260 Prof. Graham on the Diffusion of Liquids. 



of potash. The chlorides of potassium and sodium, on the 

 contrary, were not sensibly decomposed. 



It is known that a precipitation of sulphate of lime may 

 occur, with a larger proportion of sulphate of potash in lime- 

 water, in a close phial without external diffusion. As the 

 decomposition of the sulphate of potash, in the latter case, 

 has been referred to the insolubility of the sulphate of lime, 

 so the decomposition in the former circumstances is referred, 

 in a similar sense, to the high diffusibility of hydrate of potash. 



7. Diffusion of Double Salts. 



How is the diffusion of two salts affected by their condition 

 of combination as a double salt? A solution of the double 

 sulphate of magnesia and potash, in the proportion of 100 

 water to 4 anhydrous salt, was operated upon in the four- 

 ounce diffusion phials of 1*25 inch aperture, with a period of 

 diffusion of seven days, at 57°*9 F. The diffusion product 

 of the double salt was 8'09 and 7*81 grs. in two experiments : 

 mean, 7*95 grs. 



The constituent salts, sulphate of magnesia and sulphate of 

 potash, were now dissolved separately, in the proportions in 

 which they existed in the double salt, namely, 1*65 gr. anhy- 

 drous sulphate of magnesia in 100 water, and 2*35 grs. sul- 

 phate of potash in 100 water, making up together 4 parts of 

 salts. The two solutions thus contain equivalent quantities 

 of the different sulphates. 



The separate diffusion of the sulphate of magnesia was 2-09, 

 2*11 and 2-40 grs. in three cells; and of the sulphate of pot- 

 ash, 5*83, 5*97 and 5*54 grs. in three cells; the circumstances 

 of the experiments being the same as those of the double salt. 

 The means of the two salts are 2*20 and .5*78 grs.; and the 

 sum of the two means 7'98 grs. The result is, that the sepa- 

 rate diffusion of the constituent salts is almost identical with 

 their diffusion when combined as a double salt: — 



Diffusion of the double sulphate of magnesia and"] * «,. 



. i r 7'95 grs. 



potash J & 



Diffusion of equivalents of sulphate of magnesia | ^ __ 



and sulphate of potash in separate cells . . J ° 



It would thus appear that the diffusibility of this double 

 salt is the sum of the separate diffusions of its constituent salts. 



It has been a question whether a double salt is formed at 

 once when its constituent salts are dissolved together, or not 

 till the act of crystallization of the compound salt. Equiva- 

 lents of the same two sulphates, making up 4 parts, were dis- 

 solved together without heat in 100 water. Now the diffusion 

 from this mixture, which has the composition of the preceding 



