112 SCIEXCE IX SHORT CHAPTERS. 



radiometer ; and it would move backward or in opposite direc- 

 tion if equally free to move. If, by any means, we cause the 

 glass envelope of the radiometer to become the radiant source, 

 it should be repelled, and may even rotate in opposite direction 

 to the vanes, or vice versd. This has been done with floating 

 radiometers. 



Viewed thus as simple matter of fact, irrespective of any 

 preconceived kinetics of intervening media, the net result of 

 Mr. Crookes's researches become nothing less than the dis- 

 covery of a new law of nature of great magnitude and the 

 broadest possible generality viz. that the sun and all other 

 radiant bodies i.e. all the materials of the universe exert a 

 mechanical repulsive force, in addition to the calorific, lumi- 

 nous, actinic, and electrical forces with which they have hitherto 

 been credited. He has shown that this 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 companionship 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 perpetually 

 radiating heat-force, the amount of which varies, cceteris paribux, 

 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 sur- 

 face of its hottest side. If two rockets were affixed to opposite 

 sides of a pendent body, and were to exert unequal ejective 

 forces, the reaction of the stronger rocket would repel the 

 body in the opposite direction to its preponderating 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 reacting by its own radiations, or 

 otherwise as acted upon by the more powerful radiant whoso 

 rays are differentially received by the black and bright sides. 



