MlNElULO(;Yi WITJI A CLASSIFICATION OF SILICATES. 87 



to eaili other and to the species belbre him, the teuclenry of tht^ mind is to conceive this as 

 made up of identical or of similar imits or individnals. The justifuation of this mental 

 process appears. in the fact that it is in the comparison of su( h individrrals or chenrical 

 nnits that we find the chief data for the intelligent study of the chemical species. Such 

 a conception of nnits underlies the doctrine of polymerism, and that of homologous or 

 jn'ouressive series, and enables us to compare silicates, oxyds and carbon-spars in a manner, 

 the correctness of which is verified by the close relations revealed between atomic weight 

 and specific gravity. "While we may take for the chemical unit either its simplest expres- 

 sion, or some multiple thereof, we have for convenience in the study of silicates and oxyds 

 preferred the former, which for both of these may be represented as a compound of a 

 mouatomic elemental atom, or its equivalent, with an atom of oxygen, sulphur, fluorine or 

 chlorine. For compounds of metals with sulphur, arsenic, antimony, etc., the elemental 

 atom itself may be assumed as the unit. For carbonates, however, it has not seemed 

 expedient to divide the weight of the tetrad carbon beyond that portion whi<h in the 

 normal carbonate is found with a univalent metal ; while in complex sulphates like alum, 

 in orthophosphates, etc., convenience suggests the less simple individuals which we have 

 employed in these pages. 



§ 122. Having adopted for all mineral silicates and oxyds the simplest conceivable 

 chemical unit, as above indicated, the second steji in our inquiry was to determine for 

 each species, from its atomic formula, the mean weight of the unit, the atom of hydrogen 

 being taken as one and thai of oxygen at eight ; this mean weight has been designated 

 P. The next point to be considered is the relation of this chemical unit to space, a 

 relation which is the nexus between the chemical and the physical, and is determined by 

 dividing the mean unit-weight by the specific gravity of the species (water being unity) 

 represented by D. The quantity tlius obtained we liaA'e designated as the unit-volume 

 or atomic volnme, and have represented by V. The relations, alike of this unit-weight 

 and unit-volume to those of the molecule to which it belongs, an^ unknown. But this 

 molecule has, by our hypothesis, a constant volume, for which an expression is yet 

 wanting, and can, so far as now known, only be attained by assuming as unity the number 

 which corresponds to the highest discovered value of V. The true unit of molecular 

 volume will i^robably still be some multiple of this quantity, and will, at the same time, 

 be the common multiple of all the atomic volumes deduced from various chemical units. 



§ 123. In approaching the consideration of this molecular volume, it may be noted that 

 while in salts of the same type the specific gravity sometimes rises with the molecular 

 weight of the base, as when zinc replaces magnesium, or lead, strontium in carbon-spars, 

 the specific gravity of double or triple salts is essentially the same as that of the corre- 

 sponding salts with a single base, as may be seen by comjparing the densities of simple 

 and double hydrous sulphates, orthophosphates and tartrates ; so that the value of P, 

 deduced from the more complex salts, considered as chemical units, will be essentially the 

 same as that of the apparently simple salts of the type. Taking, then, of the ammonia- 

 cobalt salts (§ 115), not the simple chlorids, but Braun's complex phosphate of luteo-cobalt 

 with a unit-weight of 2540 (of which the specific gravity is undetermined), we have, if 

 we assume for it a density of 1.0*76 (which is that of the chlorid of the same base), a 

 unit-volume of not less than 1492. 



§ 124. The complex tuugstates give still higher volumes. The golden anhydrous 



