1826.] Mr. Rainy' s Reply to Br. Thomson. 191 



inquiry, to obtain sufficient data, either for refuting Dr. Prout's 

 theory, or estabhshing it on a solid basis. I do not by any means 

 object to Dr. Thomson's numbers as very good approximations, 

 probably the best which we possess at present ; but I am as yet 

 unable to see any satisfactory proof of their absolute exactness. 

 Dr. Thomson observes that in Great Britain at least, the specifio 

 gravities which he assigns for oxygen and hydrogen are univer- 

 sally admitted to be true, but I am persuaded that they are 

 admitted by no means in the rigid mathematical sense in which 

 he holds them. 



Dr. Thomson, in the conclusion of his paper, has stated some 

 other proofs, which appear to him perfectly conclusive in favour 

 of Dr. Front's hypothesis. I shall make a few remarks on each 

 in the order in which they are stated. 



1 . Dr. Thomson ** determined by actual experiment, that the 

 specific gravity of hydrogen gas is 0*0694; and the subsequent 

 determination of Berzelius and Dulong approaches very nearly to 

 this number." Now Dr. Thomson's experiments ivere made on 

 moist gas at a temperature v)hich he has not stated. We can- 

 not, therefore, ascertain from them the precise specific gravity 

 of hydrogen ; they merely prove, if correct, that the specijic gra- 

 vity of dry hydrogen is less than 0*0694. If the experiments were 

 made at 60°, it would follow from them that the specific gravity 

 of hydrogen is about 0-062, and consequently that the specific 

 gravity of oxygen is 18 times as great as that of hydrogen, and 

 that the atom of oxygen is 9 times as great aa that of hydrogen. 

 Even were the experiments made at 32°, the results would be 

 totally inconsistent with Dr. Front's hypothesis. It was in fact 

 these very experiments that first led me to question the truth of 

 Dr. Front's hypothesis. When these experiments were per- 

 formed. Dr. Thomson did not admit that moisture has any mate- 

 rial influence in modifying the specific gravity of the gases (see 

 Annals, New Series, vol. iii. p. 302 — 308) ; but now that he 

 admits that influence, I conceive he must also admit that hi» 

 experiments are hostile to Dr. Frout's views. 



2. Dr. Thomson thinks he has adduced conclusive evidence 

 in his Frinciples that atmospheric air consists of 4 volumes azote 

 to 1 volume oxygen. I am by no means satisfied with this 

 evidence. The results were not uniform. In one instance, Dr. 

 Thomson found 79-246 azote, and in another 80-927, and though 

 the mean of 10 experiments, 79-97, approaches extremely near 

 the theoretic number, it differs from previous experiments made 

 by Dr. Thomson himself, as well as by other experimenters. 

 But even were we to admit this point, I do not see how it follows 



that the specific gravity of oxygen is 1*1111, unless we pre- 

 viously know either the specific gravity of azote, or the ratio of 

 the atomic weights of oxygen and azote. Dr. Prout extricates 



