and Equivalent Volume of some Mineral Species. 205 
3asO. Silica, SiOs may also be written as 3siO, for the multiple 
equivalent weight assigned to it, is deduced from certain silicious 
species whose formulas are as simply written with the le 
equivalent ; by this means all these oxyds may be reduced to the 
type M2Oz2. 
We have further asserted, that for species crystallizing in the 
same form, the de ensity varies directly as the equivalent weight, 
so that the quantities obtained in dividing the one by the other, 
and known as the atomic or equivalent volume, will be equal. 
Such a relation is already recognized between species of the same 
genus, and we now propose, having fixed an equivalent weight 
for one species, to calculate from their densities, those of the 
ff 
anaes: thus related are homologous, or exhibit other intimate 
rela 
ig this investigation of mineral species, several circumstances 
combine to render the attainment of accurate results exceedingly 
difficult. Native crystals are often neither compact nor homoge- 
neous, and the presence of fissures filled with air, or other for- 
eign species, makes the determination of density yn 
Impurities also render the results of analysis doubtful, and if w 
employ the — generally assigned, it will be found that an 
attempt to deduce the most simple relations from the results of 
analysis, or to ike these conform to some arbitrary assumption, 
has led chemists to adopt numbers which are often considerably 
at variance with experiment, and to disregard one or two hun- 
dredths of some element, which is perhaps essential to the consti- 
tution of the species. This is probably in an especial manner 
true of the small portions of water and fluorine in certain cag ei 
with regard to the _ weight of silica is also a source of 
embarrassment in the examination and comparison of formulas. 
According to Pelouze, SiOs is 45-34 (O=8) while corrected 
determinations of. Berzelius make it 46°22. The latter number, 
adopted by Rose in the tables accompanying the sie ceditich of 
his Analytical Chemistry, will be re for the calculation of 
equivalent weights in the present 
n many artificial crystals those difficulties are pee felt, and the 
equivalent volumes are closely accordant. cite for illustra- 
tion the following results from an daboeiiey memoir on atomic 
volumes, by Playfair and Joule. (Qu. Jour. Chem. Soc., p. 121.) 
mi inations 
