236 The Rev. Jous Horwz's Observations on Arragonite, 
that, therefore, it ought not to be considered as constituting any 
anomaly in the theory of Haüy; which admits a difference of 
structure, or of mechanical division, whenever there is a change 
in the constituent parts of a simple mineral. 
The difficulty of obtaining good. specimens of Arragonite for 
some time delayed my experiments ; but having at last procured 
some very fine hexahedral crystals of that mineral, I selected 
the purest parts of these, and proceeded in their anal ysis. 
My observations were the following: A moderate degree of 
heat, when applied to a small fragment of Arragonite, is suffi- 
cient to reduce it to a white, opaque powder. The substance at 
the same time loses part of its weight. The volatile matter which 
makes its escape produces no decrepitation, but passes off silently, 
in a manner similar to that of carbonic acid when expelled by 
héat from chalk or limestone. | 
Experiment 1.—In order to ascertain the nature of this volatile 
matter, I made the following experiment : A small glass bulb 
furnished with a narrow tube was filled with 205 grains of Arra- 
gonite coarsely powdered ; but previously both the powder and 
vessels had been exposed to the heat of the sun, for the purpose 
of driving off any moisture which might adhere to their surfaces. 
The bulb was then imbedded in a crucible filled with dry sand, 
and placed in an open furnace containing a charcoal fire. The 
tube of the bulb was bent in such a manner, that its extremity 
could easily be brought under the orifice of an inverted glass 
tube filled with mercury. The fire surrounding the crucible was 
gradually raised, care being taken not to expose the subject to too 
high a temperature, for fear of separating the carbonic acid from 
its base. In thespaceof about thirty minutes an elastic fluid was 
collected over the mercury, causing it to descend about an inch. 
and a half. The mercurial trough, &c. were now removed into a 
cold 
