1829 .] Analysis of Dolomite. 267 
The specimen marked 13 was a green crystal taken from a mineralogical cabinet ; 
C. A. 
it appears to be composed of two atoms carb. lime. 70 . 8 = 30. 8 
one atom carb. mag. 29. 7 = 15. 4 
100. 5 = 46. 2 
perhaps it is the same as the rhombspar of Klaproth*. 
Unfortunately, after water had been added to the above specimens they were in- 
cautiously dried over a furnace, whence it is evident that the hydrate of lime bad 
commenced absorbing carbonic acid ; in fact this was afterwards proved by dropping 
a little acid on some of them. Thus there was an excess of weight of one or more 
per cent, beyond what the lime should have taken up to become hydrated ; never- 
theless the result, as far as regards the total incapability of magnesia to combine 
with water, was satisfactory enough, and entirely bore out the observation I had 
made on a former occasion. 
By way of removing, however, any doubt which might arise from the imperfec- 
tion of the last experiments, I repeated the proof with additional precautions. 
Three mixtures were made precisely alike, of 50 grains, rhombohedral spar and 
50 of the magnesian carbonate ; and at the same time 50 grains of Henry’s calcined 
magnesia were heated alone to examine its action on water. The first specimen 
was slaked with an excess of water, and dried in a vessel carefully closed, under a 
furnace. The second was slaked in the same way, and the third by exposure to 
steam ; and these two, and the magnesian specimen were dried in a receiver over 
sulphuric acid. 
The results were in every way coincident and satisfactory, and leave no doubt in 
my mind, that the dry analysis of mixtures of lime and magnesia is capable of much 
greater precision than thehmnid analysis, in which, according to Dauheny, five and 
even ten per cent, of difference will result from the use of different precipitants. 
With care in the process of calcination, and the check operation of slaking, I do 
not imagine that one per cent, of error should find its way into the result. And 
the method is applicable even when there is silex or any of the inalterable earths 
united to the carbonates ; provided that the proportion of these be first ascertained 
by solution in an acid. 
TABLE II. 
No. 
Composition . 
Carbonic acid 
expelled. 
Water 
absorbed. 
Carb . Lime 
deduced. 
1 . 
Carb, Lime 50, ) 
Carb. Magnesia 50, J 
50.54 
9.00 
50. 
2. 
Ditto. 
50.55 
9.03 
50.2 
3- 
Ditto. 
50.54 
8.90 
49. 6f 
4. 
Pure Magnesia 50, 
0.9 
0.2 
none 
Employing the formula m = 2.285 c — 100 -j- 0,311 the composition calculated 
r J ° r rft 
from the extrication of 50. 543 will be exactly -j ” ^q, and the mean of the three 
hydrate differs only one five hundredth from the truth. 
The loss sustained by tbc magnesia is trivial, and certainly due to a partial ab- 
sorption of hygrometric. moisture in the corked bottle, in which it had been kept : 
the access of weight in slaking must be attributed to imperfect desiccation ; for mag- 
nesia appears to be exceedingly slow in absorbing carbonic acid, scarcely taking up 
an appreciable quantity even in a very long interval of time. 
Postscript. 
Dr. Thomson has given the analysis of a native carbonate of magnesia ** forming 
whole rocks in Hindustan." He states, that “ it contains much less carbonic acid 
than it ought, hut that it will be curious to find whether the interior portions of the 
* Of the five species of lime halo'ide described by Mohs, notone agrees with this : 
we wish our correspondent had given its specific gravity and the inclinaton of its 
cleavage planes. We doubt not of the existence of such a species, from the agree- 
ment of the present with three analyses of Klaproth’s. His specimens were from 
the Apennines, Hall in the Tyrol, and Laberg in W^rmeland. — Ed. 
t A little of this specimen was absorbed by the moistened paper. 
