ON THE SOLUBILITY OP SALTS. 303 



systematically studied. The scheme of experiments proposed in the next 

 section upon the influence which salts exert upon one another's solubility in 

 virtue of their comparative morphology, may, however, afford an opportunity 

 of doing something in this direction. 



5. The fifth series of experiments will be devoted to the influence which 

 salts exert upon each other's solubility. Strictly speaking, we may consider 

 acids and bases as salts ; but it will be better for the special purposes of this 

 investigation to consider them separately. In studying the influence which 

 substances exert upon one another in solution, we must take into account, not 

 merely their chemical composition, but their crystalline form likewise. Even 

 though we may admit that a hydrated salt, when dissolved in water, parts 

 with its hydrated water, and that therefore the body which is in solution 

 belongs to a different cryslallographic system from the hydrated salt, there 

 can be no doubt that some peculiar molecular condition must exist in the 

 solution just before the separation of the hydrated salt, under the influence 

 of which the molecules of anhydrous salt combine with a definite amount of 

 water. We must also bear in mind, that in a great many instances certain 

 molecular properties intimately connected with crystalline form, such as 

 the power of circularly polarizing light, &c, are inherent in the molecules, 

 and therefore independent of their physical state. I have thought it desirable, 

 therefore, to classify the salts which I propose to experiment upon in this 

 series according to the following crystallographic scheme: — 



A. Isomeromorphous bodies, that is salts which crystallize in forms of the 

 regular system, and possess similar formula?. They may be subdivided into, — 

 a, isatomes, or those which have equal atomic volumes, made up of the same 

 number of integrant molecules; and b, poli/atomes, or those which possess 

 equal atomic volumes, but made up of an unequal number of integrant mole- 

 cules — isomorphism being supposed to be in general due to equal or approxi- 

 mate specific volume. 



B. Icono-ideomorphous substances, a name which Laurent first employed 

 to designate such substances as the laevo- and dextro-tartaric acids, which 

 are the images of one another viewed in a mirror, as Pasteur by his admirable 

 researches has shown. Under this category will be included all hemihedral 

 forms in opposite directions. 



C. Homceomorphous salts. — True isomorphism can only exist between 

 bodies crystallizing in the regular system, the term homceormorphism has 

 accordingly been proposed to designate the isomorphism of the other systems, 

 in which there is not perfect equality of angles or parameters. Laurent 

 believed that two bodies ought to be considered as isomorphous, even 

 though belonging to different systems, if their angles and parameters are 

 nearly equal. He argued that if a rhombohedron of 103° can be considered to 

 be isomorphous with one of 10i°, 105°, or even of 107°, there is no valid reason 

 why one of 89° 30', or of 90° 30' should not be isomorphous with a rhombo- 

 hedron of 90°, that is with the cube which is the limiting form between the 

 acute and obtuse rhombohedrons. Tt is in this sense I propose to use the 

 term. I likewise propose to restrict the term to such bodies as have similar 

 formula?, and, like the isomeromorphous bodies, will divide them into isa- 

 tomes and polyatomes. 



D. Many substances can have exactly the same shape, or nearly the same 

 shape, though they may wholly differ in chemical constitution. Such bodies 

 cannot be considered as isomorphous or homceormorphous in the sense above 

 contemplated. As identity of shape, however different the composition may 

 be, must imply a certain amount of similarity in the conditions of equilibrium 

 of their molecules, it is obviously of importance, in an investigation like the 



