Prof. Graham on the Diffusion of Liquids. 257 



pbate 1, and their relation to the diffusibility of chloride of 

 sodium, we should be able to judge from the proportion in 

 which the magnesia travels in company with chloride of so- 

 dium, whether it is travelling in the large proportion of chlo- 

 ride of magnesium, in the small proportion of sulphate of 

 magnesia, or in the intermediate proportion of a certain mix- 

 ture of chloride and sulphate of magnesia. But here we are 

 met by a difficulty. Do the chloride of magnesium and sul- 

 phate of magnesia necessarily pre-exist in sea-water in the 

 proportions in which they are found to diffuse? May not the 

 more easy diffusion of chlorides determine their formation in 

 the diffusive act, just as evaporation determines the formation 

 of a volatile salt — producing carbonate of ammonia, for in- 

 stance, from hydrochlorate of ammonia with carbonate of lime 

 in the same solution ? We shall see immediately that liquid 

 diffusion, as well as gaseous evaporation, can produce che- 

 mical decompositions. 



6. Decomposition of Salts by Diffusion. 



(1.) At an early period of the inquiry, a solution was dif- 

 fused of bisulphate of potash, saturated at 68° and of density 

 1*280, from the six-ounce phial of 1*175 inch aperture, into 

 20 ounces of water. The period of diffusion extended to fifty 

 days. About the middle of that period, a few small crystals 

 of sulphate of potash, amounting probably to 3 or 4 hun- 

 dredths of a grain, appeared in the diffusion cell and never 

 afterwards dissolved away. When terminated, the liquid re- 

 maining in the solution cell was found of density 1*154 ; that 

 in the water-jar 1*0.326. A portion of the latter liquid gave 

 by analysis — 



Sulphate of potash . . . 20*37 1 tj. i u . c ... 

 oil c ii A~r Bisulphate ol potash. 



Sulphate of water . . . 11*47 J * * 



Sulphate of water . . . 12*77 



44*61 



It thus appears that the bisulphate of potash undergoes 

 decomposition in diffusing, and that the acid diffuses away to 

 about double the extent, in equivalents, of the sulphate of pot- 

 ash. This greater escape of the acid will also account for the 

 deposition of crystals of the neutral sulphate in the solution 

 cell. 



(2.) A similar experiment was made with another double 

 sulphate of greater stability, common potash-alum. The so- 

 lution of 4 anhydrous alum in 100 water, was diffused from 

 the six-ounce phial into 24 ounces of water, at 64°*2, for eight 

 days. The quantity of salt diffused in that time amounted 

 only to 7*48 grs. It contained 106 gr. alumina, which is 



Phil. Mag. S. 3. Vol. 37. No. 250. Oct. 1850. S 



