Combination of Sulphuric Acid and Water. 253 



as is well known, is always opaque, and of a blackish 

 colour. This is owing to the presence of some vegetable 

 matter charred by the acid. For when the acid has been 

 kept for some time boiling, it loses its dark colour, and 

 becomes as colourless and transparent as the purest British 

 acid. Suspecting at first that this dark colour might be 

 owing to the presence of selenium or iodine, or even bro- 

 mine, I examined it for each of these bodies, but unsuccess- 

 fully. The only foreign body whose presence was dis- 

 covered, was the sulphate of lime ; of which the acid I em- 

 ployed contained y^Vo*^^ P^^^ ^^ ^^^ weight. It was 

 suspended in the acid in white flocks, and greatly facili- 

 tated the distillation, by preventing the violent ebullition 

 which is apt to take place when we attempt to distill con- 

 centrated sulphuric acid. I continued the distillation till 

 the specific gravity of the acid which came over was as 

 great as that of the acid in the retort. It was obviously 

 needless to continue the process farther ; because all farther 

 concentration of the acid became impossible.* The acid 

 remaining in the retort had a specific gravity of 1*8422 at 

 the temperature of 59°. It was obviously a compound of 



1 atom acid, . . 5 

 1 atom water, . . 1*125 



6-125 



and its atomic weight was 6* 125. It was this acid that was 

 employed in all the following experiments. The only im- 

 purity which I detected in it was (as above stated) ^^oV^*^ 

 of its weight of sulphate of lime ; a quantity so small, that 

 it could not materially affect the results. It was impossible 

 to get rid of this sulphate, while the acid retained its 



* In a first process, the acid put into the retort had a specific gravity of 

 ;1*8376, and was composed of 



Real acid, . . 77*85 or 1 atom 

 Water, . . . 22-15 1*264 atom 



100*00 



^he acid that came over had a specific gravity of 1*6598 (or was very nearly a 

 compound of 1 atom acid -f 3 atoms water.) The specific gravity of the acid 

 jmaining in the retort was 1*8396 at 64^ J. 



