1876.] 
and its Cosmical Revelations. 
519 
or otherwise stated — it exists in closer companionship with 
heat than with light, and closer with light than with 
addinism. 
According to the doddrine 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, 
c ceteris paribus, with their temperature. If we now add to 
this generalisation that all bodies are similarly radiating 
mechanical force and suffering corresponding mechanical 
reaction, the difficulties vanish. What must follow in the 
case of a freely suspended body unequally heated on opposite 
sides ? 
It must be repelled in a direddion perpendicular to the 
surface of its hottest side. If two rockets were affixed to 
opposite sides of a pendent body, and were to exert unequal 
ejeddive forces, the readdion of the stronger rocket would 
repel the body in the opposite direddion to its preponderating 
ejeddion. This represents the case of 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 addive absorption and conversion 
of light into heat, and thus the blackened face recedes. 
We may regard it thus as readting by its own radiations, 
or otherwise as added upon by the more powerful radiant 
whose rays are differentially received by the black and bright 
sides. These different modes of regarding the addion are 
perfeddly consistent with each other, and analogous to the 
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. Striddly speaking, neither 
of these descriptions is corredd, as the gravitation is mutual, 
and the total quantity exerted betwen the sun and the earth 
is equal to the sum of their energies, but it is sometimes 
convenient to regard the addion from a solar standpoint, and 
at others from a terrestrial. So with the radiometer and 
the striddly mutual repulsions bbtween it and the predomi- 
nating radiant. 
It appears to me that this unsophisticated conception of 
radiant mechanical repulsive force, and its necessary me- 
chanical readdion on the radiant body, meets all the fadds at 
present revealed by the experiments of Mr. Crookes and 
others. 
The attraddion which occurs when the disc of the radio- 
meter is surrounded with a considerable quantity of 
atmospheric matter is probably due to inequality of atmo- 
