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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 distinguished 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 heterogeneous attraction of aggregation, which is much weaker 

 and more variable than any of the others, and produces substances 

 which can no longer be called chemical combinations. In these, the 

 dissimilar ingredients may, and often do, vary in proportion, accord- 

 ing to the different 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 -,V 8 T ; 

 whereas in those kinds which have an argillaceous matrix, it is only 

 in the proportion of -r-j^. 



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 cannot 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 



