CHAP. VII.] CRYSTALLIZATION. 69 



degree of irregularity in tlie crystalline form. And if sub- 

 stances that crystallize in nearly but not quite similar Forms in- 

 forms are mixed together in solution, they will form mixed Ji.Xmr'' 

 crystals, of a form intermediate between those proper to species. 

 the two substances. 



The mention of mixed crystals brings us to a very 

 important analogy between crystals and organisms. Sub- 

 stances that crystallize in crystals of the same form are 

 called isomorphous substances : substances that crystallize 

 in crystals of nearly the same form are called plesiomor- 

 phous.i Isomorphous and plesiomorphous substances, as 

 just stated, crystallize together, forming mixed crystals; 

 but when the same water contains substances in solution 

 that crystallize in unlike forms, they do not crystallize 

 together, but form separate crystals. The fact that mixed 

 crystals can be formed when the crystalline forms are 

 similar, but not when they are unlike, is a parallel to the 

 facts, that trees may be grafted the one on the other, and Analogies 

 that organisms can breed together, when they are of the OTeanisms. 

 same or of nearly allied species, but not when the species 

 are unlike. To state these facts in terms that will apply to 

 both crystals and organisms : if the unformed but forma- 

 tive material of two similar species is mixed together, it 

 will produce an intermediate form ; but this is not the case 

 when the species are unlike. This statement postulates as 

 true the now universally received theory of generation, of 

 which I shall have to speak farther on. 



Also, as crystals of unlike species in a mixed solution 



attract to themselves each its own material, so different 



species of plants in the same soil draw each from the soil 



the nutriment it needs; and the various tissues in an 



organism separate each its own material from the sap or 



blood. 



Some substances have the property of crystallizing in Dimor- 



pliism. 



1 Isomorphism and plesiomorpliism depend on similarit}'' of chemical con- 

 stitution ; and isomorphous or plesiomorphous crystals contaia equal 

 proportions of water of crystallization. Salts are similar in constitution 

 when they contain the same number of equivalents of different bases, but 

 not when they contain different numbers of equivalents of the same base. 



