PRESIDENTIAL ADDRESSES. 553 



merely by experimental skill and acuteness of 

 observation. He was led to it by carefully de- 

 signed investigation, starting with an examination 

 of the cause of irregularities which had troubled l 

 him in his weighing of thallium ; and, going on to 

 trials for improving Cavendish's gravitational 

 measurement, in the course of which he discovered 

 that the seeming attraction by heat is only found 

 in air of greater than ^^ of ordinary density ; 2 

 and that there is repulsion increasing to a maxi- 

 mum when the density is decreased from to 



IOOO 



i oooooo an< ^ thence diminishing towards zero 

 as the rarefaction is farther extended to density 



. From this discovery Crookes came 



20,000,000 ' 



to his radiometer, first without and then with 

 electrification ; and, powerfully aided by Sir George 

 Stokes, 3 he brought all his work more and more 

 into touch with the kinetic theory of gases ; so 



1 Tribulation, not undisturbed progress, gives life and soul, and 

 leads to success when success can be reached, in the struggle for 

 natural knowledge. 



2 Crookes, " On the Viscosity of Gases at High Exhaustions, 

 655, Phil. Trans., Feb. 1881, p. 403. 



Ibid., vol. 172 (i 88 1), pp. 387, 435. 





