350 Mr. Children's Summary View of ''-V, pftAVi^ 



that is, in the proportions of oxygen contained in the fel6ments 

 of the isomorphous crystals ; and that wherever this atomic 

 similarity exists, identity of crystalline form will always be the 

 result. Thus he says the oxygen in the phosphorous and arse- 

 nious acids is to that iu the phosphoric and arsenic acids as 3 : 5. 

 In the biphosphate and binarseniate of potash, the oxygen of 

 the base is to that of the acids as 1 : 5, and to that of the water 

 of crystallization as 1 : 2. 



Hence the only difference between these salts consists in the 

 radicle of the acid of one of them being phosphorus, and that of 

 the other arsenic ; and all the salts, which diifer only in this 

 manner, are said to present identical crystalHne forms. 



Berzelius has made considerable use of Mitscherlich's hypo- 

 thesis to bring together as one species all the varieties of garnet, 

 as well as those of amphibole, mica, and several other minerals ; 

 and in vol. ix. New Series, p. 70, of the Annals of Philosophy, 

 our readers will find an abstract from Wachtmeister's paper, la 

 the Swedish Transactions, containing a description and analysis 

 of 13 varieties of garnet, all of which, with only one exception, 

 proved to be constituted of an atom of a silicate of a base con- 

 taining 3 atoms of oxygen, as alumina and peroxide of iron, 

 combined with an atom of a silicate of a base containing 2 atoms 

 of oxygen, as lime, magnesia, protoxide of iron, aua protox- 

 ide of manganese. 



M. Beudant has the following remarks on the same subject. 

 After observing that it is scarcely possible to obtain artificial 

 salts in a state of purity by crystallization from a liquid holding 

 several salts in solution, unless they differ very materially in 

 point of solubility, in which case they crystallize in succession, 

 one after the other, he says, " if on the contrary they are nearly 

 equally soluble, they all mix together in greater or less propor- 

 tion, and not one of them will be pure. These mixtures often 

 happen indifferently with every species of salts, so that they 

 appear to be the mere effect of chance, and in that case the 

 extraneous portion is always in very small quantity. But mix- 

 tures occur under certain circumstances which it is very import- 

 ant to understand, and may then take place in all sorts of pro- 

 portions, wherefore sometimes no particular ingredient sensibly 

 predominates. In general it is observed that salts of the same 

 order of composition unite most readily, especially when they 

 have nearly similar crystalline forms. Thus all the species of 

 alum have such a tendency to mix together that it is extremely 

 difficult to counteract it, and they cannot be completely sepa- 

 rated when once united, even by repeated crystallizations. Mix- 

 tures of the same kind occur between nitrate of baryta and 

 nitrate of lead ; between the nitrates of potassa and soda ; and 

 the sulphates of iron, cobalt, nickel, &c. ; also between the sul- 

 phates of zinc, soda, and magnesia, &c. &c. These mixtures 



