This process differs from the one previously described in that, in addition to the total quantity of fat and glue-yielding substance contained in the bones, the mineral salts are also obtained in a pure state, and can be further utilized.
The bones are degreased either by extraction with benzine or carbon disulphide or steaming, the operation in the latter case being continued so long as fat is yielded by the bones. The resulting glue-liquor is used in place of water for boiling the cartilage.
The bones are placed in large wooden vats furnished with well-fitting lids, and hydrochloric acid of 12 per cent. poured over them so that they are covered a few inches deep. With the use of acid of 1.04 specific gravity the greater portion of the salts contained in the bones will pass into solution in 48 to 72 hours, when the solution is drawn off as completely as possible from the vats.
The residue in the vats is treated with less concentrated hydrochloric acid and left in contact with it until the bones are soft and flexible and the thinner pieces have become translucent, this being a proof that all the mineral salts have been extracted, and nothing but pure cartilaginous substance remains behind. The solution is then drawn off, and after pouring repeatedly small quantities of pure water over the cartilage to expel the last remnants of acid liquor, it is subjected to thorough washing until the last traces of acid have been removed.
The resulting cartilage is white, translucent, and water-soaked. If left in this state it would of course soon putrefy, and it is best to work it at once, or if this cannot be done it will have to be treated with carbolic acid in the manner previously described, or dried.
Drying the cartilage is time-consuming work, and can properly be done only by artificial heat in kilns. If carefully protected from moisture, thoroughly dried cartilage may be kept without injury for any length of time. However, before being worked to glue, such material has to be again soaked in water previous to the actual boiling operation, and this process requires considerable time. It is therefore best to preserve it in carbolic acid solution, which only needs to be drawn off when the cartilage is to be worked, and may be further utilized.
If boiled in open vessels with water, 6 to 8 hours are required for the complete disintegration of the cartilage. In a closed apparatus under high pressure solution is effected in a much shorter time and the operation progresses very smoothly. With proper attention the glue obtained from bones degreased with benzine and freed from bone-earth by extraction with hydrochloric acid is, as a rule, very clear, and may be bleached with sulphurous acid.
The extraction of the phosphates from bones may be effected in a very suitable manner as follows: A number of vats filled with bones are placed in terraces one above the other, and the acid is allowed to run first into the uppermost vat. After having been for several hours in contact with the bones it is discharged into the next vat, fresh acid being run into the first one, and so on. By this process a highly concentrated solution of phosphates is in a few hours obtained in the lowest vat, solution still adhering to the bones in the other vats being finally expelled by water.
However, the process of extraction under decreased pressure is the most advantageous, it requiring least time. For this purpose the bones are brought into a vessel which can be closed air-tight and the air is expelled. When but a slight air-pressure prevails in the vessel, the cock of a reservoir filled with hydrochloric acid is opened, the external air-pressure now forcing hydrochloric acid into the extracting vessel.
Bones, as viewed under the microscope, consist of a mass permeated with numerous minute tubes or pores. When the air is expelled from the vessel containing the bones, the air in the pores of the latter is rarefied and the hollow spaces are filled with hydrochloric acid whereby solution of the phosphates is effected.