390 Conduction in Solutions and Gases, 



equation of available energy)* must be the case with any 

 system whose available energy is exactly proportional to the 

 absolute temperature. 



The advances which were effected in the last quarter of the 

 nineteenth century in regard to the conduction of electricity 

 through liquids, considerable though these advances were, may 

 be regarded as the natural development of a theory which had 

 long been before the world. It was otherwise with the kindred 

 problem of the conduction of electricity through gases : for 

 although many generations of philosophers had studied the 

 remarkable effects which are presented by the passage of a 

 current through a rarefied gas, it was not until recent times 

 that a satisfactory theory of the phenomena was discovered. 



Some of the electricians of the earlier part of the eighteenth 

 century performed experiments in vacuous spaces ; in particular, 

 Hauksbeef in 1705 observed a luminosity when glass is rubbed 

 in rarefied air. But the first investigator of the continuous 

 discharge through a Tarefied gas seems to have been Watson,! 

 who, by means of an electrical machine, sent a current through 

 an exhausted glass tube three feet long and three inches in 

 diameter. " It was," he wrote, " a most delightful spectacle, 

 when the room was darkened, to see the electricity in its 

 passage : to be able to observe not, as in the open air, its 

 brushes or pencils of rays an inch or two in length, but here 

 the coruscations were of the whole length of the tube between 

 the plates, that is to say, thirty-two inches." Its appearance 

 he described as being on different occasions " of a bright silver 

 hue," " resembling very much the most lively coruscations of 

 the aurora borealis," and " forming a continued arch of lambent 

 flame." His theoretical explanation was that the electricity " is 

 seen, without any preternatural force, pushing itself on through 

 the vacuum by its own elasticity, in order to maintain the 



* Cf. p. 241. 



t Phil. Trans, xxiv (1705), p. 2165. Fra. Hauksbee, Physico- Mechanical 

 Experiments, London, 1709. 



I Phil. Trans, xlv (1748), p. 93, xlvii (1752), p. 362. 



