424 
SECOND REPORT-1832. 
the oxygen in this compound are to one another in absolute 
weight as 277*312: 3, yet we have no means of determining 
whether the weight thus found denotes one or more atoms of 
silicium. Were we acquainted with any other compound iso- 
morphous * with silica, we should be enabled at once to come 
to a probable solution of the difficulty. In the mean time, two 
opposite views are entertained among chemists, drawn from 
considerations which do not appear to possess anything of a 
decisive character. By Berzelius it is regarded as a compound 
of three atoms of oxygen and one of metal (Si), like sulphuric 
acid ; while by Dr. Thomson it is viewed as a compound of 
the two elements, atom to atom. The former agrees best with 
the constitution of felspar, which may be considered as anhy- 
• • •• «c• ••• 
drous alum (Po + Si) + (A + 3 Si), in which the sulphuric acid 
is replaced by silica; the latter view agrees better with the 
composition of some other silicated minerals, and with the ex¬ 
periments of Dumas on the density of the chloride and fluoride 
of silicon in the gaseous state. 
Application of Isomorphism to mineral compounds .—The 
doctrine of isomorphism is susceptible of many other important 
applications in chemical science. It has proved eminently use¬ 
ful in clearing up the constitution of crystallized mineral sub¬ 
stances, and of many artificial compounds, in which the presence 
of apparently foreign bodies seemed to set at defiance the 
theory of definite proportions. Many varieties of the same 
mineral occur in nature, agreeing in form and other external 
characters, and distinguished from one another only by slight 
shades of difference ; in all of which, while the chemist found 
the same predominating ingredients, he detected in some spe¬ 
cimens the presence of small quantities of bodies not generally 
occurring in the species. As these small quantities bore no 
atomic ratio to the other constituents, they were supposed at 
first to be only accidental mechanical mixtures. But when 
more extended analyses showed that in some instances one of 
the supposed essential constituents of the regularly crystallized 
mineral might almost or altogether disappear, while its place 
was supplied in the true atomic proportion by the same sub¬ 
stance, which, when present only in small quantity, had been 
considered only as an accidental impurity, it became necessary 
to substitute some other idea for that of mechanical mixtures. 
* The analyses of the amphiboles, by Bonsdorf, led him to consider that 
three of alumina were equivalent to two of silica ; but though many other in¬ 
stances have since been found, in which the silica is replaced by alumina, no 
exact ratio between the replacing substances has yet been made out. 
