166 Dr. Thomson ou the [Sept. 



elapsed before chemists acquired any precise notions respecting 

 the true specific gravity of this gas. Mr. Cavendish, the first 

 person who attempted to determine the specific gravity of 

 hydrogen gas, fim.nd it 10*8 times lighter than common air, 

 which is about U"092.* Lavoisier, whose method of taking the 

 specific gravity of gases was better than that of Mr. Cavendish, 

 found the specific gravity of hydrogen gas only 0-0769, which is 

 considerably below the result of his predecessor, though still 

 above the truth.i' 



MM. Biot and Arago made a careful set of experiments to 

 determine the specific gravity of difterent gases, the results of 

 which were published in the Memoires de I'lnstitute for 1806. 

 They appear to have been at considerable pains in preparing 

 their gases, and their experiments, as far as appears, were made 

 with very great care. They found the specific gravity of hydro- 

 gen gas 0'0732. This is the weight of pure hydrogen gas, which 

 has been generally adopted by chemists. Dr. WoUaston, for 

 example, was decided by it in his paper on Chemical Equiva- 

 lents, and in the construction of his sliding rule, which has 

 proved of such utility in chemical researches ; for he has consi- 

 dered the specific gravity of oxygen gas as 15 times greater than 

 that of hydrogen gas, and has concluded in consequence that 

 the atom of oxygen is 7 \ times greater than that of hydrogen. 

 Sir H. Davy has adopted the same specific gravity of hydrogen 

 gas, and the same relation between the weight of the atom of 

 oxygen and hydrogen (Elements of Chemical Philosophy, p. 112), 

 swayed doubtless by the same authority ; though, as he is in the 

 habit of neglecting to state the sources of his information, the 

 reader would be apt to conclude that he had himself determined 

 the specific gravity of hydrogen gas, and found it to be the same 

 as had been before stated by Biot and Arago. Indeed 1 should 

 not wonder if Davy's statement weie really derived from an ori- 

 ginal experiment of his ov^n ; for I myself took the specific 

 gravity of hydrogen gas about 10 years ago, and obtained 0-073, 

 a result coinciding so nearly with that of Biot and Arago that I 

 considered it as quite unnecessary to publish any account of my 

 experiment. 



Thus it appears that every successive experimenter had found 

 the specific gravity of hydrogen gas lower than his predecessor. 

 This will appear at one glance from the following table in which 

 the successive results are registered : 



Specific gravity of hj'drogeu gas 0-092 Cavendish 



0-0769 Lavoisier 

 0-0732 Biot and Arago. 



Dr. Prout had the merit of discovering that even the experi- 

 ments of Biot and Arago gave the specific gravity of hydrogen 



* Piiil. Trans. 1765, p. 141. t Laviiisier's Eleincnt?, p. 569, I'ngl. Tran». 



