130 Classification of Natural Silicates. 



put forward by him in papers published in 1853 and 1854, when 

 wollastonite was referred to a polysilicic acid with HSi0 2 , and 

 pyroxene to one with 14Si0 2j or perhaps some simple multiple 

 of these numbers, with an equivalent volume, probably not less than 

 460. In such compounds the degree of complexity of the mole- 

 cule is shown by the relation to space of the chemical equivalent, 

 or, in other words, by its volume. To arrive at a term of compar- 

 ison for this relation in species of various and unknown degrees 

 of complexity, the author deduces for each silicate the mean equi- 

 valent weight of its atomic unit, corresponding to an atom of 

 ]S'aCl ; for which purpose H 2 and CaO are divided by two ; Si0 2 

 by four, and A1 2 3 by six. The mean unit-weight thus deduced 

 from any arbitrary chemical formula, when divided by the specific 

 gravity of the species gives the volume of the unit, which serves 

 to show for different species the relative condensation of the mole- 

 cule. The hardness and the chemical relations of species will be 

 found to vary with the unit-volume, as is shown in the tables 

 given below. 



The various relations just described may be illustrated by an 

 example. The simplest atomic formula representing the chemical 

 elements of meionite and zoisite (which have the same centesimal 

 composition) is (ca.al .si 3 ) o 6 ; the small leli-ers representing atoms 

 and o = 8. This gives an equivalent weight of 107, which, 

 divided by six, shows the mean weight (P) of the unit or oxyd-atom 

 in these species to be 17.83. Dividing this latter number by 2.7, 

 the specific gravity of meionite (water =1.0), we have for the 

 volume of the oxyd-atom in this species, Y=6.60. Dividing by 

 3.4. the specific gravity of zoisite, we find that V = 5.24. The true 

 formulas and equivalent weights of these two complex silicates 

 must be deduced from a comparison of their specific gravities 

 with those of other species whose equivalent weights are other- 

 wise determined. Meanwhile it will be seen that the species 

 zoisite, having the lower value of V, or the more condensed mole- 

 cule, differs from the less dense meionite in its greater hardness 

 and its superior resistance to acids. Mineralogy affords many 

 examples of the principles here illustrated. 



From the complex constitution thus assigned to silicates it 

 follows that the comparatively, simple ratios generally deduced 

 fi jt the silica and the various bases are, in many cases, but approx- 



