C. GENERAL PHYSICS. 119 



C. GENERAL PHYSICS. 



ATTRACTION AND REPULSION RESULTING FROM RADIATION. 



Mr. Crooks has lately published his investigations into the 

 phenomena known as attraction and repulsion resulting from 

 radiation. The apparatus constructed by him appears to be 

 more sensitive than the ordinary thermo-multiplier. He con- 

 siders that the experiments show that the repulsion is not en- 

 tirely due to the rays usually called heat, viz., to the extreme 

 and ultra red rays of the spectrum. The theory advanced 

 by Professor Reynolds to explain his observations is not ac- 

 ceptable to him, although, on the other hand, he has not yet 

 prepared one of his own. According to Reynolds and Bal- 

 four Stewart, these experiments constitute a direct proof, and 

 the only known direct proof of the truth of the kinetic theory 

 of gases as developed by Clausius and Maxwell. 



Mr. Crooks has adapted his results to the construction of 

 a very important instrument, which he calls a radiometer. 

 It consists of four arms suspended on a steel-pointed axle 

 resting in a cup, so that the arms are capable of revolving 

 horizontally. To the extremity of each arm is fastened a 

 thin disk of j)itb, lampblacked On one side, and the whole is 

 inclosed in a very perfect vacuum within a glass globe. Un- 

 der the influence of light, or heat, the little arms revolve with 

 considerable rapidity. Mr. Crooks states that the repulsion 

 experienced by these disks when any radiation falls upon 

 them is proportional to the length of the vibrations, and 

 varies at every point of the spectrum. Professor Guthrie 

 has remarked that Mr. Crooks's research had, in an almost 

 unequaled degree, every element of greatness. 



INCREASE OF RADIATION WITH TEMPERATURE. 



It is well known that as the temperature of a solid is grad- 

 ually increased, the refrangibility of the emitted light in- 

 creases likewise, and as the result we find red ligdit emitted 

 first ; afterward the other colored rays gradually appear as 

 the heat increases, until we reach the ultra-violet rays. This 

 correlation between refrangibility and temperature was first 



