202 Scheerer on Isomorphism. 
crystals increased in size the magnesia that was wanting was of 
necessity replaced by water. This explains how it is that the 
near ne 
as weshave seen above, cannot be called into existence where wa- 
ter is present, while senpentine for its formation requires the pres- 
ence of water, we are naturally led to the*inference that all for- 
mations ‘in which olivine eccurs could at» their origin have con- 
tained no water, whereas in all those where serpentine is met with, 
water necessarily must have. been present. And.in point of fact 
the rocks in which oliviné oceurs are not less by their petrograph- 
ic character than. by their ‘position im the series of geognostic for- 
mations, distinctly and thoroughly separated from those in which 
serpentine is met with. . The former belong to the basaltic group, 
the latter to the crystalline primary rocks. - 
nesia with reference to Polymeric Isomorphism. (From Pog- 
gendorff’s Annalen; vol. lxviii, p. 376.) 
The Chemical Constitution. of the Hydrous Carbonates of Mag- 
} P 
The different compounds of magnesia with carbonic acid and 
water have-hitherto, as.we know, been pl in the following 
six categories: (1.) Triply hydrous two-thirds carbonate of mag- 
nesia =Mg: Cc + 3H. (2.), Folurfold-hydrous ihtee-fourths ¢ar- 
bonate of magnesia =Mg‘ €* +43. (3.) Fivefold-hydrous four- 
ee ne ot oF as * a A) 
fifths carbonate of magnesia =Mg* C +5H. (4.) ‘Triply hy- 
drous, simple carbonate of magnesia =Mg C+3H,” (5.) Five- 
* ? 4.8 2 Be oh . 
fold hydrous, simple carbonate of magnesia =Mg C+5H, and 
_ (6.) Magnesia alba, thet was. held to'be a mixture of several 
of the above named Compounds, mote eSpecially Mg* C3448 
; . a cy i] . oy ’ 
and MgC+3H., : pill rd 
Since ascertaining however that, under certain circumstances, 
water plays a basic part, and this in such a manner that 1 
atom Mg is replaced by 33, the question arises whether the 
chemical constitution of the whole of these hydrous carbonates, 
when looked upon in this new point of view, may not beco 
materially modified. 'The result upon enquiry instituted in this 
sense, goes to.show that the above mentioned compounds, instead 
of being subdivided under six heads, (the sixth of them consist- 
ing itself of compounds incapable of being united under a com- 
mon formula) may very readily be comprised in two groups. — 
err te GF 
yee as 
* 
i» 
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