60 SCIENCE IN SHORT CHAPTERS. 



their hypothetical data are sound, they should be able to 

 predict the relative velocities or torsion-force of a series of 

 radiometers of similar construction in all other respects, 

 but with variable shapes and diameters of enclosing 

 vessels. 



If we divest our minds of all visions of hypothetical 

 atoms, molecules, ethers, etc., and simply look at the facts 

 of radiation with the same humility of intellect as we usually 

 regard gravitation, this primary difficulty of the radiometer 

 at once vanishes. The force of gravitation is a radiant force 

 acting somehow between, or upon, or by distant bodies ; and 

 these bodies, however far apart, act and react upon each 

 other with mutual forces, precisely equal and exactly con- 

 trary. We conceive the sun pulling the earth in a certain 

 direction, and receiving from the earth an equal pull in a 

 precisely contrary direction, and we have hitherto demanded 

 no ethereal or molecular link for the transmission of these 

 mutually attractive forces. Why, then, should we not re- 

 gard radiant repulsive energy in the same simple manner ? 



If we do this there is no difficulty in finding the ultimate 

 reaction fulcrum of the radiometer vanes. It is simply the 

 radiating body, the match, the candle, the lamp, the sun, 

 or whatever else may be the source of the impelling radia- 

 tions. According to this view, the radiant source must be 

 repelled with precisely the same energy as the arms or pen- 

 dulum of the radiometer ; and it would move backward or 

 in opposite direction if equally free to move. If, by any 

 means, we cause the glass envelope of the radiometer to be- 

 come the radiant source, it should be repelled, and may 

 even rotate in opposite direction to the vanes, or vice versa. 

 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 

 discovery 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, luminous, actinic, and electrical forces with which 

 they have hitherto been credited. He has shown that this 



