DISCOVERY BY EXAMINING THE EFFECTS OF ELECTRICITY. 555 



the negative.' l By rarefying gases, and passing electric 

 discharges through them, it has been discovered that the 

 resistance to the passage of that force through them de- 

 creases as the degree of rarefaction of the gas is increased, 

 up to a certain high degree ; but beyond that it increases 

 until the force will not pass at all. 



g. By examining the effect of mutual contact of sub- 

 stances upon each other. Numerous discoveries ^ in the 

 subjects of percussion, friction, adhesion, capillarity, 

 endosmose, osmose, absorption and diffusion of liquids 

 and gases, chemical action, the excitement of frictional 

 and voltaic electricity, have been made by this method. 

 This mode of procedure is a very common one in che- 

 mistry, and is the usual one of discovering new chemical 

 reactions and compounds ; thousands of new chemical facts 

 have been found in this way. We may always safely 

 assume the hypothesis that each substance either unites 

 chemically with, or acts in some other way upon many 

 others ; and as soon as any new elementary substance, 

 or any new acid or base is discovered, we may proceed 

 to devise and execute experiments founded upon that 

 idea. Pouillet, in 1822, discovered the general truth, 

 that when a liquid wets a solid, or is absorbed by it, heat 

 is evolved. 



Every new chemical element has been successfully 

 treated in this way, and almost the entire fabric of che- 

 mical reactions and compounds has been erected by this 

 method. Nearly the whole series of tests in analytical 

 chemistry were found by means of it ; it has also been the 

 basis of an almost unlimited number of manufacturing pro- 

 cesses, an<l to illustrate it fully would be nearly equivalent 

 to writing a history of chemistry. As almost every sub- 

 stance either combines with or acts upon nearly every 

 1 Thomson, History of Chemistry, vol. ii p. 256. 



