Species of hard Carbonate of Lime, See. 333 
each other ; at other times, to form a re-entering angle, at the 
angles of 128°. 
In another secondary form assumed by the Arragonite, there 
is a diedral summit with isosceles triangular planes, which are 
inclined upon the edges of 6 ^°, belonging to the tetraedral prism, 
and meet together in a ridge of no 0 . I have not perceived any 
trace of this form, in the kind of carbonate of lime treated of 
in this Paper; nor have I perceived the smallest trace of the 
pyramidal forms of the last-mentioned substance, among the 
crystals of the Arragonite. 
Neither the hard carbonate of lime nor the Arragonite ex- 
hibit, by their chemical analysis, any signs of the cause that 
occasions them to differ from the common carbonate of lime. 
The Arragonite has been very carefully analysed by MM. 
Klaproth, Vauquelin, and Thenard ; but their analyses did 
not show that it differed, in the smallest degree, from the com- 
mon carbonate of lime. I desired Mr. Chenevix to be so good 
as to analyse the hard carbonate of lime ; but his result was not 
more satisfactory. The cause of its difference from common 
carbonate of lime remains still undiscovered ; yet there certainly 
exists a considerable difference between the two substances, and 
one which even the chemist is compelled to admit, when he takes 
the hard carbonate of lime into his hands, and which becomes 
still more evident to him, the moment he begins to reduce it 
into powder. It is however very clear, that the said difference 
arises from a cause which has hitherto eluded the investigations 
of chemistry. It may perhaps be supposed, that it is owing to the 
constituent particles of the substance being more closely con- 
nected ; and this indeed might be sufficient to occasion a greater 
degree of specific gravity, and of hardness. But, in the first place, 
it may be asked what can produce this more intimate connection, 
MDCCCIII. X X 
