DOCTklNE OP MIXED CASES. 53 



of which admits of a mathematical demonftration, derived from 

 Newton's definitions and laws of motion : but Mr. Dalton has 

 not attempted to give ftability to his new ideas, by the aid of 

 the mathematics ; on which account the only appellation due 

 to his do£lrine, is that of a hypothefis. 



No philofopher ought to difregard the means which are The hypothefis 

 able to confirm his opinions, becaufe thefe means may be found, '^° ^^^ ^'" ' 

 upon trial, to fubvert them. This I believe to be the cafe with 

 my friend's hypothefis ; fori have endeavoured to flievv fhe 

 fallacy of it by mathematical arguments. The refult of this 

 attempt will in all probability appear in the next volume of the 

 Manchefter Memoirs ; when the merits of the eflay will be de^ 

 termined by geometricians, who are the proper judges of fuch 

 produ6lions. This want of geometrical demonfiration efcapcs 

 the notice of the cheraift, becaufe my friend has feemingly fup^ 

 plied the deficiency by a number of probabilities of an expe- 

 rimental nature ; but it is almoft fuperfluous to remind either 

 him or your readers, that a myriad of fuch proofs cannot uphold 

 a do6lrine which is repugnant to the mechanical philofophy. 



The two leading maxims which are derived from thefe proba- Conclufions 

 bilities, and form the bafis of the hypothefis, are thus briefly ^'^°?Jj^^M^ ?^^^ 

 exprefled by Mr. Henry : '* mixed gales neither altradl nor repel 

 each other, and every gas is as a vacuum to every other gas*", 

 Mr. Dalton, reafoning from thefe premifes, furrounds our 

 globe with an independent atmofphere of vapour, the prefl^ire 

 of which preferves all the water on the earth's furface in a 

 liquid ftate, and prevents the ocean itfelf from efcaping through 

 the air, by virtue of its own elaftic force ; at leaft the laft is a 

 fair inference from his own conclufions. 



Although I have demonfiraled the exiftence of an atmo-Dalton's reafon- 

 fphere of vapour to be a mechanical impoffibility, in the eflay ^"S i'i<^o""P^<^^<5* 

 mentioned above, the reality of the thing fliall be fiippofed at 

 prefent, for the purpofe of detecting the fallacy of the dodtrine 

 in the fundamental maxims of it. The truth is, Mr. Dalton 

 has difcontinued his train of reafoning too foon ; for, had his 

 arguments been purfued to their proper limits, they would 

 have difcovered the incompatibility of the hypothefis and na- 

 tural appearances. This omiflTion is eafily fupplied, and will 

 be as eafily comprehended, by a perfon who underflands the 

 laws of hydroftatics. 



• Phil. Journ. Vol. VIII. Page 298. 



If' 



