252 Dr. Thomson on the [Oct. 



of my System of Chemistry ; but I neither consider it as so 

 simple nor so satisfactory as the view now given. 



4. Carburettcd Hydrogen. — 1 have often obtained by distiUing 

 coal and whale oil, gases which possessed very nearly the 

 chemical properties of carburetted hydrogen gas; but the specific 

 gravity of the gases obtained by distilling these bodies varies so 

 much that there cannot be the least doubt of their being mix- 

 tures of various gases, some of which, in all probability, we are 

 still unacquainted with. The only method with which I am 

 acquainted by means of which we can procure pure carburetted 

 liydrogen gas in a state of purity is to collect the gas which is 

 evolved when we stir the bottom of stagnant ditches filled with 

 water in the hot weather of summer. By this simple method we 

 can collect it in abundance. It is usually as first collected con- 

 taminated with carbonic acid ; but this gas is easily removed by 

 means of milk of lime. I have sometimes also found a mixture 

 of common air, but this is not always present. 



The following are the results of the trials made in my labora- 

 tory to determine the specific gravity of this gas collected in the 

 way just mentioned : 



By first trial 0-5602 



By second trial 0*5558 



By third trial 0-5570 



Mean = 0-5576 



am not aware of any previous determination of the specific 

 gravity of this gas, except the one given by me 10 years ago, 

 in a paper which 1 published on the gaseous combinations of 

 carbon and hydrogen in the first volume of the Memoirs of the 

 Wernerian Society. I found the specific gravity of the pure 

 gas 0-5554. Mr. Dalton had indeed stated the specific gravity 

 of this gas to be 0-600 ; but from the imperfection of his appa- 

 ratus, all the specific gravities of gases which he gives can be 

 considered only as rude approximations ; but not at all precise 

 enough to found important points of doctrine on. The ditierence 

 between the experiments made in my laboratory last year and 

 my former result amounts to Tr^d part of the whole. I believe 

 the reason to be, that in my first experiments I ascertained the 

 quantity of common air mixed with carburetted hydrogen gas, 

 and made allowance for it; whereas that precaution was not 

 taken in the experiments of last summer. A mixture of 209*6 

 volumes of pure carburetted hydrogen gas and one volume of 

 connnon air would have had the specific gravity 0-5576. Now 

 I am ignorant of a good method of detecting so small a mixture 

 of common air as this in a gas of so difficult combustion as car- 

 buretted hydrogen gas. 



But it is easy from the data established in the preceding part 

 of this paper to demonstrate the absolute specific gravity of pure 



