Oil the Theory of Mixed Gases. 4 1 7 



that he has other arg^unients to answer, besidcB 

 those that '^ire fiirhished by Schmidt ; and that 

 whilst these dbjcctions to his doctrine remain in 

 force, they oblige him to consider moist air in 

 the light of a chymical compound. Now any 

 attempt to calculate the forte of vapour in a 

 mixture df thi^ description is visionary, and 

 must prove a sourceof error, if put in practice; 

 because it is inipossible to discover the qualities 

 of a chymical compound from the known, pro^ 

 perties of its ingredients. Mr. Dalton's con- 

 fidence in his theorem is evidently connected 

 with a notion, that vapour, mixing v^^ith per- 

 fectly dry air increases the spring of it, to the 

 exact amount of its own force.* But there 

 are strong reasons to suspect his manometers 

 of being very treacherous guides :^ a tube of^^ 

 of an inch internal diiameter i^ too narrow : the 

 mercury cannot pTay freely in it ; and the ex- 

 perimenter is at liberty to estimate at pleasure 

 the errors, arising from its impeded motion. 

 The supposed power of vapour to increase in 

 appearance the elastic force of air confined in 

 a manometer, is mentioned and explained in 

 such expressive words by Mr. Dalton, that 

 nobody can mistake his meaning. Indeed his 

 style is in general too correct and perspicuous 



* Manchester Memoirs, Vol. 5, p. 537, • 



