A manual of elementary chemistry : theoretical and practical / by George Fownes.

  • Fownes, George, 1815-1849.
Date:
1873
Catalogue details

Licence: Public Domain Mark

Credit: A manual of elementary chemistry : theoretical and practical / by George Fownes. Source: Wellcome Collection.

    992/1062 (page 964)
    remains. The purified albuminate is then dissolved in boiling water or spirits of wine, in which it ought to give a clear solution. An albuminate is more simply obtained by shaking milk with caustic soda and ether, pouring off the clear alkaline lower layer of liquid, precipitating it with acetic acid, and washing it with water. Dried casein and albuminate are yellow, transparent, and hygroscopic, swelling up in water, but not dissolving. When precipitated in a flocky state, they dissolve easdy in water if it contains a little alkali. The precipitate which forms on neutralis- ing the alkaline solution, dissolves easily in an excess of acetic acid or dilute hydrochloric acid. On the addition of an excess of mineral acid, or on neutralisation with an alkali, these solutions give a precipitate. The neutral or feebly alkaline albuminate, and casein in alkaline solution, are precipitated in the cold by alcohol: when hot thejr are dissolved. Albuminates are precipitated by copper sulphate, silver nitrate, and barium chloride. Lieberkuhn gives as their formula C72H112Il2N18Q23S, where R denotes an atom of univalent metal. According to him, potassium albuminate has the same composition. Meissner says that by boiling casein continuously, lactic acid and creatine are formed. By fusion with potassium hydrate, casein yields valeric and butyric acids, besides other products. The most striking property of casein is its coagulability by certain animal membranes. This is well seen, in the process of cheesemaking, in preparing the curd. A piece of the stomach of the calf, with its mucous membrane, is slightly washed, put into a large quantity of milk, and the whole slowly heated to about 53°. In a short time after this temperature has been attained, the milk is observed to separate into a solid, white coagulum, or mass of curd, and a yellowish, translucent liquid called wlmij. The curd contains all the casein of the milk, much of the fat, and much of the inorganic matter : the whey retains the milk-sugar and tin- soluble salts. It is just possible that this mysterious change may be really clue to the formation of a little lactic acid from the milk- sugar, under the joint influence of a slowly decomposing membrane and the elevated temperature, and that this acid may be sufficient in quantity to withdraw the alkali which holds the casein in solu- tion, and thus occasion its precipitation in the insoluble state. The loss of weight the membrane itself suffers in this operation is very small : it has been found not to exceed r^nr part- Casein differs greatly from albumin in the quantity of am- monia which it yields when oxidised with an alkaline solution of potassium permanganate, casein yielding only 65 p. c, ammonia, whereas albumin yields 10 p. c * Class IV.—Fibrin.—Insoluble in water; sparingly soluble in dilute acids and alkalis, and in neutral saline solutions. * Wiinklyn, Chom. Soc. J. [2], ix. 837.