X 



tain water, mostly in different proportions, appeared peculiarly well fitted 

 for this purpose. He found that the following were similar in form : — 

 (1) sulphate of copper and sulphate of protoxide of manganese ; (2) sul- 

 phate of protoxide of iron and sulphate of oxide of cobalt ; (3) sulphate 

 of magnesia, sulphate of oxide of zinc, and sulphate of oxide of nickel. He 

 also found that the Siilts which had dissimilar forms contained a different 

 number of equivalents of water, and that those which had similar forms 

 contained the same number. He then mixed the solutions of the different 

 sulphates, and found that the resulting crystals had the form and the same 

 number of equivalents of water as some one of the unmixed sulphates. 

 Lastly, he examined the combinations of these sulphates with sulphate of 

 potash, and showed that the double salts had all similar forms belonging to 

 the oblique system, and that they were composed of one equivalent of the 

 earthy or metallic sulphate, one equivalent of sulphate of potash, and six 

 equivalents of water. The memoir in which these observations are recorded 

 was presented to the Berlin Academy on the 9th of December 1819. In the 

 course of the preceding August Berzelius came to Berlin, on his way from 

 Paris to Stockholm. He became acquainted with Mitscherlich, and con- 

 ceived such an opinion of his talents, that he suggested him to the 

 Minister Altenstein as the most fitting successor to Klaproth in the chair 

 of Chemistry in the University of Berlin. Altenstein did not at the 

 moment act upon this suggestion, but consented to the proposal that 

 Mitscherlich should perfect his chemical education by working for some 

 time under the guidance of Berzelius. In Stockholm he continued and 

 extended his researches on the phosphates and arseniates, and wrote a 

 memoir on the subject, which appeared in the Transactions of the 

 Swedish Academy. In it he described with great care the forms of the 

 acid and neutral phosphates and arseniates of potash, soda, and ammonia, 

 the neutral double salts of potash and soda, and of ammonia and soda, and 

 the phosphates and arseniates of oxide of lead. He showed in every case 

 that the phosphates and arseniates have similar forms and analogous com- 

 positions. Urged by Berzelius to give a name to this newly detected 

 property of the chemical elements, he designated it by the term iso- 

 morphism. This discovery was of the highest importance to the theory of 

 chemical equivalents, inasmuch as it explained the exceptions to the law 

 of definite proportions in the mineral system of Berzelius. It appeared 

 moreover, from the crystallization of the mixtures of the different sulphates, 

 that isomorphous substances combine in all proportions ; and that they re- 

 place one another in indefinite proportions in the composition of minerals 

 was proved by Mitscherlich' s fellow- students, Heinrich Rose and Bons- 

 dorff, in the cases of augite and amphibole. 



The doctrine of isomorphism, moreover, was an admirable test of the 

 determination of the equivalents of the different elements, whilst the 

 smallness of the number of changes in the equivalents of the simple sub- 

 stances that followed the discovery of isomorphism, is an indication of the 



