both in quantity and quality; and while it depends on the temperature it is entirely independent of the materials or shape of the enclosure.

2. This stream is unpolarised.

3. The absorption of a surface in such an enclosure is equal to its radiation, and this holds for every kind of

heat.

205. Heat equilibrium of plates. Returning once more to our chamber of constant temperature, let us suspend in it a thin plate of rock salt. Now since the temperature of this plate remains constant, the plate must radiate just as much heat as it absorbs. But rock salt (Art. 191) absorbs very little heat, hence also it will radiate very little. Moreover, a thick plate will absorb more than a thin one, and hence also it will radiate more. Both of these conclusions have been verified by the author of this work. By making use of the thermo-pile he has found that the radiation from a thin plate of rock salt is only 15 per cent. of the total lamp-black radiation for the same temperature, and that the radiation from a thick plate of rock salt is greater than from a thin one.

206. Suppose, however, that instead of rock salt we had suspended two plates of glass of unequal thickness. Since this substance is extremely athermanous, either of these plates would probably absorb nearly all the heat which fell upon it, and hence the radiation of both plates (radiation being equal to absorption) would be sensibly the same, and would be very great-in fact it would be much the same as if they stopped the whole heat, or were covered with lamp-black.

207. From this we see what it was that misled the early experimentalists on this subject, and induced them to think that radiation was confined, if not to the surface of a body, at least to a very small depth beneath it.

They found that

when a metallic surface was coated with varnish its radiative power was very much increased, but that very soon this increase attained its maximum, after which an additional coating produced no further effect. But the reason of this was, not that radiation is in all cases confined to a very small distance beneath the surface, but that these coatings were of a very athermanous substance, so that a very small thickness was practically equivalent to a surface of lampblack. Could it have been possible to apply a coating of transparent rock salt, the result would have been very different.

208. Let us now take our thermometer and cover its bulb once more with a substance having a selective absorption for heat, and, further, let us hang up before it in the enclosure a plate of rock salt. No change in its indication will take place; but in order that the temperature of this thermometer may remain without change it is obviously necessary that this plate of rock salt should change neither in quantity nor in quality the stream of heat which impinges against the bulb; that is to say, this stream after it has passed through the salt must be precisely the same both in quantity and in quality as before it entered it. In order that this may be the case it is necessary that the absorption of the rock salt should be equal to its radiation for every kind of heat.

This result has also been verified experimentally by the author of this work. If the kind of heat which rock salt radiates be the same as that which it absorbs, it would follow that a cold plate of rock salt ought to be exceedingly opaque to the radiation from heated rock salt. This was found to be the case. A moderately thick plate of this substance was found to stop at least three-fourths of the heat from a thin plate of heated rock salt, whereas it will only stop a small proportion of ordinary heat.

A

с

A

CE

209. This affords an explanation of the fact that two plates of rock salt placed the one behind the other, or a single plate of double thickness, do not radiate twice as much as a single plate. For let EF be the front surface of such a double plate, of which CD represents a line midway between the two surfaces. Now, while as much heat will cross the line

B

D

B

D

Fig. 47.,

CD from the hinder half of the plate as would be radiated from the single plate ABCD, a great proportion of this heat (probably three-fourths) will be absorbed by the front half in its passage through it, since we have seen that rock salt absorbs intensely the heat which it radiates. Hence, if the radiation of the single plate be = 1, that of the double plate, instead of 2, will probably not be more than 1.

210. Our readers will thus be prepared to see that radiation is a thing which goes on in the interior of a plate just as much as near the surface; and they will also see that it does not necessarily follow from this that the radiation of a plate should be proportional to its thickness, but very much the reverse;-indeed, had the substance of the plate in Fig. 47 been glass instead of rock salt, the single plate would have given out sensibly the same amount of heat as the double plate, since in the latter the heat from ABCD would all have been stopped by CDEF. We are thus prepared to see that in the interior of substances, as well as in air or vacuo, a stream of radiant heat is constantly passing and repassing in all directions, and in the case of constant temperature, as this stream of heat passes any layer of particles it is just as much diminished by the absorbing action of these particles as it is recruited by their radiation, so that the stream flows on virtually unchanged

both in quantity and quality until at last it reaches the surface.

211. Amount of internal radiation. We have now to consider a more difficult question, which may be thus stated. Supposing we have several different substances all remaining at the same constant temperature, will the streams of radiant heat continually passing and repassing in the interior of these substances be equal to each other? In the first place, and before attempting to answer this question, we must shew how the intensity of a stream of radiant heat may best be measured. For this purpose let us suppose a small square surface representing unity. of area to be placed in the interior of an enclosure, or of a substance surrounded by an enclosure kept at an uniform temperature. In accordance with our views, streams of radiant heat will be continually passing through this surface in all directions; let us confine our attention to these rays, which are as nearly as possible perpendicular to the plane of our square unit. But it may be said, why not confine our attention to rays strictly perpendicular to this plane? In answer to this, we remark that in our present investigation (the reason will afterwards appear) we must regard a ray in the sense in which a straight line is regarded. And just as a line is in reality always part of the boundary of a solid, so a ray is always in reality part of the boundary of a beam or pencil of light. We may satisfy ourselves that this is the case in nature by considering the light which reaches the eye from a star or other object apparently very small; this would seem to be the nearest approach to a geometrical line of light, whereas since a star has a certain real though very minute angular diameter, the light from it is in reality a converging pencil, although no doubt the angle of convergence is very small.

We will confine our attention therefore to rays as nearly

Let B (Fig. 48)

as possible perpendicular to our unit area. represent this area, and let CBD be a very small pencil or cone of rays nearly perpendicular to the plane of B, the central line AB being strictly perpendicular to this plane. Now if we suppose the angle CBD as well as our unit area to remain constant while we pass from one substance to another, then the quantity of heat radiated in unit of time upon this unit area B through directions comprised within the small cone CBD will denote the intensity of internal radiation of the substance in question.

C

B

Fig. 48.

212. The circle CAD may in fact be compared to the disk of a small star whose diameter CD subtends with the eye the angle CBD (greatly exaggerated in the diagram for the purpose of demonstration), and from which a beam or pencil of light represented by the cone CBD reaches the eye of the terrestrial observer at B. Now imagine, for the sake of demonstration, that it is possible to place the eye in the interior of a substance of constant temperature, and also that the eye is sensible to all the rays which compose, according to our hypothesis, the entire radiation of the substance, then it is evident that if the eye look in the direction of CAD, the brightness of the field of view in front of the eye or of any given detached portion of it, such as the area CAD, will indicate the internal radiation; so that, if the eye be now removed to the interior of another substance of greater internal radiation, more rays will strike it in front from CAD in one second of time, and the field of view will therefore appear brighter in the very same proportion in which the internal radiation is increased.

213. Let us now direct our attention to an enclosure, such as a sphere (Fig. 49), kept at a uniform temperature,