THE PHILOSOPHY OF THE RADIOMETER. 61 



force is refrangible and dispersible, that it is outspread with 

 the spectrum, but is most concentrated, or active, in the 

 region of the ultra-red rays, and progressively feeblest in the 

 violet ; or, otherwise stated, it exists in closer companion- 

 ship with heat than with light, and closer with light than 

 with actinism. 



According to the doctrine of exchanges, which has now 

 passed from the domain of theory to that of demonstrated 

 law, all bodies, whatever be their temperature, are per- 

 petually radiating heat-force, the amount of which varies, 

 cwteris paribus, with their temperature. If we now add 

 to this generalization that all bodies are similarly radiating 

 mechanical force and suffering corresponding mechanical 

 reaction, the theoretical difficulties of the radiometer vanish. 

 What must follow in the case of a freely suspended body 

 unequally heated on opposite sides? 



It must be repelled in a direction perpendicular to the 

 surface of its hottest side. If two rockets were affixed to 

 opposite sides of a pendant body, and were to exert unequal 

 ejective forces, the reaction of the stronger rocket would 

 repel the body in the opposite direction to its preponderat- 

 ing ejection. This represents the radiometer vane with one 

 side blackened and the other side bright. When exposed 

 to luminous rays the black side becomes warmer than the 

 bright side by its active absorption and conversion of light 

 into heat, and thus the blackened face radiates in excess and 

 recedes. 



We may regard it thus as acting by its own radiations, 

 or otherwise as acted upon by the more powerful radiant 

 whose rays are differentially received by the black and 

 bright sides. These different modes of regarding the action 

 are perfectly consistent with each other, and analogous to 

 the two different modes of regarding gravitation, when we 

 describe the sun as attracting the earth, or, otherwise, the 

 earth as gravitating to the sun. Strictly speaking, neither 

 of these descriptions is correct, as the gravitation is mu- 

 tual, and the total quantity exerted between the sun and 

 the earth is equal to the sum of their energies, but it is 

 sometimes convenient to regard the action from a solar 

 standpoint, and at others from a terrestrial. So with the 



