92 FORMATION OF SHELLS OF ANIMALS, ETC., 
amine the chemical conditions under which the natural 
products are formed, and to compare them with those of 
the artificial process. It will be remembered that, for the 
production of the artificial calculi, two separate solutions 
are necessary, one containing subcarbonate of potash or 
soda with a glutinous material, the other a salt of lime, 
and that these solutions are to be gradually mixed with 
each other. Now, as it will be easy to show that these 
conditions exist as perfectly in the shells of crustaceans as 
they were described to exist in the artificial process, ex- 
actly the same results may naturally be expected in both 
cases. In the first place, I may observe, that an alkaline 
solution containing subcarbonate of soda and albumen, 
can easily be demonstrated on that surface of these animals 
which is covered with a calcareous shell. In the fluid 
taken from this part in the crab or lobster albumen is 
thrown down by boiling, and subcarbonate of soda can be 
detected by the proper tests, and where the quantity of 
this fluid is insufficient to admit of the application of the 
ordinary reactives, the surface in contact with the shell 
can always be shown to be alkaline by simply holding a 
piece of reddened litmus paper for a short time in contact 
with it. I may observe, also, that this fact applies equally 
to molluses and other animals which I have examined, 
whose surface is covered with a shell of carbonate of lime. 
The production of an alkaline carbonate in these instances 
may then be looked upon as the effect of a natural pro- 
cess, and results, without doubt, from the decomposition 
of some of the salts of potash or soda contained in the 
medium in which these animals live, or with which their 
surface is frequently brought into contact. Under what 
agency this decomposition is immediately effected is not 
apparent, but it will probably be classed with vital pheno- 
