on the Atomic Theory. 123 



which, though still inexact, yet being applied to his former expe- 

 riments, throws them all beyond the pale of the atomic theory. 

 To square his experiments with Dr. Prout's numbers, he, on that 

 occasion, estimated the weight of aqueous vapour in his gases, 

 at about one-sixtieth of what they really contained; and now, 

 when he admits that there must be nearly fifty times more mois- 

 ture in them, still he cites his old results as true. His new ideas 

 relative to aqueous vapour, we shall presently discuss. Meanwhile, 

 let us consider " the attempts which he has made to determine 

 the specific gravity of oxygen gas in quite a different way." 



He dissolved in dilute sulphuric acid, 100 grains of distilled 

 zinc, contained in a small glass retort. At the end of twenty-four 

 hours, the metal being dissolved, and the apparatus, <§-c, having 

 taken the temperature of the room, he obtained in each of 2 ex- 

 periments, a volume of moist hydrogen gas, which, reduced by 

 calculation, gave at 60° Fahr., 138.755 cubic inches. He con- 

 cludes, that one half of this volume or 69.3775 cubic inches of 

 oxygen gas, have entered into union with the zinc. But from his 

 researches in the preceding section, it appears, " that 100 grains 

 of zinc when converted into an oxide, combine with 23.5294 grains 

 of oxygen. Hence, the weight of 69.3775 cubic inches of oxygen 

 gas, is 23.5294 grains. Consequently, 100 cubic inches weigh 

 33.9 15 grains *. Estimating with Sir George Shuckburgh Evelyn, 

 the weight of 100 cubic inches of dry air to be 30.5. grains, the 

 specific gravity of oxygen comes out 1.1111, or Dr. Prout's theo- 

 retic number exactly. This concurrence of numbers, in the sequel 

 of such experiments, we may venture to predict will hardly be 

 realized by any other chemist. 



Having settled this point to his entire satisfaction, he con- 

 cludes the section by a formula, to shew what influence the pre- 

 sence of moisture must exercise on the specific gravity of oxygen 

 gas. But his formula is rendered altogether erroneous by his 

 adopting 0.00772, as the specific gravity of aqueous vapour at 

 60° Fahr. This being a matter of fundamental importance in 

 pneumatic chemistry, and one much misunderstood, we shall de- 

 vote a paragraph or two to its elucidation. 



M. de Saussure took extreme pains to determine, by direct ex- 

 periment, the weight of aqueous vapour contained in a given por- 

 tion of moist air. He ascertained that the same volume of dif- 

 ferent gases, standing over water at a given temperature, afford, 

 on being dried by muriate of lime, the same weight of water ; 

 which, for 100 cubic inches of aeriform matter, amounted to 0.35 

 of a grain. Now the subsequent researches of M. Gay Lussac, 

 and Mr. Dalton, concur to shew that the same volume and weight 



* Thomson's First Principles, i. 65. Taking his own data, the above num- 

 ber comes out 69.S765, but this error is of no consequence. 



