The action of reagents on gelatine is in some cases of high interest, it is not precipitated by solutions of either ordinary or basic alum, but if a solution of common salt be also mixed, the gelatine falls down, combined with alumina, as it decomposes the muriate of ammonia which is then formed. On this principle is founded the manufacture of white leather, by a kind of tanning with alum. The most important compound of gelatine is that with tannic acid, which constitutes ordinary leather; this reaction is so distinct that one part of gelatine in five thousand of water is at once detected by the infusion of galls. Many chemists consider that gelatine is merely a product of the decomposition of albumen and fibrine by boiling water, and not a true constituent of the tissues; but this idea is thought to be incorrect, on the following grounds: First, pure fibrine or albumen gives no gelatine by boiling; second, in the process of tanning, the tannic acid combines with gelatine in a skin which has never been boiled; and third, that we can easily understand why some tissues give more gelatine than others by the different degrees of condensation of their structure. But it is rather considered that gelatine bears the same relation to the tissues of the skin or cellular membrane that protein does to the fibrine of the blood, being really a product of its death and decomposition, _though the only representative of it which we can have_. _Of the Fatty Constituents of the Tissues._—The fatty bodies, although contributing essentially to the support of the animal frame, are mere secretions, and do not form any portion of its organized tissues. The substances properly included under the present head are the constituents of the nervous tissue, such as it is found in the brain, the spinal cord and the nerves. In the composition of the brain, it is easy to distinguish three, perhaps five, distinct substances of a fatty nature; the most characteristic and important is called cerebrote; in composition it resembles albumen, containing a large quantity of nitrogen. _Saline, and Extractive Constituents of the Tissues._—We find in all the animal tissues small quantities of a great variety of salts, the same as those which will be hereafter noticed as existing in the blood, to the presence of which in the substance of the tissues they are probably due. In the tissue of the bones and teeth, however, these saline matters are deposited in much greater quantities, and in disease and old age bony deposits occur in all those tissues, which yield true gelatine on boiling. The composition of the bones will be hereafter noticed.