TRANSACTIOXS OF SECTION B. 629 



and vapours to polymeric vapours, and thence to liquids and solids, and remembering 

 that none of tbese forms are stable beyond certain ranges of temperature and 

 pressure, we proceed to determine the specific gravity of all such bodies in terms of 

 the same gaseous unit ; the number thus obtained being for each body its equivalent 

 weight. We thus find, as has long been suspected, that the equivalent (or so-called 

 molecular) weights of liquid and solid species are exceedingly elevated. That of 

 water, a litre of which at 100° (its temperature of formation under a pressure of 

 760 muj.) weighs !)58'78 grams, corresponds to 1192 volumes of water vapour at 

 standard temperature and pressure (H,jO = 17'flG) condensed into a single volume; 

 or to 1102 X 17'96 = 21,408, approximately 21,400. Representing by p the empiri- 

 cal equivalent weight, which is really the specific gravity on the hydrogen basis 

 (IIo = 2'0), and by d the specific gravity taking water = 21,400 as unity, we obtain 

 by the formula j)-^d = v, the reciprocal of the coefficient of the condensation which 

 talies place in the passage of a normal gaseous species, by intrinsic contraction or 

 polymerisation, into the liquid or solid species, the specific gravity of which we 

 have determined by comparison with water. 



§ 4. The reciprocal number thus got is, as we shall show, one of great signifi- 

 cance. In determining the specific weight of any given liquid or solid species, the 

 fact of prime importance is not simply its specific gravity as compared with water, 

 but the relation of the value thus determined to the equivalent weight, or, in other 

 words, to its specific gravity on the hydrogen basis. It is not d, nor yet^, but the 

 relation p : d,&a expressed by v. In the case of volatile species the true value of jo 

 may be known, but for the comparison of fixed solids, as oxyds, carbonates, and 

 silicates, we deduce from the received formulas an arbitrary value iov p by dividing 

 the value calculated therefi'om by twice the number of oxygen portions. Thus 

 for MgO, p = 40^2 ; for SiO,, p = 60^4 ; for AlX)„ _p = 102 -^ 6 ; for SiMg^O^, 

 ^=140-^8; for CCa03, J) = 100 -=- 0. For metalline minerals, including metals, 

 and their compounds with S, Se, Te, As, Sb, Bi, the value assumed iovp is that got 

 by dividing the empirical equivalent weight by the sum of the valencies. 



"While the specitic gravity of liquid and solid species is represented by d, the 

 hardness, infusibdity and insolubility or resistance to chemical change are, for 

 related species, directly as the condensation, or inversely as the value of v. This 

 may be seen in comparing colourless ordinary phosphorus, v = 172, with the 

 metalloidal form, ?; = 13'2; the isomeric silicates, meionite, « = 6*5, and zoisite, 

 V = 53 ; or calcite, v = 6-2, with dolomite, chalybite and diallogite, v = 5'2, and with 

 magnesite and smithsonite, v = 4'7 ; for aragonite, v = 5'55. These examples will 

 serve to show the relations between sensible characters and chemical constitution, 

 the interdependence of which must be taken into account in a natural system of 

 mineralogical classification. The diiierences in hardness and in solubility of the 

 different species just named are familiar to chemists. The behaviour of native 

 silicates with fluorhydric acid, lately studied by J. B. Mackintosh, illustrates in a 

 striking manner the relations between condensation and solubility. 



§ 5. The successive forms imposed upon matter give us the order in which such 

 a system of mineralogy should be built up. First, the form which we may call the 

 chemical form of the species, either elemental or compound, due to the unknown 

 stochiogenic process, or to subsequent chemical metagenesis. Second, what may be 

 called the mineralogical form, which involves the greater or less intrinsic contrac- 

 tion (polymeric condensation) of the normal chemical species — often gaseous or 

 volatile but frequently unknown to us — and the assumption by it of a liquid or 

 solid state, having greater or less specific gravity, hardness, fixity and insolubility, 

 and being metallic or non-metallic, colloidal or crystalline. Third, the crystalline 

 form, being the geometric shape assumed by the crystalline individual, which con- 

 notes a certain structure, apparent in the cleavage, the varying hardness, and the 

 thermic, optical and electrical relations, of the crystal, but is, notwithstanding its 

 value iu determinative mineralogy, the least essential or most accidental form of 

 the mineral species. The significance involved in the note of metallicity is very 

 apparent when we consider the metallic and non-metallic conditions of selenium 

 and of phosphorus, the similar dual conditions of the sulphids of mercury and 

 antimony, the non-metallic and sparry characters of the native sulphids of zinc, 



