230 PROCEEDINGS OF THE AMERICAN ACADEMY 



Specific Gravity of Carbonic Acid. Hydrogen = 1. 



Wt. of C0 2 . Wt. of H 2 . H T Sp. Gr. 



B No. 1 9.0321 0.4111 756.73 22°.40 21.971 

 C No. 1 8.9808 0.4088 760.50 25°.56 21.971 

 C No. 2 8.9335 0.4066 759.13 26°.60 21.971 



Average value 21.971 



Average value by chemical process 21.957 



The close agreement, indeed the essential identity, of these results 

 should be noticed, as this fully substantiates the extreme accuracy of 

 the method of recovering the lost tare of the empty balloon described 

 on page 224. It is true that, as the values of the specific gravity of 

 carbonic acid have been calculated with an average value of the sev- 

 eral reduced weights of hydrogen or air, an error in these data would 

 not affect the results ; but any error in the observed weights of car- 

 bonic acid would appear to its full extent. 



Moreover, the striking fact should not be overlooked that the specific 

 gravity of carbonic acid gas referred to hydrogen approaches much 

 more nearly 22, the half molecular weight of carbonic acid gas as 

 generally assumed, than does that of oxygen gas when referred to the 

 same standard, the corresponding whole number 16, and the signifi- 

 cance of the circumstance is obvious. If we assume that the atomic 

 weight of oxygen as determined by Dr. Richards under my direc- 

 tion is 15.87,* then the corresponding atomic weight of carbon would 

 be 11.90, and the half molecular weight of carbonic acid 21.82. 

 Theoretically, this number ought also to define the specific gravity of 

 carbonic acid gas referred to hydrogen gas, if the two gases were com- 

 pared under the same conditions of high temperature and indefinite 

 expansion. But under the great pressure of our atmosphere the 

 molecular volume of carbonic acid gas is known to be condensed to 

 a measurably greater degree than the potentially equal molecular vol- 

 ume of hydrogen gas, and the result must be a proportionally increased 

 density ; and, moreover, the inequality in the condensation of the two 

 gases must be increased by the circumstance that at the ordinary tem- 

 perature of the air carbonic acid gas is below the critical point, while 

 hydrogen gas is very far above it. If, however, this is true in the case 

 of carbonic acid gas, our knowledge of the deviations from Mariotte's 

 law compels us to infer that the same must be true in some small 

 measure, although in a much less degree, in the case of oxygen gas ; 



* These Proceedings, vol. xxiii. p. 185. 



