easily made incandescent by external heating was noticed by Salet (Spectroscopie, p. 173); and in the article above mentioned Prof. Smithells concludes that this luminosity can only be due to heat, and that chemical action plays no part in the phenomenon. It seemed, therefore, of interest to determine, first, the character of this iodine emission, and then to discover whether the property of glowing by mere heating was peculiar to iodine, or was shared in greater or less degree by other allied elements when vaporized, particularly by those which exhibit strong absorptive properties on light. After having thus gained some experience with the less easily oxidizable metalloids, I proposed finally to approach the question of the radiation of heated metallic vapours, to determine whether or not their characteristic spectra can be produced by mere heating.

It will perhaps be worth while at the outset to describe briefly a few simple experiments made to determine whether the iodine glow is affected in any way by the nature of the gas in which it is heated. I will describe them as nearly as possible in the order in which they were performed.

Experiment I.-A little iodine was placed near the closed end of a piece of hard glass tube of about 7 mm. bore, the


other end being open to the atmosphere. This was suspended horizontally, and a Bunsen flame brought under the central part ; when about 5 or 6 cm. of the tube had become nearly red-hot, the flame was held for a few seconds under the part containing the iodine, which immediately volatilized, filling the whole tube with coloured vapour.

A bright reddish glow then appeared in the heated portion of the tube.

Exp. II. - Iodine was heated in a tube as in I. but closed to the atmosphere, and in which air was replaced by nitrogen. No difference was seen in the emission of light.

Exp. III.—The same as II., excepting that hydrogen was substituted for nitrogen. In this case also the glowing appeared, the same as before, but it could not be maintained as combination took place between the hydrogen and iodine, and the colourless hydriodic acid formed emitted no light.

Exp. IV.-In CO, the same result was obtained as in nitrogen.

Exp. V.--A thick glass tube containing iodine was exhausted with an air-pump until the pressure fell to about 20 mm. It was then heated so as to expel the remaining air by volatilizing a portion of the iodine, and then sealed off. The whole length of the exhausted tube was next warmed up until the density of the iodine vapour was sufficient to give the usual deep violet colour. On strongly heating a short section of the tube at this stage, the usual reddish-yellow glow appeared exactly as in the other experiments.

These preliminary experiments, therefore, as far as they go, entirely favour the view that the glowing is determined by heat alone, and not by chemical “luminescence. For the phenomenon appears to be quite independent of the gas in which the iodine is heated, and in Ill., where chemical combination actually occurs, the light, instead of being intensified, is extinguished.

To determine the character of the light, things were arranged as in I., the glowing tube being observed with a spectroscope, having a lens in front to throw an image of the hot part of the tube across the slit of the instrument. Before heating the iodine a faint streaky spectrum, due to glowing opaque particles in the glass, was all that could be seen; but immediately the iodine was vaporized a bright spectrum shone out. This appeared perfectly continuous, and similar to that given by a red-hot iron wire, with which it was directly compared. No sign of resolution into lines could be made out even with a highly dispersive train of five prisms.

Next a ray of white light from the flame of a paraffin lamp was made to pass through the glowing tube and enter the spectroscope, after having suffered absorption by the heated vapour.




The dark bands, characteristic of cooler iodine vapour, were seen to be unchanged, and there was no sign of any continuous absorption. Thus at the temperature of the experiment iodine emits a continuous spectrum, and does not emit only those rays which it absorbs.

The question whether iodine is the only element having this property of glowing with a continuous spectrum at comparatively low temperatures was next investigated. A series of experiments with the allied element bromine immediately showed that this was not so, for the vapour of this substance was also found to glow brightly under similar circumstances.

The bromine glow seems indeed to be quite as conspicuous as that of iodine: the spectrum also was found to be perfectly continuous.

It was then thought that chlorine, being also a coloured gas and allied to the others, would probably be found to emit light in the same way. Accordingly arrangements were made for generating and thoroughly drying this gas and heating it in a glass tube, as in the foregoing experiments. The first attempts to see the glow were made, as before, from outside the tube; but they led to negative, or, at the most, very doubtful results. A very slight alteration in the mode of viewing the heated gas soon, however, revealed a distinct and unmistakable, though faint, luminosity. The experiment was arranged in the following way :-Chlorine from a generating flask, after being freed from HCl and dried by passing slowly through a long tube of calcium chloride, was led into a straight piece of combustion-tube of about 7 mm. bore, heated strongly in the middle. This tube connected on to a larger glass tube containing a total-reflexion prism placed in line with the central axis of the heated tube : this enabled one to observe the heated gas “end on” without the interposition of the red-hot glass, and a dark background was obtained by covering up the further end of the combustiontube with opaque material. .

In this way observations were made, first, with common air only passed through the apparatus, and afterwards with dried chlorine, about 5 cm. of the tube being kept meanwhile at the highest temperature attainable with an ordinary Bunsen burner. With air, the centre of the tube remained perfectly dark, and no trace of glowing solid matter in the form of dust could be seen. With chlorine, on the other hand, a faint greenish glow gradually filled up the previously dark central bore of the tube, appearing first on the lower or hottest part. The light, although brighter on the lower side, appeared perfectly uniform in texture so to speak, unlike the glow produced by incandescent solid particles, which can be seen




under certain circumstances as wreaths and streaky clouds moving with the convection-currents in the tube *.

Being satisfied by repeated trials that the glow was really due to incandescent chlorine, an endeavour was made to determine the spectroscopic character of the light. It seemed almost hopeless to observe the spectrum directly, although with a spectroscope of very low dispersion and wide slit the spectrum appeared to be continuous; but it could only be traced between the positions of D and F of the solar spectrum, probably owing to the greater sensitiveness of the eye

to that region. An indirect method was, however, devised. The depth of the layer of cool chlorine traversed by the light from the hot gas was, in the first instance, about 20 cm., that being the distance between the prism and the hot part of the tube. The absorbing effect of this thickness of gas was tested by varying the distance, heating the tube successively at 15 and 30 cm. from the prism. If the spectrum of the light emitted consisted of lines or bands corresponding to the absorption-lines of chlorine, there should be a marked increase of absorption after traversing 30 cm. as compared with 15 cm. But no difference was perceptible in the intensity of the glow whatever the distance traversed, showing that the cool gas exercises very little absorption on the light coming from the hot.

The inference, therefore, is that chlorine, like iodine and bromine, emits other rays than those absorbed, and probably shines with continuous light; the selective absorption of the cool gas merely giving the glow a greenish tint.

There seems no reason to doubt that the luminosity of chlorine, as of its allied elements, is directly due to the heating, and that chemical changes are not concerned in the phenomenon.

The Sulphur Group.—Similar results have been obtained with sulphur and selenium. The glowing may be observed either “end on ” in a red-hot porcelain tube filled with hydrogen or nitrogen, or the element may be simply sealed in a hard glass tube in air. If a small portion of the tube be then heated strongly, a faint glow can be seen while distillation is going on, and the heated space is filled with vapour.

Phosphorus.—Experiments with this substance have so far led to negative results. After all the phosphorescence due to traces of oxygen has disappeared from the tube, the vapour appears to give no light.

* The glowing of heated chlorine may be more conveniently observed in a porcelain tube connected with a T-tube having a glass plate cemented in one end of the T. If the Bunsen is concentrated on the tube by means of a fire-clay arch placed over it, the glow uniformly illuminates the bore of the tube,


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