From the bone-glue industry, the grease is similarly a valuable bye-product, but there is also another of equal importance, viz. the phosphate of lime, which comprises about half the raw material. As previously described in Part IV., Section II., this is usually extracted after the grease, by solution in weak hydrochloric acid. The solution is neutralized in lead-lined vats with milk of lime, a precipitate of di- and tri-calcium phosphates being obtained. Calcium chloride is left in solution, and the precipitate should be, therefore, well washed if it be desired to have dry phosphate. The bone-glue industry is, generally speaking, much more intimately connected with the fertilizer trades than the skin-glue trades, indeed the extraction of the bones for glue is not always advisable, in which case the protein matter as well as the phosphatic matter of the bones are employed for making "bone manures." For details of this industry the reader is referred to a companion volume in this series on "Chemical Fertilizers." REFERENCES. "Chemical Fertilizers and Parasiticides," S.H. Collins, M.Sc. "Bones," Part II., Section V., p. 72. "Precipitated Bone Phosphate," Part III., Section III., p. 157. "Bone Manures," Part III., Section V., p. 173. "Gelatine, Glue, and Allied Products," T. Lambert. SECTION III.--FOOD PROTEINS Although there are those who consider that animal proteins are both undesirable and unnecessary as foods, it is nevertheless true that man is almost universally a carnivorous animal. The animal world provides mankind with one of its chief sources of food, and especially of protein foods. Protein foods are unquestionably essential, and animal protein foods differ chiefly from those of vegetable origin in the fact that they contain generally much more protein. Of the proteins noted in our Introduction, the keratins have no value as foods; the gelatins have some value as culinary material, but little actual food value; whilst the albumins comprise practically all the useful animal food proteins. Whilst the actual flesh of animals is the principal source of food proteins--both as to quantity and food value--other parts of animals, _e.g._ kidneys, liver, blood, brains, tongue, are used and relished. The most important sources of animal food proteins are from fish, fowl, sheep, cattle, and pigs, the meat from these being roughly in the same sequence as to digestibility. There are, however, many other animals of which the flesh is quite edible, but most of the above are specially farmed and propagated primarily for their food value.