WILHELM HITTORF AND WILLIAM CROOKES 349 



Hittorf also began the thorough investigation of the con- 

 duction of electricity by flame, for which purpose he was 

 able to make use of the Bunsen burner for exact experi- 

 menting. 



Crookes was the first who attempted to follow up experi- 

 mentally the pressure of light given by Maxwell's equation. 

 This pressure, exerted by light or ether wave radiation on 

 any surface which it meets, had been calculated, as also 

 had been its magnitude, by Maxwell. It is extremely small, 

 even with the greatest available intensity of light, so that 

 very small currents of air are able to mask it. Crookes made 

 experiments in exhausted vessels. Here, it is true, he ac- 

 tually found a pressure upon surfaces exposed to radiation; 

 but it was not the pressure of radiation, but a new phe- 

 nomenon caused by the gas molecules which were still 

 present in large numbers in the best vacuum that could be 

 obtained in those days. Crookes thus made his 'radiometer,' 

 or light mill, in which a particular demonstration of„ the 

 motion of gas molecules was given, but the pressure of light 

 still remained masked on account of its smallness. 



Later on, success in proving the pressure of light was ob- 

 tained by improved pumps, increased intensity of light, and 

 above all, by taking into account the radiometer effect; the 

 pressure was measured and shown to be in agreement with 

 Maxwell's theories; it has proved to be of no small import- 

 ance for further reasoning. In cosmic space, where every- 

 where great intensity of radiation and large surfaces exposed 

 to it occur, the effect of radiation pressure may be con- 

 siderable; it may even outweigh the effect of gravitation. 



Crookes was also the first to continue Bunsen's discovery of 



to Wiirzburg, and in 1888 to Strasbourg. From 1894 to 1905 he was 

 president of the Reichsanstalt. Kohlrausch exercised a very great in- 

 fluence for good upon the progress of electrical measurement; he was 

 also responsible for the exact standard of electric current (ampere or 

 weber) generally used to-day, and depending upon a measurement of the 

 deposition of silver, made by him with the highest accuracy in the year 

 1881. 



