70 MR B. STEWART ON RADIANT HEAT—SECOND SERIES. 
Dutone and Petit’s law; it may even be that the radiation of a glass particle is 
proportional to its absolute temperature. 
40. But all substances (with the exception of black mica and black glass, 
whose peculiarity may perhaps be otherwise explained) have the same properties 
as glass with regard to heat; that is, they are more diathermanous for heat of 
high than for heat of low temperature. The radiation of thin plates or particles 
of all substances will therefore increase less rapidly with temperature than that 
of black surfaces. It may therefore be, that the same law of radiation is common 
to very thin plates or particles of all bodies; this law (whatever it be) giving, in 
all cases, a less rapid increase of radiation with temperature than is indicated 
by Dutone and Petit’s law. Had, however, the diathermancy of thin plates 
of different substances in some cases diminished, and in others increased for 
heat of high temperature, the law of radiation of a particle could not have been 
the same for all bodies. 
The generality of this law of increased diathermancy of all bodies for heat of 
high temperatures seems. therefore, to me to argue in favour of the universality 
of the unknown law of particle radiation which depends upon the former. 
41. What, then, does Dutone and PeEtrt’s law express? The answer is, It 
expresses the law of radiation of indefinitely thick plates, and we have shown 
that it increases faster than the law of radiation of a material particle. 
To facilitate the comprehension of this subject as much as possible, I have put 
it in the following shape. Suppose we have two substances opposite one another, 
the one having the temperature of 0’, and the other of 100°, the latter will of 
course lose heat to the former—let us call its velocity of cooling 100. 
Suppose, now, that (the first surface still retaining its temperature of 0°) the 
second has acquired the temperature of 400°; then we should naturally expect 
the velocity of cooling to be denoted by 400; but by Dutone and Petit’s law, it 
is much greater. The reason of the increase may be thus explained. At the 
temperature of 100°, we may suppose that only the exterior row of particles of 
the body supplied the radiation, the heat from the interior particles being all 
stopped by the exterior ones, as the substance is very opaque for heat of 100°; 
while at 400° we may imagine that part of the heat from the interior particles is 
allowed to pass,: thereby swelling up the total radiation to that which it is by 
Duotone and Petit’s law. 
42. We have thus ascertained—1s¢, That DuLone and Pertir’s law is not the 
law of radiation of a material particle; and, 2d, That this law increases less 
rapidly with the temperature than DuLone and Perir’s law. But now the ques- 
tion arises, Can any method be indicated of ascertaining experimentally the law 
of radiation of a material particle ? 
Now, by continually diminishing the thickness of the plate whose radiation at 
different temperatures we are ascertaining, we certainly approach nearer and 
