1820.] Specific Gravity of Gases. 251 



have only to add together twice the specific gravity of hydrogen 

 gas, and twice the specific gravity of carbon vapour. 



Specific gravity of hydrogen 0-0694 and 0-0ri94 x 2 = 0-1388 

 carbon . . 0-4166 and 0-4166 x 2 = 0-8333 



olefiant gas. = 0-9722 



Thus we see that the true specific gravity of olefiant gas is 0-9722* 

 This specific gravity neither agrees with my former determina- 

 tion nor with our late results ; but if we take the mean of the 

 two sets of experiments, we obtain a number which approaches 

 very near the truth. 



I Sp gr. by my former experiments . . . 0-9745 

 recent experiments . . . 0-9709 



Mean = 0-9727 



This differs from the truth only -yVTtli part, and is, therefore, 

 greatly within the limits of unavoidable error from experiment. 

 1 have no doubt that the excess of my first specific gravity, 

 which amounts to -^^d part, was owing to the gas having been 

 imperfectly freed from carbonic acid. A mixture of 239-5 

 volumes of olefiant gas and one volume of carbonic acid gas 

 would have exactly the specific gravity which I obtained. It is 

 not so easy to account for the error in defect of the last experi- 

 riment, which amounts to ,j4yth part, unless we suppose that a 

 small portion of carburetted hydrogen gas had been generated. 

 A mixture of 318-i- volumes of olefiant gas and 1 volume of 

 carburetted hydrogen would have precisely the specific gravity 

 of 0-9709, which was the result obtained. 



Upon the whole, there does not seem the least reason for 

 hesitating to admit 0-9722 for the true specific gravity of olefiant 

 gas. It is a compound of two atoms of hydrogen and two atoms 

 of carbon. Hence the weight of an atom of it is 1-75. Now 

 this number is just double the specific gravity of olefiant gas, 

 reckoning the specific gravity of oxygen gas unity ; for its 

 specific gravity in that case will be reduced to 0-875 ; and 

 0-875 X 2 = 1-75. 



It deserves attention that the specific gravities and the 

 weights of the atoms of azotic gas, carbonic oxide gas, and ole- 

 fiant gas, are all the same. The specific gravity of each is 

 0-9722, and the weight of an atom of each is 1-75, or 14 times 

 the weight of an atom of hydrogen. 



We might indeed consider olefiant gas as a compound of one 

 atom hydrogen and one atom carbon. This would reduce the 

 atomic weight to 0-875, which is precisely equal to the specific 

 gravity, reckoning oxygen unity. This was the view of its con- 

 stitution which I entertained when I published the fifth edition 



