MOLECULAR COALESCENCE. 71 
investigated experimentally, and the physical principles 
concerned in their formation clearly and rigidly demon- 
strated, it will be necessary, for the full comprehension of 
the physiological division of this paper, that the physical 
part should have been carefully considered, and generally 
understood. As in the physical part of this paper I have 
described minutely the different processes for making 
artificial calculi, and the mode of performing all the 
necessary experiments, in order that those who wish to 
prepare their own specimens may be able to judge for 
themselves of the truth and accuracy of the various state- 
ments contained in these accounts, so in the physiolo- 
gical part I shall also adopt a parallel course of procedure, 
by describing the mode of examining the natural structures, 
and pointing out the parts best adapted for displaying the 
various stages of their development. This has been espe- 
cially neglected in the examination of calcareous tissues, thin 
sections being too exclusively employed for that purpose, 
which, although they display well the number and arrange- 
ment of the lamine of perfectly formed parts, and so far 
are indispensable, yet shows little or nothing of the earliest 
forms of the earthy deposit, especially as this is all re- 
moved by the subsequent grinding, unless care be taken 
to prevent it. Nothing can be more dissimilar in appear- 
ance than the different forms assumed by the globular 
carbonate of lime in the various parts of the same shell, 
and it is only by examining the passage of the one form 
into the other that their identity can be determined. 
This observation strikingly applies to the analagous forms 
of the artificially prepared compound of the same compo- 
sition, in which, as in the natural products, there are the 
molecular, globular, and laminated forms, resembling one 
another in no respect excepting in their composition. 
