Eloge of Aleocander VoUa. 7 



scale, and decrease in the elevated degrees. He proved, in short, 

 by delicate measurements, that atmospheric air, if shut up in a 

 vase perfectly dry, dilates in proportion to its temperature, when 

 this is measured u{X)n a mercurial thermometer equally divided : 

 now, as the works of Deluc and Crawford appear to establish 

 that a similar thermometer gives the true measurements of the 

 quantity of heat, Volta thought himself authorised to announce 

 the very simple law which flowed from his experiments, in these 

 new terms, of which every one will appreciate the importance ; — 

 that the elasticity of a volume of air is proportionate to its heat 



Air on being heated when at a low temperature, and contain- 

 ing the same quantity of moisture, has its elastic force augmented 

 like dry air. Volta thence concluded that steam and air expand 

 in the same degree. Every one now knows that this result is 

 correct ; but Volta's experiment left some reason to doubt ; for 

 at ordinary temperatures, the vapour of water mingles with at- 

 mospheric air in very small proportions. 



The work which I have just analyzed, which Volta called a 

 mere rough draught, he intended to incorporate with many other 

 researches of the same kind, in a memoir which was never pub- 

 lished. Our knowledge of the subject is now complete, owing 

 to the exertions of Gay-Lussac, and Dalton. The experi- 

 ments of these ingenious chemists, made at a time when the me- 

 moir of Volta, although published, was not known either in 

 France or England, shewed that the law laid down by the 

 learned Italian extended to all other gases. 



I shall not examine the researches of Volta on atmospheric 

 electricity, until I have taken a rapid survey of analogous experi- 

 ments that preceded his. In order to judge correctly of the route 

 a traveller has traversed, it is often useful to see likewise the 

 point from which he set out. 



Dr Wall, who wrote in 1708, should be first mentioned, for 

 in one of his memoirs this ingenious reflection occurs : " The 

 light and crackling noise of electrified bodies, seem, in a certain 

 degree, to represent thunder and lightning.*" Stephen Grey pub- 

 lished, in 1735, a similar remark. " It is probable," says this 

 illustrious natural philosopher, " that in time means will be found 

 of concentrating large quantities of electrical fire, and of increas- 

 ing the power of an agent which, according to many of my experi- 



