Rutic Acid, C10H2002.-A white crystalline solid, fusing about 29°.5. This acid, in combination with glycerin, also occurs in very minute quantity in milk-fat. It is even less soluble than caprylic acid.

$115. The Albuminoids of Milk.—The albuminoids of milk comprise at least three principles,-viz., casein, albumen, and nuclein. To these a fourth substance used to be added-viz., lacto-protein; but this, as the author has shown,* is not a simple substance, and it is probable that it is a mixture of peptones. Casein appears but little, if at all, different from alkali-albuminate, the minor differences which exist being, with probability, ascribed to impurities. It is true that when milk is filtered through a porous cell, casein, for the most part, is left behind; while, if a solution of alkali-albuminate is similarly treated, it passes through. Experiment has, however, shown that a solution of alkali-albuminate shaken up with butter fat behaves exactly like casein; and similarly, Soxhlet has proved that a concentrated solution of sodic carbonate precipitates both casein and alkalialbuminate, provided they are under the same conditions, and that it does not (as asserted by Zahn) leave alkali-albuminate in solution.t

Hoppe-Seyler does not adopt altogether this view; for although he states that without doubt no other albuminoid, in its properties, stands so near casein as alkali-albuminate, yet the notable difference in its power of rotating a ray of polarised light, and its behaviour to rennet, separates casein as a distinct substance. The casein of either cows' or goats' milk, not only in acid fluids, but in perfectly neutral solutions, is coagulated by gastric juice or a watery extract of the stomach. This coagulation takes place slowly at common temperatures, rapidly on warming; and the cause of it, according to Hammersten, is a body distinct from pepsin. Hammersten finds that the casein of cows' milk, in the absence of lime salts, is coagulated by the addition of an acid, but not by rennet. A solution of casein which has been precipitated by an acid, run into lime water neutralised by very dilute phosphoric acid, quickly coagulates on the addition of rennet, but without rennet neither on the addition of an acid, nor on boiling. Hammersten considers that the chemical change produced in the coagulation of casein by

1879.

Composition of Cows' Milk in Health and Disease, Jour. Chem. Soc.,

+"Beiträge zur physiologischen Chemie der Milch," von Dr. F. Soxhlet. Journal für praktische Chemie, vol. 6, p. 1, 1872.

Hoppe-Seyler, Physiologische Chemie, p. 930.

rennet, is the splitting up of the casein into two bodies, one of which is precipitated, and an albuminoid, which remains in solution, and is neither precipitated by boiling, nor by any of the following reagents-acetic acid, potassic ferrocyanide, or nitric acid; but is precipitated by mercuric chloride, and also by Millon's reagent.

Casein is precipitated by a variety of substances-lead acetate, cupric sulphate, alum, mercuric chloride, tannic acid, rennet, sulphate of magnesia, and mineral acids, if not too dilute; but none of these precipitate casein in a pure state, the precipitate usually containing fat, nuclein, and phosphate of lime, the latter, as already stated (p. 236), in the proportion of from 1 to 1.5 per cent. of casein. The best precipitant is sulphate of magnesia, which leaves the nuclein to a great extent in solution. The fat may then be extracted by ether; but the phosphate of lime is in true combination with the casein, and only a portion of it can be removed. A solution of casein in combination with sulphate of magnesia, and freed from fat, turns a ray of polarised light in weak alkaline solution, -87°; in very dilute alkaline solution, - 87°; in strong alkaline solution, - 91°. Pure casein is a perfectly white, brittle, transparent substance, insoluble in water, but soluble in very dilute acid solution, as well as in very dilute alkaline solution; in each case there is little doubt that a true chemical combination is formed. The presence of phosphate of soda in a solution of casein (as, for example, in the milk itself), prevents the precipitation by simple neutralisation by an acid, the casein not falling down until the acidity of the liquid is decided. It has been shown by Schutzenberger that, on sealing up casein in a tube and heating with baryta water, it behaves like albumen, and is resolved into the following substances :The elements of urea (ammonia and carbon dioxide), traces of sulphurous acid, of sulphuretted hydrogen, of oxalic and acetic acids, tyrosin, CH,,NOg, the amido-acids of the series CH2n+1NO2, corresponding to the fatty acids, CH2O2, from amido-cenanthylic acid to amido-propionic acid-leucin, CHINO butalanin, C,H,,NO2, and amido-butyric acid, CHNO, with a few less known or identified products. Many of these substances may be identified in putrid milk.

