256 -Dr. Thomson on the [Oct. 



indicates a gas composed of 207"5 volumes of chlorocarbonic 

 acid and one volume of common air. This is as good an approx- 

 imation to accuracy as could have been looked for in such an 

 experiment. 



We see that chlorocarbonic acid is a compound of one atom 

 chlorine + one atom carbonic oxide. Hence the weight of an 

 integrant particle of it is 6-25 ; for 



1 atom chlorine = 4*50 



1 atom carbonic oxide = 1"7.5 



6-2 



= an atom chlorocarbonic acid. 



If we reckon the specific gravity of oxygen gas unity, then 

 the specific gravity of chlorocarbonic acid will be 3-125. Now 

 3-125 X 2 = 6-25; so that the atomic weight of this gas, like 

 so many others, is just double its specific gravity. 



Whoever considers attentively the preceding discussion will, 

 I think, entertain no doubt that the specific gravity of carbon 

 vapour is 0*4166, and that the weight of an atom of carbon is 

 0-75. 



II. Sulphur. 



Sulphur does not enter into so many gaseous compounds as 

 carbon, or at least we are acquainted at present with only two 

 permanently elastic fluids, into which it enters as a constituent. 

 These are 



1. Sulphurous acid composed of sulphur and oxygen. 



2. Sulphuretted hydrogen composed of sulphur and hydrogen. 



These two, hov/ever, will put it in our power to determine the 

 weight of an atom of sulphur, with as much precision as the 

 atom ofcarbon. 



1. Siilp/niroiis Acid. — This acid gas is easily obtained bv 

 mixing togetlter a mixture of mercury and concentrated sulphuric 

 acid in a retort, and heating the mixture b)^ an Argand's lamp. 

 The mercury is oxidized at the expense of the sulphuric acid ; 

 which, being deprived of one-third of its oxvgen, flies off" in the 

 state of sulphurous acid gas. It must be collected over mercury. 

 A sufficient quantity of pure gas being prepared in this way, its 

 specific gravity was found to be as follows : 



By first trial 2-2221 



By second trial 2*2221 



By third trial 2-2223 



Mean 2-22216 



We have two previous sets of experiments to determine the 

 specific gravity of this gas. 



