Professor Apjohn on a new Variety of Alum. 161 



of manganese ; magnesia, and protoxide of manganese, being Isomorphous sub- 

 stances. This hypothesis, it must be admitted, does not receive any support 

 from the analytic results, as there is no deficiency of manganese ; but the amount 

 of the sulphate of magnesia is very small, and the discrepancy in question may 

 well be due to errors of experiment. 



As the alkali of alum may be replaced by the protoxide of manganese, and 

 since, as Mitscherlich has shown, the alumina may be replaced by sesqui-oxide of 

 manganese, it is obviously theoretically possible that an alum should exist con- 

 taining no metal but manganese. I have not as yet had time to bring this 

 anticipation to the test of experiment. 



I shall conclude with one or two remarks, naturally suggested here, upon the 

 important doctrines of isomorphism, first promulgated by Professor Mitscherlich 

 of Berlin. These doctrines are generally considered as supported and well illus- 

 trated by the constitution and form of the different kinds of alum. This, however, 

 would appear to be only partially true. Alumina, peroxide of iron, and the sesqui- 

 oxides of manganese and chrome, having a similar composition, and being supposed 

 (the two first certainly are so) isomorphous, we can understand how they might 

 replace each other in alum without affecting its crystalline form. This is quite 

 intelligible, and squares with the doctrines of Mitscherlich. But soda may, we 

 know, be substituted for potash and ammonia, with neither of which it is iso- 

 morphous, and the octohedral form still subsist. Moreover, ammonia, potash, 

 and soda alums contain — the two first twenty-four, the last twenty-six atoms of 

 water, and nevertheless the crystal of each is a regular octohedron. These facts 

 do not appear to be in accordance with the laws of isomorphism, as far as these 

 have been hitherto developed, but I am far from thinking that they do not 

 admit of explanation. The latter difficulty, for example, may be removed by 

 supposing that the different varieties of alum have, as Professor Graham sup- 

 poses, in reality the same quantity of combined water, and attributing the diflPerent 

 proportions given by experiment to inevitable errors of manipulation, or to water 

 mechanically interposed between the plates of the crystals. 



In conclusion, I may observe, that upon the principles under consideration, 

 the alum which I have described ought not to crystallize as an octohe- 

 dron, inasmuch as the protoxide of manganese is not isomorphous with the 

 alkalies. 



z 2 



