polarized. It will be found to be polarized in a plane at right angles to that in which light is polarized as it passes through the same tourmaline when cold and similarly placed.

224. Experiment IV.—We have seen that in an enclosure of uniform temperature the flow of radiant heat is the same in all directions, both as regards quantity and quality, whatever be the substances with which the enclosure is filled. Now with regard to light a good coal fire may be viewed as an enclosure of approximately uniform temperature, and accordingly we ought to find that, whatever substances be put into this fire, when these ultimately become of the same temperature as the fire, they will not alter the nature of the light which is given out. We may prove this by throwing coloured glasses into the fire, and when these become sufficiently heated they will be found to have lost all their colour. The red glass, for instance, which we have thrown in will, as we have already seen, give out a greenish light on its own account; but it will pass red light from the coals behind it in such a manner that the light which it radiates precisely makes up for that which it absorbs; so that we have virtually a coal radiation coming partly from and partly through the glass.

225. We cannot conclude this subject without alluding to a very interesting experiment first made by Foucault, but afterwards revived and extended by Kirchhoff, in which the equality between radiation and absorption is extended to individual rays of the spectrum.

Foucault found that the voltaic arc formed between charcoal points often emits the ray D of the solar spectrum on its own account, and at the same time absorbs it when it comes from another quarter. Kirchhoff, again, found that coloured flames, in the spectra of which bright sharp lines present themselves, weaken rays of the colour of these lines when such rays pass through the flames. We thus

see that the same media which in a heated state emit rays of a certain refrangibility in great abundance have also the power of stopping these rays when they fall upon them from another source.

CONCLUDING REMARKS.

226. We have thus arrived both theoretically and experimentally at a law which may be enunciated as follows:— Bodies when cold absorb the same rays which they give out when hot. The reader will at once be struck with an analogy between sound and light in this respect. A musical string when at rest takes to itself and therefore absorbs the very note (given out by another instrument) which it will itself give out when in a state of vibration.

Reasoning from this analogy Professor Stokes had suggested beforehand the probability of a connexion between the absorption and radiation of bodies for particular rays of the spectrum, and he also imagined that this suggestion would account for the dark lines in the solar spectrum.

The prediction of this philosopher has been abundantly confirmed by the labours of Kirchhoff; but the striking conclusions with regard to the constitution of the sun and stars which Kirchhoff has experimentally arrived at must be deferred till another chapter.

We cannot, however, refrain from remarking that the law developed in this chapter affords a valuable confirmation by analogy of the truth of the undulatory theory of light. The likeness between a vibrating string and a heated particle has been remarked above, and we have seen that a particle (just as a string with regard to sound) absorbs the same kind of ray which it gives out. It is, perhaps, allowable to infer that light, like sound, consists of undulations which are propagated in a medium surrounding bodies, and that when heat or light is absorbed by a particle, the motion is con

veyed from the medium to the particle, just as when a string takes up a note passing through the air the motion is conveyed from the air to the string; and that, again, when heat or light is radiated by a particle it is similar to the giving out by a string of its note to the air.

CHAPTER IV.

Radiation at Different Temperatures.

227. It has already been shewn (Art. 204) that the stream of radiant heat continually proceeding through an enclosure of which the walls are kept at a constant temperature depends only on the temperature of the walls, and not on the nature of the various substances of which they are composed; the only difference being that for metals this stream is composed partly of radiated and partly also of reflected heat, while for lamp-black it is composed wholly of radiated heat. This may be expressed by saying that this stream depends upon or is a function of the temperature, and of it alone; but there is the following very important difference between a reflecting and a lamp-black surface, as representing this stream of radiant heat.

It is only when a reflecting surface forms part of a complete enclosure of the same temperature as itself, that the radiated and reflected heat from this surface together represent the whole stream of heat; for if we bring it for a moment into another enclosure of lower temperature, the reflected heat is altered, and although the radiation will for a short time continue nearly constant, yet this radiation will not represent the whole stream of heat due to the temperature of the surface.

In he other and f 1 mr-lack unce be placed in the incre position, since he stream of seat which tows from : 3 entreiv intenendent of the redecon ize to

bergineung nities, he heat which it mates when brought for a moment into in enclosure of over temperatore than tseif vill my represent the stream of am heat me o the temperature of he amp-ics.

228 Suppose now that we have a thermometer with 1 blackened bult, and that this is placed in a backeneti enclosure of 1 over temperature han self the heat vinch it radiates vill represent the total adanon me to the temperature of the bulb, valle hat which I receives vill represent the total radiation ine to The Temperature of the enclosure, and the fference between these o vil tus be represented by the loss of heat pertenced by the ther

mcmeter.

Thus, f he he temperature of the encicsure, and that of the bulb, then, since the stream of radiant heat Art. 201 is a function of the temperature nir, ve shall have this stream represented by F :-A må For nese vo temperatures, and the rate a vich the hermometer loses heat will be denoted by F :-D −F 9.

This is the rate at which the instrument lcses radiant heat, and t will also represent the rate at which I lcses temperature, or the velocity of cociing, as this is termed, if we suppose that the specific heat or heat required to produce a change of 1) of the mercury of the thermometer remains the same for all the temperatures of the experiment. This, though not precisely, is very nearly the case. and hence the velocity of cooling of a thermometer placed m these circumstances may be regarded as representing with great accuracy the intensity of radiation.

With these remarks we shall now discuss the experiments hat have been made on velocity of cooling.

VELOCITY OF COOLING; VARIATION WITH TEMPERATURE OF QUANTITY OF RADIATION.

229. Newton was the first to enunciate his views on the cooling of bodies. He supposed that a heated body exposed to a certain cooling cause would lose at each instant a quantity of heat proportional to the excess of its temperature above that of the surrounding air. It was, however, soon found that this law was not exactly followed, and several philosophers made experiments on the subject with more or less success, until the time of MM. Dulong and Petit, who made a very complete and successful investigation of the velocity of cooling of a thermometer both in vacuo and in air. It is with their experiments in vacuo that we have now to do.

230. The apparatus used by these experimentalists consisted of a hollow globe of thin copper with the interior blackened, which could be immersed in a vessel of water of known temperature. Through an orifice in this globe inserted, so as to have its bulb in The temperature of this thermo

a thermometer could be the centre of the globe. meter was always higher than that of the globe, and the number of degrees that the mercury would sink in a minute, supposing the cooling to be uniform during that time, was taken to denote the velocity of cooling.

A preliminary set of experiments was first made, from which it appeared that the law of cooling of a liquid mass is independent of the nature of the liquid and of the form and size of the vessel which contains it. Having determined this, MM. Dulong and Petit proceeded to make their final experiments with a thermometer containing about 3 lbs. of mercury. In the first instance this thermometer preserved its natural vitreous surface, but since glass is exceedingly opaque towards the heat radiated at all the temperatures of the