Law of Volumes in Chemistry. 263 



water at the same temperature, 1,628 volumes of it are con- 

 densed into a single volume, having a specific gravity of 

 0.9588, which at 4° 0. becomes 1.0000. Water is thus 1,628 

 = (H 2 0) ;. and the weight of its volume at the temperature 

 of formation, as compared with an equal volume of hydro- 

 gen gas or of steam, in other words, its equivalent weight, 

 is 1,628X18=29,304, which thus corresponds to a specific 

 gravity of 1.000; ice, at its temperature of formation, with 

 a specific gravity of 0.9167, being 1,487=(H 2 0) with an 

 equivalent weight of 26,766. The hydrocarbon, C 4 H 10 =58, 

 condenses to a liquid having, according to Pelouze and 

 Cahours, a specific gravity of 0.600, which corresponds to 

 an equivalent weight, as compared with that of water, of 

 17,582, or approximately 303 (O 4 H 10 ), with a calculated spe- 

 cific gravity of 0.5997. The reciprocal of the co-efficient of 

 condensation (or so-called molecular volume) of steam is 18, 

 while that of the gaseous hydrocarbon is 600 : 1000 :: 58 : x= 

 96.66. 



The chemical unit for bodies, which, like these, volatilize 

 integrally, is fixed by the density of their vapors ; while 

 for fixed species, like anhydrous oxides and silicates, or for 

 those which by heat undergo heterogeneous dissociation, as 

 for example calcite and hydrous silicates, the unit may be 

 the simplest formula deduced from analysis, or, for greater 

 convenience in calculation in the case of oxides and silicates, 

 may have a value corresponding to H=l, or 0=8. The 

 unit for silica thus becomes Si 2 -r4 = 15 ; that for alumina, 

 Al 2 3 -7-6 = 17; and that for the magnesian silicate, Si Mg 2 

 4 -r 8= 17.5. Such unit- weights as these have been 

 employed by the writer in his late essay on " A Natural 

 Sytem in Mineralogy," in the tables of which they are 

 represented by P ; while the values got by dividing these 

 numbers by the specific gravity of the species have been 

 designated unit-volumes, and represented by V. The 

 writer of that essay, in deference to the general usage of 

 chemists, therein adopted the received terminology of 

 " molecular weights " and "molecular volumes," and, fail- 

 ing at the time to grasp the full significance of his own 

 earlier teachings as to the universality of the law of vol- 



