ISOMORPHISM. 141 



odour, although the one is a metal and the other a non-metallic 

 body, while the corresponding arseniates and phosphates agree 

 in taste,, in solubility, in the degree of force with which they 

 retain water of crystallization, and in various other properties. 

 The seleniate and sulphate of soda, which are isomorphous, 

 are both efflorescent salts, and correspond in solubility, even 

 so far as to agree in an unwonted deviation from the usually 

 observed increasing rate of solubility at high temperatures, 

 both salts being more soluble in water at 100 than at 212. 

 In fact, isomorphism appears to be always accompanied by 

 many common properties, and to be the feature which indicates 

 the closest relationship between bodies. 



It will afterwards appear that the more nearly bodies agree 

 in composition, they are the more likely to act as solvents of 

 each other, or to be miscible in the liquid form. An attraction 

 for each other of the same character is probably the cause of 

 the easy blending together of the particles of isomorphous 

 bodies, and of the difficulty of separating them after they are 

 once dissolved in a common menstruum ; such isomorphous 

 salts as the hypermanganate and hyperchlorate of potash, may, 

 indeed^ crystallize apart from the same solution, owing to a 

 considerable difference of solubility ; and potash-alum may be 

 purified, in a great measure, by crystallization, from iron-alum, 

 which is more soluble and remains in the mother-liquor ; but 

 most isomorphous salts, such as the sulphates of iron and 

 copper, when once dissolved together, do not crystallize apart, 

 but compose homogeneous crystals, which are mixtures of the 

 two salts in indefinite proportions. This intermixture of 

 isomorphous compounds is of frequent occurrence in minerals, 

 and was quite inexplicable and appeared to militate against the 

 doctrine of combination in definite proportions, till the power 

 of isomorphous bodies to replace each other in compounds was 

 recognized as a law of nature. Thus in garnet, which is a sili- 

 cate of alumina and lime (Al Al Si 3 + 3 Ca Si 3 ) the alumina is 

 found often wholly or in part replaced by an equivalent quan- 

 tity of peroxide of iron ; while the lime at the same time may 

 be exchanged for protoxide of iron or for magnesia, without 

 the proper crystalline character of the mineral being destroyed. 



The extent to which the isomorphous relations of bodies 

 have been traced, will appear on reviewing the groups or na- 



