346 REPORT—1905. 
temperature represents the energy associated with an individual disturbance. 
Such a view of the nature of heat would enable us to express the principles and 
conceptions of thermodynamics in very simple mechanical terms. The writer 
supported the view that radiant heat can be analysed into elementary proportions 
by considerations based upon the known physical properties of radiation, and also 
showed how a similar resolution of the thermal energy of a body into elementary 
proportions is the logical outcome of the electron theory of matter. 
2. On the Kinetic and Statistical Equilibrium of Ether in Ponderable 
Matter at any Temperature.’ By Lord Ketvin, G.C.V.0., FR.S. 
1. Consider first the simplest possible case: a piece of solid matter of a few 
millimétres or a few centimétres greatest diameter, placed in space at the earth’s 
distance from the sun—say 150 million kilométres; for particular example, 
suppose two globes of metal, or rock, or glass, or the bulbs of two thermometers, 
of one centimétre diameter, one of them coated with black cloth and the other 
with white cloth, side by side, at a distance of a few centimétres or métres 
asunder, For the most extreme simplification suppose no other matter in the 
universe than ether, the sun, and our test globes. From our knowledge of the 
properties of matter it is obvious that each of the test globes will, in a few 
minutes of time, come to a steady temperature. In these circumstances, each 
globe sends out by radiation as much energy of waves in ether as it takes in from 
the sun, after it has been long enough exposed to come to a steady temperature. 
2. The internal mechanism in each globe consists of atoms of ponderable 
matter, with ether permeating through the whole volume of the globe, and locally 
condensed and rarefied in the space around the centre of each atom; asthe author 
has assumed, with explanations, in §§ 162, 163, 164, of pp. 412,413, and in § 3 of 
pp. 487, 488, of his volume of ‘ Baltimore Lectures.’ 
3. The action of this mechanism in the case under consideration involves the: 
communication of energy from the incident waves of sunlight to the atoms of the 
solid in the surface of the hemisphere illuminated by the sun, and the communi- 
cation of energy from the atoms to ether in the form of waves travelling out in all 
directions from the surface of the globe. The travelling of this energy through 
the volume of the globe is carried on according to the laws of the conduction of 
heat through solids (modified, scarcely perceptibly, by convection currents in the 
case in which the globe is the bulb of a mercury thermometer). 
4. Our present knowledge of the radiational properties of matter does not quite 
suffice to let us pronounce for certain which of the two globes will have the higher 
steady temperature, as this depends not only on the well-known higher recep- 
tivity of the black surface than of the white for sun-heat, but also on the difference 
of radiational emissivities of the two surfaces on their hemispheres not exposed to 
the sun. It seems most probable that the black globe will be steadily warmer 
than the white ; but it cannot be said with certainty that thisis true. Suppose for a 
moment that the steady temperature of! the two is the same; and then whiten the 
hemisphere of the black globe facing from the sun. This will cause the black-and- 
white globe to be warmer than if was, because it will radiate less into void 
ether than it did when it was all black. . 
5. Then blacken the hemisphere facing from the sun of the globe which was 
originally all white. Its steady temperature obviously will be lowered. There are 
now, side by side, two globes, each with a white hemisphere and a black hemi- 
sphere, facing respectively towards and from the sun. The globe of which the 
black hemisphere is towards the sun will certainly be warmer than the other, when 
a few minutes of time have been given for the temperature of each to become 
steady. 
6. It is not possible for a human experimenter to attain to the extreme simplicity 
ideally prescribed in §§ 1-5 above. But it has occurred to the author (and probably 
* See Philasophioal Magazine, vol. x., p."285 (1905). 
