62 FORMATION OF SHELLS OF ANIMALS, ETC., 
In this case all the carbonate separates from the vegetable 
basis, and becomes collected into small globular particles 
of different sizes, mixed with crystals of carbonate of lime. 
These are scattered about the vegetable residue, which 
retains the place, figure, and laminated appearance of the 
original calculus, but when examined by polarized lhght 
exhibits no trace of earthy carbonate ;.excepting im the 
centre of each circular portion of residuum, there are two 
imperfectly-formed crystals joined together, and presenting 
somewhat the appearance of nuclei in process of division. 
I have specimens of such calculi, which have been in 
solution of gum for about nine months. It may not be 
irrelevant to state that these experiments and examinations 
have not been hastily made, or made only on scanty 
materials, but have been repeated over and over again 
with due care and attention to everything at all likely to 
affect the results. In considering the cause of disinte- 
gration as displayed in these calcul, it may be observed, 
first, that their structure is less dense, as shown by the 
absence of transparency observable in the other calculi ; 
and, secondly, that insoluble vegetable matter enters 
largely into their composition. <A part of this matter may 
have been suspended in the solution of gum, which had not 
stood sufficiently long to allow of its complete subsidence ; 
but I believe the principal part of it is set free durmg the 
decomposition of the salt of lime contained in the gum, 
which, bemg then placed under circumstances particularly 
well fitted for the production of large calculi (the experi- 
ment being performed on a large scale), combines mole- 
cularly with a small portion only of the carbonate of lime, 
to form the calculi in question. ‘The other portion of the 
carbonate of lime, combining with the gum, forms the 
same description of calculi as those formed of carbonate of 
