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

  • George Fownes
Date:
1869
Catalogue details

Licence: Public Domain Mark

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

    820/882 page 816
    6 per cent. As much as 1 per cent, of cholesterin has been found to be present; but neither by this nor by any other character can the passage of mucus into pus be determined. MILK.—The peculiar special secretion destined for the nourishment of the young is, so far as is known, very much the same in flesh-eating ani- mals and in those which live exclusively on vegetable food. The propor- tions of the constituents may, however, sometimes differ to a considerable extent. The specific gravity varies from 1 018 to 1-045. It will be seen hereafter that the substances present in milk are wonderfully adapted to the office of providing materials for the rapid growth and development of the animal frame. It contains an azotized matter, casein or potassium al- buminate, fatty principles, and a peculiar sugar, and lastly, various salts, among which may be mentioned calcium phosphate, held in complete solu- tion in a slightly alkaline liquid. This last is especially important to a process then in activity, the formation of bone. The white, and almost opaque, appearance of milk is an optical illusion: examined by a microscope of even moderate power, it is seen to consist of a perfectly transparent fluid, in which float about numbers of transparent globules: these consist of fat, surrounded by an albuminous envelope, which can be broken mechanically, as in the churning, or dissolved by the chemi- cal action of caustic potash, after which, on agitating the milk with ether, the fat can be dissolved. When milk is suffered to remain at rest some hours at the ordinary tem- perature of the air, a large proportion of the fat-globules collect at the surface into a layer of cream; if this be now removed and exposed for some time to strong agitation, the fat-globules coalesce into a mass, and the re- maining watery liquid is expelled from between them and separated. The butter so produced must be thoroughly washed with cold water, to remove, as far as possible, the last traces of casein, which readily putrefies, and would in that case spoil the whole. A little salt is usually added. Ordinary butter still, however, contains some butter-milk, and when in- tended for keeping should be clarified, as it is termed, by fusion. The watery part then subsides, and carries with it the residue of the azotized matter. The flavor is unfortunately somewhat impaired by this process. The consistence of butter, in other words, the proportions of solid fat and olein, is dependent upon the season, or more probably upon the kind of food; in summer the oily portion is always more considerable than in win- ter. The volatile odoriferous principle of butter, butyrin, has been already referred to. The casein of milk, in the state of cheese, is in many countries an im- portant article of food. The milk is usually heated to about 49° C. (120° F.), and coagulated by rennet, or an infusion of the stomach of the calf in water: the curd is carefully separated by a sieve from the whey, mixed with a due proportion of salt, and sometimes some coloring matter, and then subjected to strong and increasing pressure. The fresh cheese so prepared, being constantly kept cool and dry, undergoes a particular kind of putrefactive fermentation, very little understood, by which principles are generated which communicate a particular taste and odor. The good- ness of cheese, as well as much of the difference of flavor perceptible in different samples, depends in great measure upon the manipulation: the best kinds contain a considerable quantity of fat, and are made with new milk: the inferior descriptions are made with skimmed milk. Some of the Tartar tribes prepare a kind of spirit from milk by suffering it to ferment, with frequent agitation. The casein converts a part of the milk-sugar into lactic acid, and another part into grape-sugar, which in turn becomes converted into alcohol. Mare's milk is said to answer better for this purpose thai* that of the cow.
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