CASE-HARDENING is the process of converting the surface of certain kinds of mal leable•iron goods into steel, thereby making them harder, less liable to rust, and capable of taking on a better polish. Fire-irons, portions of fine grate-fronts, gun-locks, and other articles of limited size, are very commonly so treated, but the process is sometimes applied to large objects. such as iron railway-bars. The articles are first formed of bar iron. and being heated to redness, are sprinkled with a little powdered yellow prussiate of potash, and heated again. The result is, that the heat decomposes the prussiatc of potash, and the liberated carbon combines with the iron, forming a coating of steel on the surface of the articles. Another mode of case-hardening is to heat the articles along with some animal matter, such as the parings of horns and a little common salt, from one half to several hours; the articles are then cooled in cold water, or in oil, when they are of a delicte nature. Charcoal alone is also employed. The coating of steel is very thin, seldom exceeding nth of an inch. Where it is wanted to be thicker, the articles are treated several times. A Swedish iron-master has found that a very excellent case hardening is obtained by treating iron objects with a mixture of animal matter and arsenious acid dissolved in hydrochloric acid, and heating as usual.
or CASEUM, is an organic compound allied to albumen (q.v.), found in the milk of the manimalia, and in pease, beans, and other leguminous seeds, when it receives the name of LEGUMIN. The proportion of C. in milk (q.v.) varies, but averages about 3 per cent, and it may be coagulated and separated therefrom by the addition of a little rennet (q.v.), as in the manufacture of cheese (q.v.), or by the employment of a few drops of a mineral acid, such as dilute sulphuric acid. In either case, the C. sepa rates as curd, which still retains attached to it some oil and earthy salts, though the greater portion of these substances, along with the sugar, remains in the watery liquid or whey. The elementary bodies which enter into the composition of C., and the pro
portion in which these are present iu 100 parts, are—carbon, 53.83; hydrogen, 7.15; nitrogen, 15.65; oxygen, 22.52; and sulphur, 0.S5. The properties of C. are, that it is not coagulated by heat, as is well evidenced in the heating of milk, but is coagulated on the addition of rennet; sulphuric, hydrochloric, or nitric acids; alcohol, creosote, or infusion of galls, but not by acetic acid. It also forms insoluble precipitates with solu tions of the poisonous salts, acetate of lead, nitrate of silver, and bichloride of mercury (corrosive sublimate), and hence the efficacy of taking large doses of milk in cases of poisoning by those deadly salts, as the C. in the milk, forming an insoluble compound with the poison, keeps it from exerting its deadly powers.
The form of C. obtained from plants, and termed legumin, is generally procured from leguminous seeds, like pease or beans, though it can also be extracted from the majority of vegetable substances, especially from sweet and bitter almonds, and even from tea and coffee. Dried pease contain a fourth of their weight of legumin, and this can be extracted by bruising the pease to powder, and digesting in warm water for two or three hours. The liquid is then strained through cloth, which retains the insoluble matters, and allows the water with the leguinin dissolved therein, and with starch mechanically suspended, to pass through. On settling, the starch falls to the bottom of the vessel, and the clear liquid holding tlic legumin in solution, on the addition of a small amount of acetic acid, yields a precipitate of legumin or vegetable Caseine. So perfectly does the vegetable C. resemble the C. from milk, that the one can hardly be distinguished from the other by chemical tests or by taste; and at the present time there is regularly prepared in various parts of China, especially near Canton, a form of cheese from pease, which is sold to the populace in the streets of Canton under the name of taofoo. C. is a most important article of food. See NUTRITION.