The composition of bone-tissue varies considerably, and depends on the age and kind of animal to which it belongs, as well as to the part of the animal frame from which it is taken. Bones are made up of an organic and an inorganic part. By steeping a piece of bone in a dilute acid solution, the inorganic portion of the bone is dissolved out, and the organic portion, which forms the framework of the bone, is alone left. On the other hand, by submitting a bone to the action of great heat, the organic portion of the bone is driven off, and all that remains is a quantity of ash. The proportion of the organic to the inorganic matter varies considerably in different bones. The bones of young animals contain more organic matter than those of old animals. In compact bones, also, the organic matter is greater than in spongy bones. The thigh-bone, of all the bones, contains most inorganic matter. In short, bones which have to bear the greatest strain are richest in inorganic matter. Of the bones of animals, fish-bones exhibit the greatest variety of composition, some being almost entirely made up of organic matter, while others are similar in their composition to the bones of quadrupeds. _The Organic Matter of Bones._ The organic portion of bones is almost entirely made up of a substance to which the name _ossein_ has been given, and which, when boiled for a long time, is converted into gelatine. This ossein, which forms on an average from 25 to 30 per cent of the weight of bones, is extremely rich in nitrogen, containing over 18 per cent. _Inorganic Portion of Bones._ The inorganic portion, which forms about 70 per cent, is made up chiefly of phosphate of lime. The dry leg-bones of oxen and sheep, according to Heintz, have the following percentage composition:-- Per cent. Phosphate of lime 58 to 63 Carbonate of lime 6 to 7 Phosphate of magnesia 1 to 2 Fluoride of calcium 2 Organic matter 25 to 30 According to Payen and Boussingault, raw bones contain 6-1/4 per cent of nitrogen and 8 per cent of water. Pure bones are thus seen to contain about 29 per cent of phosphoric acid and 6-1/4 per cent of nitrogen. The composition of the commercial article, however, differs very widely. This is due to the fact that bones collected from India and America, where they have been long exposed to atmospheric influences, have lost much of their organic matter. The amount of sand and earthy impurities also varies very considerably. _Treatment of Bones._