250 jDr Thomson on the [Oct. 



four parts of sulphuric acid and one part of alcohol by weight, 

 and exposing the mixture in a retort to the heat of a lamp. The 

 gas comes over, and may be received over water. The only 

 precaution is to take care that all the common air in the retort 

 be driven oft' before we begin to collect the olefiant gas. If the 

 mixture of sulphuric acid and alcohol be kept hot after it has 

 become black, there is always evolved a considerable portion of 

 carbonic acid gas mixed with some sulphurous acid gas. The 

 sulphurous acid is immediately absorbed by the water ; but to 

 get rid of all the carbonic acid, the best way is to wash the de- 

 fiant gas in some milk of lime. All these precautions were 

 attended to in tiie experiments made in my laboratory to deter- 

 mine the specific gravity of olefiant gas. The following were 

 the results obtained : 



By first trial 0-9709 



Bysecond trial 0-9709 



By third trial 0-9709 



We have two previous sets of experiments to determine the 

 specific gravity of this gas. The Dutch chemists who disco- 

 vered the gas found its specific gravity 0-909. In a set of 

 experiments which I made on it ni 1810, I determined its 

 specific gravity to be 0-9745. Both of these, results differ from 

 what has been just given ; but from the great pains taken to 

 obtain the gas pure, there can be no doubt that our present 

 results are much nearer the truth than those of the Dutch che- 

 mists, and at least as near as my own former ones. 



As we know the specific gravity of hydrogen gas and carbon 

 vapour, the two constituents of this gas, and as we are in posses- 

 sion of accurate experiments respecting the proportion in which 

 these two elements are combined in it, we can easily determine 

 its true specific gravity by calculation. 



It has been ascertained by experiment that one volume of 

 pure olefiant gas requires for complete combustion three volumes 

 of oxygen gas, and that the residue after combustion amounts to 

 two volumes of carbonic acid. Now the two volumes of carbo- 

 nic acid required for their formation two volumes of oxygen gas. 

 The other volume of oxygen gas consumed must have combined 

 with hydrogen and formed water, and it must have combined 

 with a quantity of hydrogen which, had it been in the gaseous 

 state, would have amounted to two volumes. Two volumes of 

 carbonic acid gas contain just two volumes of carbon. We see 

 from this experimental result that a volume of olefiant gas is 

 composed of 



2 volumes carbon 1 , , . , 



hvdrooer I ^°'^"^"s^d J"to one volume. 



Therefore, to obtain the specific gravity of olefiant gas, we 



