148 ON CRYSTALLOGRAPHY. 



Thus the primitive form of iluate of lime is the octalu dron ; but 

 that salt is often found crystallized in cubes, in rhomboidal dude- 

 cahedrons, and in other forms. All these different forms which 

 a body assumes, the primitive excepted, have been denominated 

 by Hauy secondary forms. Now what is the reason of this lati- 

 tude in crystallizing ? why do bodies assume so often these se- 

 condary forms ? 



7. To this it may be answered : 



1st, That these secondary forms are sometimes owing to varia- 

 tions in the ingredients which compose the integrant particles of 

 any particular body. Alum, for instance, crystallizes in octahe. 

 drons 5 but when a quantity of alumina is added, it crystallizes in 

 cubes ; and when there is an excess of alumina, it does not crys- 

 tallize at all. If the proportion of alumina varies between that 

 which produces octahedrons and what produces cubic 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 crystal put into a solution that would afford cubic crys- 

 tals 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 crystallized, it assumes the form of cubes ; 

 but when crystallized in urine, it assumes the form, not of cubes, 

 but of regular octahedrons. On the other hand, muriate of am. 

 monia, when crystallized in water, assumes the octahedral form, 

 but in urine it crystallizes in cubes +. 



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 ex. 



* Leblanc, Ann. de Chim. xiv. 149. 



t Fourcroy aad Vauquelio, ibid, xiv, 149. 



