298 report— 1859. 



however, that the formation of the crystallized salt is the result of double 

 decomposition,, we admit the existence of a specific molecular arrangement 

 of salt and water in the solution, which under the same conditions of tempera- 

 ture, &c. always yields the same salt. That such a molecular arrangement 

 does in reality exist, is proved by the fact that the same salt crystalizes with 

 different amounts of hydrated water according to the temperature, a good 

 example of which is afforded by sulphate of manganese. The lower the 

 temperature, the larger will be in general the quantity of hydrated water 

 which attaches itself to the salt molecule. Even salts which usually crystal- 

 lize anhydrous, such as common salt, frequently separate in a hydrated 

 form from solutions at very low temperatures. A solution of common salt 

 cooled to — 10° yields transparent oblique rhombic prisms of the monoclinic 

 system containing 4 equivalents of water. The large flat six-sided tables 

 which Ehrenberg and Frankenheim observed to form under the microscope 

 at the temperature of +15°, were most probably crystals of another hydrate. 

 The formation of the double sulphates of the magnesian series is another 

 striking proof of the pre-existence in a saline solution of a special molecular 

 arrangement. The equivalent of constitutional water in MgO, S0 3 , HO, 

 6HO is substituted in the double salts by an equivalent of an alkaline sul- 

 phate ; this substitution must take place by double decomposition, and so 

 far supports the hypothesis that all crystallizations are due to that process. 

 As the substitution takes place in solution, we must I think admit, either 

 that the salt has this equivalent of constitutional water attached to it in solu- 

 tion, or that some combination does occur there, which by double decompo- 

 sition would give a crystalline compound in which it would exist. The 

 existence of special molecular modifications in the constitution of salts, is 

 assumed by Lowel to be the cause of the difference of solubility which the 

 same salt sometimes presents at the same temperature, and not the amount 

 of water which it takes up in crystallizing. This must necessarily follow as 

 a relation of cause and effect if we adopt his view, that a salt is always pre- 

 sent in solution in its anhydrous state. Such a view, however, compels us 

 to admit that the salt molecule itself undergoes as many molecular modifica- 

 tions as there are distinct hydrates, and this under the influence of very slight 

 causes ; on the other hand, it does not satisfactorily account for the formation 

 of such isomeric salts as those alluded to above, or for the effect of heat upon 

 solubility. Gay-Lussac's view, that the hydrated water of the crystallized 

 salt remains attached to the salt molecule in solution, affords no more satis- 

 factory explanation of the phenomena in question, while that which it gives 

 of supersaturation — the inertia of the valine molecules — is wholly untenable. 

 Lowel' s view, if we could account for the necessary molecular changes, 

 would explain the phenomenon of supersaturation satisfactorily. It assumes 

 that, strictly speaking, there is no such thing, and that solutions considered to 

 be supersaturated merely contain salts having a different molecular consti- 

 tution, and a different solubility from that ordinarily present at that tempe- 

 rature. The hypothesis that solutions are unstable compounds of salt mole- 

 cules and water molecules, and crystalline salts compounds formed by their 

 double decomposition, affords a simple explanation of the molecular changes 

 required for accounting for supersaturation; and it especially enables us to 

 distinguish between the molecular modifications which give rise to isomeric 

 hydrated salts, and those offered by the green and violet modifications of 

 chrome alum — the former may be assumed to affect the arrangement of the 

 molecules of water and salt in the unstable compounds in solution; the latter 

 and more profound, the salt molecules themselves. 



Equal solubility does not necessarily imply that two salts are held by the 



