THOMSON AND DE SAUSSURE. 309 



generated during the conversion of water into 

 vapour by heat ; that it combines with vapour, 

 and contributes to its formation : "II monte 

 invisible et inactif, cache dans le sein de ces 

 vapeurs ; mais deployant ensuite son energie 

 lorsque ces vapeurs ont chang de forme, il 

 redescend actif, anim6 de sa force penetrante 

 et expansive." (Voyages dans les Alpes, Tom. 

 II. p. 244.) But it is quite evident that his ideas 

 were vague and unsettled in regard to the ori- 

 gin of atmospheric electricity : for he observes 

 in another volume of the same work, that it 

 seemed to be produced by the friction of clouds 

 against the air ; by the action of the solar rays ; 

 or by these two causes united. (Vol. i. p. 241 .) 



Dr. Thomson has expressed nearly the same 

 opinion in his late work on heat and electricity, 

 " that atmospheric electricity may result from 

 the friction of two currents of dry air moving 

 different ways." (page 441.) 



De Saussure supposed that there were two 

 kinds of vapour ; the one vesicular* and visible, 

 which contained electricity ; while transparent 

 elastic vapour was combined with, and sustained 

 by, the expansive agency of heat alone, (sans le 

 secours d'aucun autre agent.) (Vol. ii. p. 259.) 



* The vesicular theory originated with Derham, who supposed 

 that vapour was composed of hollow sphericles of water filled 

 with highly attenuated air, which Mr. Leslie terms a " fanciful 

 notion, countenanced by certain dubious optical appearances." 



