300 report — 1859. 



exist in solution, and we find that many of them are undoubtedly decom- 

 posed when dissolved in water. The simple double salts formed by non-iso- 

 morphous salts, such as those formed by the magnesian sulphates, the alums, 

 &c, are compounds of a higher degree, and are held together with much 

 more force, and in most cases are not decomposed by water at common 

 temperatures. If the constituent salts of alum existed free in solution, 

 it would be difficult to account for certain phenomena to which Chevreul 

 first drew attention. If cotton be immersed in a solution of alum of a 

 given strength, it will absorb some of it, but it will be found that the 

 solution absorbed docs not contain as much alum as the original one, 

 that is, the cloth absorbs the water molecules more readily than the saline 

 ones. Unless porous bodies, such as vegetable fibre, exert exactly the same 

 adhesive power for sulphate of potash as they do for sulphate of alumina, 

 which does not seem probable, the mere passage of a solution of alum 

 through cotton, supposing the two sulphates not to exist in a state of combi- 

 nation in solution, would be sufficient to alter their relative proportions. 

 The mere fact of vegetable fibre exerting unequal attraction for the consti- 

 tuent salts of double salts in solution, which in some cases at least it appears 

 to do, would not, however, of itself constitute a proof that double salts did 

 not exist, as such, in solution. It is very probable that the force binding the 

 constituent salts of a double salt, and which is different for each one, is 

 greatly diminished when the salt is dissolved in water; and that under those 

 circumstances, the superior attraction of vegetable fibre, or porous bodies for 

 one constituent salt, may be sufficient to overcome the chemical force by 

 which the double salt is formed. The full investigation of this very import- 

 ant problem forms part of the series of experiments to be made on the 

 action of charcoal, spongy platinum, and other porous bodies on saline 

 solutions, already alluded to. 



The preceding discussion on the condition of double salts in solution, 

 clearly shows that we could not determine their solubility without taking 

 into consideration the whole subject of the constitution of salts, the actiou 

 of salts in solution upon one another, and the nature of solution. It fully 

 justifies me therefore in viewing the subject which I proposed to investigate 

 from the general point of view which I have done, instead of confining my- 

 self to the construction of a few imperfect tables, and which, though they 

 may, if carefully constructed, be practically useful, could give very little aid 

 towards the advancement of chemical theory. 



If I have succeeded in the preceding pages in sketching so much of the 

 general outlines of the subject of investigation as to convey an adequate 

 idea of its character and scope, the classification which I have made of the 

 several groups into which the experiments to be made may be divided for 

 greater convenience of study, and the general character of the experimental 

 processes which I have employed, or propose to employ, will be at once 

 understood. A very brief account then of these two matters will complete 

 what I proposed to do in this preliminary report : and first of the classifi- 

 cation. 



In an investigation involving so great a variety of detail, and in which 

 so many physical phenomena must necessarily be employed as tests of 

 molecular changes, the experiments require to be so classified that the 

 results obtained in connexion with each class of phenomena shall be com- 

 parable, and that the results of one series of experiments shall throw light 

 upon, and assist in carrying out the next, and lastly, that all shall converge 

 to the main problem. The following scheme of experiments, if made upon 

 an extensive scale sufficient to enable us to eliminate errors and anomalies, 