27

The amount of casein in milk is fairly constant, being about 3.9 per cent.; and the author has never known it exceed 5 per cent.

Serum-albumen occurs in milk, in no respect differing from the albumen of the blood. By careful addition of an alkali, this albumen may be changed into alkali-albuminate-that is, into casein; therefore, according to this view, the albumen in milk

may be considered the residue of an incomplete reaction. Albumen is not precipitated by acetic, carbonic, phosphoric, or tartaric acids. A small quantity of a dilute mineral acid does not precipitate; with a larger quantity of concentrated mineral acid the solution becomes troubled, and the deviation of a ray of polarised light increased; a still larger quantity of acid precipitates it as acid albumen. The best method to obtain a solution of pure albumen is to precipitate a solution by basic acetate of lead, pass carbon dioxide through the mixture, separate the carbonate of lead by filtration, and, lastly, pass through it hydric sulphide, to remove the trace of lead still existing. Albumen is then in solution, but with a little acetic acid, on evaporation, it may be obtained in the solid state contaminated slightly with acetic acid.*

Another method of obtaining albumen pure is by dialysis. The physical characters of solid albumen differ according to the method of separation. Albumen obtained by dialysis is in the form of a yellow transparent mass, specific gravity 1314; but albumen. separated in the ordinary way from milk, for the purpose of quantitative determination, is in yellowish flakes, brittle, without taste or smell, insoluble in water, alcohol, and ether, soluble in dilute caustic alkali, if gently warmed, and from this alkaline solution precipitable by an acid. The amount of albumen in milk is really fairly constant, and averages 7 per cent. In healthy cows it is a very constant quantity, the chief deviation occurring directly after calving, when the amount may rise as high as 3 per cent., but this is always accompanied by a corresponding rise in the casein. According to the author's experience, the albumen preserves a very constant relation to the casein, the quantity of the latter being five times that of the albumen; so that if either the amount of casein or albumen is known, the one may be calculated from the other with great accuracy.

Nuclein.-Nuclein is the organic phosphorus compound of milk, containing, according to Miescher, 96 per cent. of phosphorus. Its formula is CH49 N9 P5O92. It is by no means peculiar to milk, but has been found in the blood, in pus, in the yelk of eggs, in the liver cells, and in yeast cells. When freshly precipitated, it is a white amorphous body, somewhat soluble in water; freely soluble in ammonia, soda solution, and phosphate of soda. The special test distinguishing nuclein from other albuminoids is the presence of phosphorus, and the production Meggenhofen appears to have been one of the first who detected the presence of albumen in milk. He estimated the amount in cows' milk as '59 per cent. Dissertatio Inauguralis sistens indagationem Lactis Muliebris Chemicam. C. Aug. Meggenhofen. Frankfort, 1826.

of no red colour, either by Millon's reagent, or by a copper salt, added to a solution of nuclein alkalised by soda lye: it forms a very definite compound with lead, the lead and phosphorus being in the proportion of Pb to P.

The method adopted by Hoppe-Seyler* to separate nuclein from pus, was isolation of the pus cells by Glauber's salts, washing with very dilute hydrochloric acid and much water; then extracting the nuclein by the aid of a very weak alkaline solution of caustic soda, and filtering (which in this case proves a troublesome operation), and precipitating by a mineral acid. The precipitate is again dissolved in weak alkaline solution, and again precipitated, and the process repeated until the nuclein is supposed to be in a fairly pure condition. Nuclein may be separated from milk on the same principles, first exhausting the solids by alcohol and ether to remove fat.

