S56 Mr. Children's Summary View of [May, 



iron a right rhomboidal prism, and that of copper a rectangular 

 octohedron ; and so of the rest." 



It does not appear that M. Haiiy has taken into consideration 

 the difference that water, chemically combined, may produce in 

 crystalline forms ; at least lie says nothing about it in his argu- 

 ments against isomorphism in the passages we have quoted, lie 

 continues thus : — " According to these observations, if the 

 opinion of MM. Berzelius and Mitscherlich with regard to pyrpx- 

 ene, a natural mineral, be correct, it follows that its constitution 

 is an exception to the general results of tl^ crystallization of 

 natural bodies, and appears to be inexplicable. 



" I must add, that on the preceding'idea, it would be very 

 difficult to form a clear idea of what constitutes the species, 

 pyroxene, in a chemical point of view. The different silicates 

 which occur as constituent parts of that mineral have nothing 

 fixed, either in respect to their number in the same individual, 

 nor in their proportions. Supposing all the combinations of 

 which they are capable, taken one and one, two and two, three 

 and three, to exist in nature, we shall have fifteen different modi- 

 fications of pyroxene ; and if we reflect that in the analyses 

 hitherto made of different pyroxenes, the quantity of magnesia 

 varies from 4*5 per cent, to 30, that of iron from 1*08 to 17-38, 

 and that of manganese from 0*09 to 3, what a series 'of shades 

 shall we obtain if we multiply those analyses !" 



According to Haliy's views, all the pyroxenes contain a conO'- 

 mon basis of elementary molecules, which determines their true 

 composition, and by a necessary consequence the invariable form 

 of their integrant molecule, and all the other ingredients, which 

 he considers as purely accidental, are only interposed amongst 

 the molecules of the essential substance without affecting its 

 characteristic form. That substance he assumes to be silicate 

 of lime, for in fourteen analyses the quantity of lime was nearly 

 constant, and in the proportion of about 20 per cent, on the 

 whole mass. " I do not know," says he, " why M. Berzelius 

 has supposed that it may be replaced by magnesia ; how can it 

 yield a place to that substance which it has never abandoned?" 



More lately Mr. Brooke has also questioned the stability Qf 

 this hypothesis,* and has asserted (as we have seen that Haiiy 

 had done before), that the supposed identity of isomorphous 

 bases does not exist, and that the apparently similar forms 

 belonging to substances which differ in composition, do really 

 (lifer from each other in measurememt, although in some cases 

 by only so small a quantity as not to be appreciable by the 

 goniometer. Mr. Brooke remarks, that " the instances which 

 M. Mitscherhch has adduced in support of his theory, or we 



• Edinburgh PhUesophical Jpurual, voj. jui. p. JS. - j^ 



