168 Dr. Thomson on the [Sept. 



gas.* It deserves to be recoictod, and the fact cannot but strike 

 posterity with surprise, that though Dr. Front's demonstration is 

 complete, and though it has been before the pubhc these five 

 years, I am not aware of any British chemist who has adopted 

 the true specific gravity of hydrogen gas, except myself. Every 

 writer, either of elementary chemictil books, or of experimental 

 investigations (as far as my reading has gone), has uniformly 

 contented himself with the sjiecitic gravity as determined by 

 Eiot and Arago. The same observation applies to the French 

 chemists ; but I have seen papers which have been published 

 both in Germany and in the United States of America in which 

 the true specific gravity of hydrogen gas, as determined by Dr. 

 Prout, has been adopted. 



The first persons who attempted to verify Dr. Front's conclu- 

 sion were Berzelius and Dulong. They found the specific gravity 

 of hydrogen gas only 0-06(S8, or 0'0689.t As Berzelius gives no 

 details of the way in which his experiments were made, we are 

 nnable to point out the reason why his gas was lighter than that 

 which had been weighed by preceding experimenters. Those in 

 my laboratory, of Avhich I am going to give an account, were 

 made before 1 had heard that the subject had attracted the atten- 

 tion of Berzelius; but 1 think it not unlikely that he preceded me 

 in point of time. 



The hydrogen which we used was obtained from zinc. In 

 order to get rid of the impurities with which common zinc is 

 always contaminated, and which from previous experiments there 

 was "reason to believe added somewhat to the weight of the 

 hydrogen evolved, the zinc was distilled over by exposing it to a 

 white lieat in a stoneware retort. The requisite precautions were 

 taken in employing pure water and pure sulphuric acid, and 

 every particle oi" common air was carefully excluded. The 

 specific gravity of- hydrogen gas thus prepared was taken three 

 different times. The following were the results obtained : 



By first trial , 0-0G954 



By second trial 0-06933 



By third trial 0-06933 



Now tlie mean of these three experiments gives us 0'06940, 

 which is precisely the number already deduced by Dr. Prout, 

 from the specific gravity of ammoniacal gas. 1 conceive then 

 that there cannot be the" least hesitation m adopting 0-0694 as 

 the true specific gravity of pure hydrogen gas ; consequently 

 oxygen gas is precisely 16 times heavier than hydrogen gas, and 

 if the weight of an atom of oxygen be reckoned unity, then that 

 of hydrogen is 0-126 ; and 0-125 x 8 = I ; so that the atom of 

 oxygen is a multiple of the atom of hydrogen. 



» ^4unah of rUilosophij, vi._^322. 



t Kt;iii sur la Tliturie dt's Vroporlioni, Cliimiqiics, p. 129. 



