Some of those who advocate the chemical theory suppose that an acid fluid is secreted by the sponge, and that the carbonate of lime forming the shell or stone is thereby dissolved; but such advocates have, as yet, failed to detect the presence of any acid substance in the body of the animal. Others ascribe the action to the solvent power of carbonic acid gas. This gas certainly has the power of dissolving carbonate of lime, as may be proved by a very simple experiment: Pour a little lime water into a glass, and blow into it through a glass tube. The lime water speedily becomes milky in appearance, the lime having been converted into particles of chalk or carbonate of lime by union with the carbonic acid gas from the lungs. Continue to blow into the liquid for some time, and the carbonate of lime will slowly disappear, being gradually dissolved by the excess of the gas--the gas over and above that required for the formation of the carbonate. Thus, it has been said, the carbonic acid gas evolved as a product of the respiration of the sponge is the agent by which the channels are excavated. Whatever be the acid to which this power is ascribed, whether it be the carbonic acid or a special acid fluid secreted for the purpose, there is still this difficulty in the way of accepting the theory, namely, that an acid, though it has the power of dissolving the mineral matter of a shell--the carbonate of lime--has no action on the laminæ of animal substance that form part of the structure. If we put the shell of a mollusc in hydrochloric or _dilute_ nitric acid, we obtain, after the complete solution of the carbonate of lime, a substantial residue of animal matter which the acid does not touch, but in the case of _Cliona_ both animal and mineral substances yield to its power. [Illustration: FIG. 76.--SPICULES OF _Cliona_] Those who favour the mechanical theory assert that the material is worn away by siliceous particles developed by the sponge, and kept in constant motion as long as the animal lives; and the theory is supported by the statement that, in addition to the spicules of silica, which are pin-shaped, and occupy the interior of the animal, there are little siliceous granules scattered on the surface of the sponge which are kept in constant motion resembling that of cilia; and the minute particles of carbonate of lime that form a dusty deposit within the galleries are supposed to be the product of the rasping or drilling action of these granules. The pin-shaped spicules of _Cliona_ may be obtained for microscopic examination by breaking any old oyster shell that has formed its home, and brushing out the dust from the galleries; or, a part of the shell may be dissolved in acid, and the sediment examined for spicules on a slip of glass. CHAPTER IX _THE CŒLENTERATES--JELLY-FISHES, ANEMONES, AND THEIR ALLIES_