3820.J Specific Gravity of Gases. 245 



carbonic acid gas, 1*587. Bat these data are too loose to enable 

 us to come to precise conclusions. 



Lavoisier gives us the result of his experiments on the specific 

 gravity of carbonic acid gas in his Elements.* He found it to 

 be r4993. Biot and Arago, in the year 1806, published the 

 result of a set of experiments undertaken to determine the specific 

 gravity of different gases. They fix the specific gravity of car- 

 bonic acid gas at 1-5196.^ Thus we have four different deter- 

 minations of the specific gravity of this gas, each founded on 

 experiment. According to 



Cavendish, it is 1"587 



Lavoisier 1-4993 



Biot and Arago 1-5196 



Thomson 1-52673 



Now, if we except the determination of Cavendish, which, 

 from the reasons above stated, does not seem entitled to much 

 confidence, it is obvious that each experimenter has found the 

 specific gravity of this gas higher than his predecessor. Lavoisier, 

 who was first in order, makes it the lightest; Biot and Arago, 

 who followed him at an interval of nearly 20 years, found it 

 heavier. The experiments in my laboratory were made 14 years 

 later, and we found the gas heaviest of all. Now this is just 

 what ought to have happened, supposing that every succeeding 

 experimenter employed greater care than his predecessor to 

 exclude common air from his carbonic acid gas as completely as 

 possible ; for common air being much lighter than carbonic acid 

 gas, it is obvious that the intermixture of a very small portion of 

 it would produce a sensible change on the specific gravity of the 

 carbonic acid gas subjected to experiment. Had Lavoisier's 

 carbonic acid been mixed with a very little more than five per 

 cent, of common air, the specific gravity of such a mixture 

 would have been just what he states his carbonic acid to be. 

 Now whoever has paid the least attention to the manner of 

 making experiments on gases 30 years ago will have no hesita- 

 tion in admitting that Lavoisier's carbonic acid gas might very- 

 likely be contaminated with even a greater proportion of air 

 than we have here supposed. The carbonic acid gas of Biot 

 and Arago was probably contaminated with a mixture of 1^ per 

 cent, of common air. 



Even the carbonic acid gas, v/hose specific gravity was taken 

 in my laboratory, was not absolutely pure. It was collected over 

 common river water. Now such water always contains some 

 common air, part of which is disengaged when any other gas is 

 placed over it. Of course a small portion of air would of neces- 

 sity mix itself with the carbonic acid gas, and of consequence 

 diminish its specific gravity somewhat. Upon trying the good- 



• 



tn-lijii Tranilatioii, p. 5fi9. + Mrm. dc rinslituf. 1806, \>. 320. 



