Messrs. Wanklyn and Robinson on Diffusion of Vapours. 547 



ception of the vapour to be operated upon ; the upper flask is for 

 the atmosphere into which the vapour is to be diffused. The atmo- 

 sphere of dry air, or other suitable gas, is kept constantly renewed 

 by the transmission of a slow stream of gas, which enters the upper 

 flask by the narrow tube above, and passes out by the space between 

 the two necks, which, as we have said, do not fit air-tight. When 

 in use, the whole apparatus is kept at a temperature above the con- 

 densing-point of the vapour by means of an air-bath. 



After a diffusion has gone on for a sufficient length of time the 

 apparatus is allowed to cool, and the contents of the lower flask are 

 analysed, by which means it is seen whether diffusion has effected 

 any alteration in the composition of the vapour. 



We have used a lower flask of about 500 cubic centimetres capa- 

 city, with a mouth 10 millimetres in diameter; the capacity of the 

 upper flask was 100 cubic centimetres. 



The first substance taken for experiment was sulphuric acid, which 

 is converted at high temperatures into vapour of sulphuric anhydride 

 and vapour of water. Inasmuch as vapour of water is lighter than 

 vapour of sulphuric anhydride, the former should diffuse more 

 rapidly than the latter. Accordingly, the residue after diffusion 

 should be richer in sulphuric anhydride than the acid before diffusion. 



In one experiment we took an acid composed of 

 95 Mono-hydrated sulphuric acid. 

 5 Water. 



100 

 After diffusion for an hour at about 520° C, the residue was 

 composed of 



60 Mono-hydrated sulphuric acid. 



40 Sulphuric anhydride. 



100 



In another experiment we took an acid containing 



99 Mono-hydrate. 

 1 Water. 



100 



and after diffusion for a shorter time at 445° C. found the residue to 

 consist of 



75 Mono-hydrate. 



25 Anhydride. 



100 



In both cases the residues after diffusion fumed strongly on 

 exposure to the air, and consisted partly of crystals and partly of 

 liquid. 



The substance next submitted to diffusion was pentachloride of 

 phosphorus, which is decomposed by heat into terchloride and free 

 chlorine. 



