Page:Proceedings of the Royal Society of London Vol 1.djvu/156

Rh being composed of an aggregate of irregular grains, will appear to consist of small crystals of a very regular form, as may be very fre- quently seen in manganesian carbonate of lime. At other times, again, the molecules, instead of uniting together by the inﬂuence of the crystalline attraction of aggregation, are precipitated in a de- tached but confused manner; and then a simple aggregation takes place, which banishes all appearances of crystallization, and affords the aspect which has been distingm'shed by the name of compact or earthy.

This last, or the amorphous species of crystallization, is here de- scribed as being that kind which, though it produces no determinate form, is nevertheless the result of a regular aggregation. This is thought to depend on the peculiar form of the primitive molecules; such as, for instance, the globular, or others approaching to it. Cal- cedony, girasol, and pure transparent steatite, appear to owe their origin to this mode of formation. And here the author enters into a minute disquisition concerning the cause of the deception which has often taken place, and has led several experienced mineralogists to ascribe to these substances, especially the chalcedony, a certain degree of regular crystallization.

Besides these attractions between similar molecules of substances, there exists another, between such similar molecules and others which are dissimilar, or of a different nature. This introduces what may be called an heterogeneaus attraction qf aggregation, which is much weaker and more variable than any of the others, and produces substances which can no longer be (nailed chemical combinations. In these, the dissimilar ingredients may, and often do, vary in proportion, accord- ing to the diﬁ'erent circumstances under which they are formed; as, for instance, in those kinds of tremolite which have the dolomite for matrix where the carbonate of lime is in the proportion of ﬁrst: whereas in those kinds which have an arg'illaceous matrix, it is only in the proportion of «re-r.

Although these heterogeneous compounds be not attended with any changes in the chemical nature of their ingredients, yet they frequently, as has already been observed, admit of an alteration in their physical construction, and very often induce variations in such of their characters as more immediately depend upon that construc-

'tion; such as, their speciﬁc gravity, their hardness, their transparency, and even (particularly in the class of stones) their colour. Hence it appears that the mineralogist carmot pay too much attention to this mode of attraction; since, by a due application of it, he will be en- abled to understand the accidental causes of the variations to which those substances are liable. Nor should the chemist be less mindful of such an agent; since, by neglecting that precaution, he will be constantly exposed to confound those products which really belong to the chemical composition of the substances which he examines with those which are foreign to it.

At the close of this part of the paper, the author acknowledges that the different kinds of attraction here described may be nothing