286 Lord Kelvin on the Statistical Kinetic 



have assumed, with explanations, in §§ 162, 163, 164 of 

 pp. 412, 413, and in § 3 of pp. 487, 488 of my volume of 

 Baltimore Lectures. 



§ 3. The action of this mechanism in our case under 

 consideration, involves the communication of energy from 

 incident waves of sunlight to the atoms of the solid, in the 

 surface of the hemisphere illuminated by the sun ; and the 

 communication of energy from the atoms to ether outside the 

 globe, 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, 

 but scarcely perceptibly modified, 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 tempe- 

 rature : as this depends, not only on the well-known higher 

 receptivity of the black surface than of the white for sun- 

 heat; but also, on the difference of radiational emissivities 

 of all parts of the two surfaces to surrounding space. 

 It seems most probable that the black globe will be steadily 

 warmer than the white ; but we cannot say with certainty 

 that this is true. Suppose for a moment that the steady 

 temperatures of the two are the same ; and now whiten the 

 hemisphere of the black globe remote from the sun. This 

 will cause the globe which is now black and white, to be 

 warmer than it was, because it will radiate less into void ether 

 than it did when it was all black. 



§ 5. Now blacken the hemisphere facing from the sun, of 

 the globe originally all white. Its temperature obviously 

 will be lowered. Thus we have, side by side, two globes 

 each with a white hemisphere and a black hemisphere; 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 has 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 me (and probably to many others) 

 that instructive experiments might be made by observing the 

 temperatures of two equal and similar thermometers, placed 

 beside one another on a wooden table (or on two similar 

 tables of the same materials) or on a cushion or layer of very 

 fine cotton wool : each thermometer between the folds of a 

 doubled sheet ; one of white cloth and the other of black; both 