$116. Milk-Sugar, CHOH,O.-Milk-sugar, so far as is known, is only found in human milk, the milk of the herbivora, and of the bitch. It is easily distinguished from other sugars; its specific gravity is 1-53; and its solution turns a ray of polarised light to the right at 20° 53°.

The separate researches of Erdmann + and Schmager have shown the existence of four modifications of milk-sugar, exhibiting a different rotation to normal sugar, viz.:-1. Crystallised milk-sugar exhibiting in solution strong bi-rotation. 2. Anhydrous milk-sugar obtained by dehydrating crystallised sugar at 130°, showing in solution also strong bi-rotation. 3. Anhydrous milk-sugar obtained by quickly evaporating a solution of milk-sugar in the presence of sand, or other finely divided substance, so as to ensure a large surface; the solution shows slight bi-rotation. 4. Anhydrous milk-sugar evaporated down from a solution quickly, but without the addition of sand or other substance. All these modifications are at once transformed into milk-sugar of normal rotation by boiling their solutions, or gradually, without the application of heat.

Milk-sugar is soluble in six parts of cold, and 2.5 parts of boiling water; it is insoluble in absolute alcohol and in perfectly dry ether, but in dilute alcohol and commercial ether it is slightly soluble, the solubility in amount depending mainly on the percentage of water which the ether contains. At 150° it loses an atom of water without further decomposition; its watery solution is perfectly neutral, and has a sweet taste: the sweetening power of milk as compared with cane sugar is but feeble.

It reduces Fehling's copper solution in a proportion different from that of grape-sugar (see p. 139). Milk-sugar undergoes lactic fermentation readily (see p. 251), but alcoholic with some difficulty. Milk-sugar is precipitated by acetate of lead and ammonia; neutral acetate of lead, even at a boiling temperature, neither precipitates nor changes it. The oxides of copper, of bismuth, and silver are reduced by solutions of milk-sugar, and * Med. Chem. Untersuch., Hoppe-Seyler. Berlin, 4 Heft. † Bericht, xiii. 2180-2184.

Ibid. xiii. 1915-1931.

indigo is decolourised; these latter reactions are similar to those of grape sugar. When oxidised by nitric acid, milk-sugar yields mucic acid, acetic acid, and tartaric acids, and on further decomposition oxalic acid may be obtained.

By boiling milk-sugar for several hours with 4 parts of water and 2 per cent. sulphuric acid, neutralising with carbonate of lime, evaporating the filtrate to a syrup, a different sugar from lactose may be obtained in microscopical crystals. To this altered milk-sugar, the name of galactose has been given. Its action on polarised light is expressed as +83·22 at 15°; it is a fermentable sugar, and yields, on oxidation with nitric acid, twice as much mucic acid as milk-sugar.

The amount of milk-sugar in normal milk preserves a very constant relation to the percentage of casein, being about 1 grm. per every 100 cc. in excess of the casein. Its average is about 4 per cent.

§ 117. Mineral Constituents of Milk.-The mineral constituents of milk have been fully and early investigated, and the following may be considered a very close approximation to their actual amount and character :

Four analyses of milk ash by R. Weber and Haidlen give

the following:

The chlorine is in combination with the alkalies, the iron and the earths occur as phosphate, as well as the potassium oxide. So that the mineral constituents of cows' milk are, phosphate of potash, phosphate of lime and magnesia, common salt, and a trace of phosphate of iron. Other mineral inorganic constituents have been found in small quantity. If sufficient milk be used, it is not difficult to obtain a fluorine reaction, and since fluorides form an essential constituent of the teeth, it is easy to see their impor tance. A minute quantity of sulphuric acid as sulphates exists