\Q^ CONSTlTUtlON OF MIXED GASES. ' 



bis 6\vn except this: If a particle of vapour can pafs freefy 

 through the air, a fecond can alfo fucceed it at any given 

 diftance. My antagonift will certainly grant me this con- 

 ceffion ; becaufc the falvation of his aqueous atmofphere obliges 

 him not to fufped it. Should this indulgence be extended to 

 my Stridures, the demonftration in queftion muft be re-exa- 

 mined v^^ith greater diligence by Mr. Dalton ; for if it cannot 

 be refuted, it will manifeftly do away his atmofphere of free 

 vapour, by proving the abfurdity of his fecond poftulate in the 

 cafe of air and water ; and the fame obfervation may alfo be 

 extended to all the permanent gafes which are not abforbed by 

 water ; becaufe no fluid of this defcription forces water to pafs 

 through its pores by preffing upon it. 



The ufeof the After all, if we may judge from Mr, Dalton's reply, he ap- 



anafyfis^ *^'^* pears not to comprehend my arguments on this fubjed : they 

 have however nothing of novelty in them ; for the bell rea. 

 foners in all ages have not hentated to admit a hypolhefis is for 

 the fole purpofe of difcovering its worth, by comparing the 

 confequences refulting from it with known fads. This kind 

 of argumentation was in great efteem with the Greek mathe- 

 maticians, and Pappus of Alexandria has defcribed and re- 

 commended this fpecies of logic under the name of the con- 

 templative analyfis. 



Faftsoppofing Mr. Dalton fays, he is acquainted with no fads that con* 

 ypo e }s. ^^g^jj^ what I call his experimental probabilities. I can fur- 

 niih him with one to which perhaps he is a ftranger, and re- 

 mind him of another, which he will not difpute : it will there- 

 fore be his bufinefs to reconcile them to the hypothecs of an 

 aqueous atmofphere. 



An experiment Fifji, Mr. Kirwan, in the courfe of his flatical experiments 

 y r. irwan.Q^ ^j^.^ conftantly found a given bulk of it to be lighter, ccEtens 

 paribus, when De Sauflure's hygrometer was at 90"^, than 

 when the atmofphere was lefs humid. Now according to 

 Mr. D.'s hypothefis, all the permanent gafes, of this given 

 bulk, had the fame weight both in the dry and moift ftate of 

 the air ; becaufe they were of the fame denfity on account of 

 the ccettris paribus : but the aqueous vapour was moft abun- 

 dant when the weight of the aggregate was leaft ; that is, the 

 weight of vapour dirainifhes, while its denfity increafes. Will 

 Mr. D. admit the truth of the preceding conclufion, to the 

 detriment of his own dodrine; or will he refute the infer* 

 ence, by denying the fad ? 



4 StcQtid, 



