284 



SALTS. 



gles is employed not alone as a means of inference of 

 perfect from imperfect shapes, but as the simplest means 

 of accurate description. For, as before stated, it is the 

 size of the corresponding angles of a crystal which 

 form its characteristic. 

 Have different 70S. ISOMORPHISM. Many substances 



substances ever whi(jh are aljke in ^ , mmber and arrange . 



Iflv SGL-iflv CTlj& m ^ 



tallineform? ment of their atoms, although these atoms 

 are different in kind, have the same crystalline form. 

 This is the case with common alum, and other alums 

 to be hereafter mentioned. The similar arrangement 

 of atoms will be best seen by inspecting the formulas 

 which represent them. These are given in the appendix. 

 The term expresses their likeness in form. Besides 

 this series there are many other isomorphous groups. 

 Give the pro- 709. It is to be regarded as probable, 

 babie reason. t h at fa Q sna pe and size of the molecules 

 thus similarly composed, is exactly the same, and that 

 it is for this reason that they may be used in building 

 up crystals of the same form. The different alums will 

 even unite when they crystallize, in building up one 

 and the same crystal. Substances which are thus si- 

 milar in composition, and crystallize in the same form, 

 are called isomorphous. There are many cases of simi- 

 lar crystalline form in substances which are not thus 

 related in other respects. Such bodies are not called 

 isomorphous, notwithstanding their identity of crys- 

 talline form. Certain substances crystallize in forms 

 belonging to two or even three different systems, ac- 

 cording to the temperature, or other circumstances 

 under which their crystallization occurs. Such sub- 

 stances are called dimorphous or trimorphous. 



