1868. | Mineralogy. 417 
hydrogen in 12 molecules of water (H,O) have been replaced by 
4 atoms of tetratomic silicon, 1 atom of hexatomic aluminium, and 
2 atoms of monatomic potassium. But although this mode of 
representing the constitution of chemical compounds has been of 
eminent service in organic chemistry, it would seem, even from this 
single illustration, that it is by no means of equal value in mineral 
chemistry. 
From this consideration of typical formule, we turn to the 
second question discussed in Von Kobell’s paper. At the present 
time the mineralogist endeavours to express the manner in which 
the constituent elements may be grouped together in any given 
mineral, by what is termed a rational formula. Many chemists, 
however, would have us confess our ignorance of this mode of 
grouping, and would simply write the elements side by side, with- 
out regard to the manner in which they may be associated: such 
formule are said to be empirical. Thus, the antimonial sulphide of 
lead called plagionite, has a rational formula, as follows :—4 Pb 
S-+3 Sb §,; an expression which plainly shows that the mineral is 
composed of 4 molecules of galena and 3 of antimonite. The 
would-be innovator objects, however, to this theory, and gives us 
therefore an empirical formula written in this fashion: Pb, Sb, 8::. 
From such examples as these, Von Kobell concludes that there 
is no necessity, at present, for supplanting our old-fashioned expres- 
sions either by typical or by empirical formulee—a conclusion by no 
means distasteful to so conservative a creature as the mineralogist. 
“ Agates, I think, of all stones, confess most of their past 
history.” Such are the words of Mr. Ruskin in his pleasing little 
work, ‘The Ethics of the Dust ;’ and under this belief he has, of 
late, set himself the task of studying their history, and interpreting 
it to the readers of the ‘Geological Magazine.* In one paper he 
describes a group of agate-like structures which he calls “ Dipartite 
Jaspers,” and in a second communication notices the class of “ Folded 
Agates,” while he hints that other papers are forthcoming on 
“Mural Agates” and “Involute Agates.” Certainly the most 
attractive features of these articles are the admirable tinted engray- 
ings by which they are illustrated. 
It has always been a moot-point whether the peculiar markings 
observable in the so-called moss-agates and Mocha stones are truly 
of organic origin. As bearing upon this point, we call attention to 
a subject recently brought before the Chemical Society by Mr. W. 
C. Roberts.t Certain specimens of colloid silica prepared in 
Graham’s dialyser, and evaporated in air, exhibited singular den- 
* © Geological Magazine,’ April, 1868, p. 156; May, p. 208. 
+ ‘Chemical News,’ April 10, 1868, p. 175; April 24, p. 195. 
