The explanation of the method is as follows. The lime which is added neutralizes the carbonic acid combined with the calcium bicarbonate, and the result is the same as in the former case where this carbonic acid was decomposed by heating, for calcium carbonate is thrown out of solution. For domestic purposes, water is softened by the addition of washing soda. Since this reacts with all the calcium and magnesium compounds forming the insoluble carbonates, all hardness, both temporary and permanent, is removed. CHAPTER VI PHOSPHORIC, BORIC, AND SILICIC ACIDS The acids which are grouped in this chapter are not in themselves of much interest, though some of their salts are extremely important compounds. Bone. Much of the refuse bone, sooner or later, reaches the marine store, and from that point starts on a career of usefulness in the industrial world. “Green bone,” as it is then called, may have fat adhering to it or confined in its hollow interior as marrow. This is recovered by treatment with benzine, and after that the bone is subjected to the action of superheated steam in order to convert cartilage into glue. In some cases, the residue is then ground up to make bone meal, which is valuable as a manure because of the calcium phosphate which it contains. In this way, the phosphate returns again to the animal kingdom, for it supplies plants with the phosphates that they require, and from the vegetable kingdom it passes to animals and helps to build up bone again. Calcium Phosphate and Bone Black. Instead of being ground up, bone may be heated in a retort in much the same way as coal is treated for the manufacture of coal gas; bone oil is distilled off, and a non-volatile residue, called bone black or animal charcoal, remains. This contains about 90 per cent. of calcium phosphate and 10 per cent. of finely divided carbon disseminated throughout the mass. It has the peculiar property of absorbing colouring matter, and is used for this purpose in the sugar industry and in the preparation of fine chemicals. Phosphoric Acid. After being some time in use, bone black loses the property of absorbing colouring matter; and though it can be “revived” several times by heating it strongly in a closed retort, it ultimately becomes spent and of no further use to the sugar refiner. It is then heated again, this time in an open vessel, until all the carbon is burnt away. The residue is now a greyish solid consisting mainly of calcium phosphate. This, supplemented with native phosphate, which is probably fossilized bone, is used for the preparation of phosphoric acid.