406 Scientific Intelligence. 
The following facts will show the grounds upon which this idea is 
supported. It is known that magnetic iron, spinel, chromic iron, gahn- 
ite, pleonaste and franklinite have the same crystalline form ; these are 
regarded by chemists as combinations of an oxy with three atoms of 
oxygen (alumina, chromic and ferric oxyds) with an oxyd of one yews 
alent of oxygen, as magnesia, protoxyd of iron and zinc. Thus w 
the equivalents of Berzelius 
eet iron will be a ferrate of iron, ‘ Fe, 0,4-Fe O. 
Chromic iron will be a chromite of iron, é Cr, ore O. 
Spinel will be an aluminate of magnesia, Al, O,+-Mg0. 
Pleonaste will be a ferro-aluminate of magnesia, { a } O,-+Mg0. 
2 
Franklinite te be a ferro-aluminate of zinc my Al Fe, ato ne dy 24 Or 
3 
and magne 
Gahnite will Ya! an aluminate of z 1,0 E00. 
Gahnite ; . . : Al, to Al,;--Zn. 
Spinel, | . ‘ ‘ : Al., Al,, Al, Mg. 
Pleonaste,  . ‘ . \ oa } --Mg. 
In chromic iron the Fe O varies from 18 to 36 per cent., Cr.0, from 
36 to 60 per cent., and Al,O, from 6 to 20 per cent. In the magnetic 
iron we find from Fe 20 sLFe 0 to 4Fe,0,-+3Fe 
How is it viii: demands M. Laurent, in view of such facts to 
suppose that in all these minerals the ratio of. the oxygen in the two 
species of oxyds is as 3: 1? ow on the other hand, can we conceive 
that all these minerals or even the different spinels, should have the 
same form? But admit the divisibility of the molecules, and give to all 
these oxyds the same formula: the seeming contradiction which exists 
between the constancy of form and the change in the constitution of 
these minerals, then vanishes entirely. They-may then all be referred 
to a type, the general formula of which.is O(AIf, Fe?, Mg, Zn, Fe)»: 
M. Laurent applies the same reasoning to the silicates ; the formulas 
which he propeses include all the silicates known, and are at the same 
time exceedingly simple. He admits besides the a types, some 
thers of basic salts, (the preceding formulas --nOM,,) but it hap- 
fei ae thus that two or three formulas become applicable to the same sili- 
have— 
= Bisilicates, Si,0,(R,). 
Basic trisilicates, Si 0 ;(R,)-+-O(R,)=Si,0,(R,)=148i,0 (Ry). 
Bibasic eat si, 0,(R,)-+20(R, )=Si,O, (Ry )=28i2% (Ry). 
Si,0,(Ry)-+40(Ry)-=8i,0, o(Ry2)=B5i204(Ba): 
 sexsilicates, 
ie gia nas 
* See M. Gerhardt’s considerations upon the atomic yolume of these oxyds, in 
this Journal, Nov., 1847, p. 405. 
