58 



Experiments on Vapour-density. 



[May 8, 



It was useless to estimate the residual amount of trioxide, as sul- 

 phuric acid had collected in the lower part of the globe while the 

 trioxide was being sublimed into it. The vapour-density at the 

 lowest pressure was therefore assumed to be normal (corresponding 

 to S0 3 ), as ifc would be in all probability, and on this assumption the 

 other vapour- densities are also normal. If the third vapour-density 

 were actually above normal, the second would be less above normal, 

 and the first still less; while, if the third vapour-density were actually 

 below normal, the others would also be below normal ; these alterna- 

 tives are very improbable, and I regard it as fairly proved that the 

 formula of sulphuric anhydride is S0 3 , and not S 2 6 . 



Vapour-density of Aqueous Hydrochloric Acid. 



Commercial hydrochloric acid was distilled until the boiling point 

 became constant, at 108'2 (pressure, 745 mm.). The distillate was 

 then collected and used for vapour-density determinations ; to find 

 the percentage of real acid, weighed quantities were titrated with a 

 standard soda solution. Tho acid used in Series I contained 

 20'82 per cent. HC1 ; that in Series II, 21'33 per cent. ; and Series III, 

 20'45 per cent. The last two samples were distilled from pure acid. 

 The vapour-density was found by means of the apparatus used for 

 iodine. The acid coming over on lowering the pressure was absorbed 

 by a standard soda solution in the absorption-tube ; much of it, how- 

 ever, condensed above the stopcock, and caused some difficulty. 



Results. 



