BY MOLECULAR COALESCENCE. 97 
and similarity of physical and chemical conditions. With 
respect to the alkaline carbonate, it can be demonstrated 
on the surface of the parts connected with these septa ; 
hence this chemical condition is the same as in the other 
parts of the shell, so that the difficulty exists only with 
regard to the source from which the salt of lime proceeds, 
and the route by which it arrives at the surface of the shell, 
moistened by the subcarbonate of soda. But this question 
admits at once of simplification and the removal of the 
first part of the difficulty, there being but one source by 
which the salt of lime can be furnished, which is the water 
taken into the system of the animal through the imstru- 
mentality of its digestive apparatus. Now, as to theroute, 
this can only be the vascular system, which, being that 
through whose medium the elementary constituents of all 
the other structures are conveyed to their various points 
of destination, may be considered also to convey the soluble 
salts of hme to the parts where they are required. Now, it 
ean be shown, by the application of chemical tests to these 
septa, that an alkaline carbonate is formed on their surface, 
and, by the microscope, that minute as well as larger 
globules of carbonate of lime exist in the same situation ; 
and, therefore, it may be fairly concluded that this carbonate 
of lime is formed in the place where it is found, and not in 
the interior of the adjacent blood-vessels, and, more 
especially, as bemg insoluble it would not pass through 
their coats in particles of such a size. Hence, taking all 
these circumstances into consideration, it seems in the 
highest degree probable that all the conditions, both 
mechanical and chemical, necessary for the formation of 
carbonate of lime in the case of the deeply-seated shell- 
structure, are essentially the same as in the superficial ones. 
The fluid contaiing these substances in different states of 
a 
