172 
Book III. 
In the sol 
vents, and 
AFFINITY. 
tity of alumina is added, it crystallizes in cubes ; and 
when there is an excess of alumina, it does not crystal 
lize at all. If the proportion of alumina varies between 
that which produces octahedrons and what produces cu 
bic crystals, the crystals become figures with fourteen 
sides ; six of which are parallel to those of the cube 
and eight to those of the octahedron ; and according as 
the proportions approach nearer to those which form 
cubes or octahedrons, the crystals assume more or less 
of the form of cubes or octahedrons. What is still 
more, if a cubic crystal of alum be put into a solution 
that would afford octahedral crystals, it passes into an 
octahedron : and, on the other hand, an octahedral cry 
stal put into a solution that would afford cubic crystals 
becomes itself a cube *. 
Now, how difficult a matter. 
it is to proportion the different ingredients with absolute 
exactness must appear evident to all. 
2d, The secondary forms are sometimes owing to 
the solvent in which the crystals are formed. Thus if 
common salt be dissolved in water, and then crystalli 
zed, it assumes the form of cubes ; but when crystalli 
zed in urine, it assumes the form not of cubes, but of 
regular octahedrons. On the other hand, muriat of am 
monia, when crystallized in water, assumes the octa 
hedral form, but in urine it crystallizes in cubes t. 
3d, But even when the solvent is the same, and the 
proportion of ingredients, as far as can be ascertained, 
exactly the same, still there are a variety of secondary 
forms which usually make their appearance. These 
secondary forms have been happily explained by the 
X Le Blanc, nn. de Chim. xiv. 149. 
1 Fourcroy and Vauquelin, Ibid.