258 Prof. Graham on the Diffusion of Liquids, 



equivalent to 5*33 grs. of alum. The diffused salt gave off 

 no acid vapours at 600°. We may therefore suppose the ex- 

 cess of salt which is diffused to be sulphate of potash. The 

 diffusion product of alum, at 64°, appears to be— 



Alum 5*33 71*26 



Sulphate of potash . . 2*15 28*74 



7*48 100*00 



In a second experiment, the diffusion product amounted to 

 6*39 grs., of which 0*95 gr. was alumina ; and it is represented 

 by 4*77 alum and 1*52 sulphate of potash. 



In connexion with the low diffusibility of the sulphate of 

 alumina of alum, it was found that the addition of caustic 

 potash to the alum solution, so as to convert it into an alumi- 

 nate of potash, increased the diffusibility of the alumina. The 

 diffusion product from the 4 per cent, solution of alum so 

 treated contained 1*62 gr. of alumina in one experiment and 

 1*54 in another. 



As alum is a salt of great stability, it presents a severe test 

 of the influence in question. The decomposition of this double 

 salt by diffusion was further confirmed therefore in experi- 

 ments made by means of the four-ounce diffusion phials of 

 1-25 inch aperture, and the alteration which the salt undergoes 

 in the process more exactly ascertained. The experiments 

 were made at a mean temperature of 57°*9, and lasted seven 

 days; the solution employed being of 4 anhydrous alum to 

 100 water, as before. 



In three experiments, the salt diffused out amounted to 

 5*73, 5*80 and 5'65 grs. ; of which the mean is 5*73 grs. The 

 latter quantity gave 0*82 alumina and 3*22 sulphuric acid, 

 which correspond to 4*11 anhydrous alum and 1*62 neutral 

 sulphate of potash. Or, we have as the diffusion product of 

 alum, in 100 parts — 



Alum 71*73 



Sulphate of potash .... 28*27 



100-00 

 This analysis corresponds closely with the diffusion product 

 of the former experiments, which gave 71*26 per cent, of alum. 

 The solution of alum which remains behind in the solution 

 phials must of course acquire an excess of sulphate of alumina. 

 The salt, sulphate of alumina, did not appear to be decom- 

 posed when diffused alone. A four per cent, solution of the 

 hydrated sulphate of alumina, which is manufactured at New- 

 castle, when diffused in the same circumstances as the prece- 

 ding solutions of alum, gave 3*40 grs. of anhydrous sulphate 

 of alumina, in which the acid was to the alumina as 2'95 equi- 

 valents of the former to 1 equivalent of the latter, or as nearly 



