2 FORMATION OF SHELLS OF ANIMALS, ETC., 
vitality, belongs to physics. The other, though connected 
with structures exhibiting the same class of appearances, 
and being of a similar composition, but occurring in living 
parts, and therefore variously interfered with in their 
development by vitality, belongs to physiology. 
I shall discuss the physical part of this paper as _ briefly 
as the extent and importance of the subject will permit, 
and introduce into it nothing which is not either directly 
or indirectly connected with physiology; but it may not 
be amiss to observe, that the connexion of the two parts 
is so intimate, that the latter will not be intelligible 
without an adequate acquaintance with the former. 
The chemical substances to be employed in the pro- 
duction of the artificial calculi are, a soluble compound of 
lime, and carbonate (sub-carbonate of the old pharma- 
copeias) of potash or soda, dissolved in separate portions 
of water; and some viscid vegetable or animal substance, 
such as gum or albumen, to be mixed with each of these 
solutions. And the mechanical conditions required to act 
in conjunction with the chemical means, are the presence 
of such a quantity of the viscid material in each solution 
as will be sufficient to make the two solutions, when mixed 
together, of about the same density as that of the nascent 
carbonate of lime, and a state of perfect rest of the fluid 
in which the decomposition is gomg on; so that the 
newly-formed compound may be interfered with as little 
as possible in its subsidence to the sides and bottom of 
the vessel. This will require two or three weeks, or 
longer, according to the size and completeness of the 
calculi. But I have not found that they increase at all 
after six weeks. 
Now, from this process there results the simple though 
very umportant fact, that when the carbonate of lime thus 
