to dryness, and freed from any trace of fat by exhaustion with pure ether. The glycerin is now dissolved out by a mixture of alcohol and ether, the alcohol-ether evaporated off, and the glycerin identified by its physical characters and the production of acrolein fumes when heated with sulphuric acid.* Use may also be made of the fact that glycerin sets free boracic acid from borax. A little borax, therefore, may be moistened with the syrupy drops supposed to be glycerin, heated in a Bunsen flame, and examined before the spectroscope for the boracic acid bands.† Salicylic acid is used occasionally as a preservative of milk, and it is easily detected by shaking up milk whey (first acidified by hydrochloric acid) with ether. The ethereal solution on evaporation leaves the acid in a pure enough state to permit the successful application of reagents. The best test for salicylic acid is the beautiful violet colour which it gives with a neutral solution of ferric chloride. Besides this test, a minute portion may be placed in the subliming cell, when a well-marked sublimate is obtained at about 100°. The crystalline form of this sublimate may be compared with one obtained from a known pure sample of salicylic acid. Benzoic acid is also occasionally used, and it may be detected as follows:-200 cc. or more of milk are alkalised with baryta water and evaporated down to one-fourth; the thickened residue is next mixed with calcic sulphate to a paste and dried on the water-bath. The mass is finely powdered, moistened with dilute sulphuric acid, and extracted with cold 50 per cent. alcohol. The alcoholic extract is neutralised with baryta water, and concentrated by evaporation to a small volume. The liquid is now acidulated with dilute sulphuric acid and extracted with ether. On separating and evaporating the ether, any benzoic acid is left sufficiently pure to respond to the usual tests. That with ferric chloride succeeds best if to an aqueous solution of the acid a little sodic acetate is first added. For a quantitative estimation of the acid, it is best to sublime the acid, and weigh the sublimate, checking this weight by ascertaining the loss the residue has experienced. *A test for glycerin has been proposed by E. Donath and J. Mayshofer (Zeitschrift für Anal. Chem., xx., 79). The supposed glycerin is heated to 120° with two drops of phenol and the same quantity of concentrated sulphuric acid. The whole is then treated with a little water, and the insoluble portion rendered slightly ammoniacal. Under these circumstances glycerin gives a beautiful red colour. A. Senier and A. J. G. Lowe: Chem. Soc. Journal, clxxxix., Sept., PRESERVATION OF MILK. § 163. It has already been stated that the lactic fermentation and the putrid, or butyric, fermentation of milk are both due to mysteriously minute bacteroid bodies, ever present in the atmosphere. Milk boiled, or raised to a sufficiently high temperature to destroy any germs which may be already in the milk, and then kept by any process whatsoever in such a manner that germs cannot gain access to it, remains sweet for an indefinite time. If, for example, a flask of milk is taken, heated up to its boilingpoint for some time, and then, while boiling, plugged in the neck with a good compact piece of fibrous asbestos, which itself has been made for a few minutes red hot, the milk will neither decompose nor ferment. Similarly, with suitable precautions, the long thin neck of a flask may be bent in an N shape, the milk boiled as before, and allowed to cool; in this case, also, there will be no decomposition. The explanation in the one instance being that the germs have been filtered; in the other, that they have settled in the bend of the N, not being able to turn corners readily. Similar experiments (all of which have been essayed over and over again by Tyndall, Pasteur, and others) all point to the same simple conclusion-viz., that it is only necessary to destroy the existing germs, and then put the organic substances or fluids under such conditions as will shield them from renewed infection, in order to preserve the most complex substances and fluids from further change. The various processes which have been proposed for the preservation of milk fall under the following heads : (1.) Evaporating Processes,-in which the milk is reduced to a dry powder, and generally mixed with sugar, the evaporation taking place either in a vacuum or in a stream of warm, dry air. (2.) Chemical Additions,—such as formalin, glycerin, or other antiferments. (3.) Application of Cold. (4.) Application first of Heat, and then of Cold. § 164. (1.) Evaporating Processes. All putrefactive and fermentative change is reduced to a minimum when organic substances are deprived of water, and milk is no exception to the rule. The dried milk solids, without any addition whatever, will often keep for many weeks although freely exposed to the air; while with certain additions, such as sugar, the preservation may be called for practical purposes permanent. The Swiss Company's Condensed Milk may be cited as a very successful experiment of this kind, the milk being what it pre tends to be-viz., evaporated to a certain point with the addition of sugar. Numerous patents have also been taken out in this country with the same end in view. A few of the more important are as follows:- În William Newton's patent [No. 6787, 1837] the milk was evaporated as rapidly as possible, either by warm or cold air or in a vacuum, and then pulverised and mixed with powdered loaf-sugar. In 1847, a process was proposed by T. S. Grimwade [patent No. 11703]. The milk was concentrated in vacuo, and four grains of saltpetre were added to every quart; the milk was then transferred into vacuous bottles, the arrangement for corking these bottles being particularly ingenious. Jules Jean Baptiste Martin de Lignac [patent No. 11892, 1847] evaporated in simple open pans, continually breaking the scum up by mechanical means. A little sugar was added, and the product preserved in hermetically-closed vessels. A patent taken out by Grimwade in 1855 [patent No. 2430], was a combined process. The milk, immediately on being received from the cow, was heated to 110° Fahr.; and 5 ozs. of refined sugar and 1.25 ozs. of milk-sugar were added to every gallon of milk. The whole was now evaporated in a particular pan with double bottom, through which hot water was made to circulate during the evaporation. These pans were kept in a continual oscillation by means of machinery, and the resulting dry solids were ground to powder by rollers. In Clark's patent [No. 3675, 1837], for the first time, is mentioned the heating of milk to the boiling point of water, with the avowed object of destroying germs. The milk is evaporated in a vacuum without the addition of sugar. Stephens has an ingenious specification [No. 1342, 1872], according to which the milk is rapidly condensed in a continuous manner by successively passing through a series of twenty-four pans, each pan being raised a little above the next in order, anu the whole being in a line. The bottoms of the pans are serrated, heated by steam, and oscillation by machinery is kept up. The milk flows in a slow shallow stream, and the evaporation is finished by the time the milk reaches the last pan. § 165. (2.) Additions to Milk.--The ordinary additions have been sugar, milk-sugar, glucose,* carbonate of soda, and nitre. Bethall, in 1848, preserved cream and milk by first expelling the air, and then saturating the liquids by carbonic dioxide. The gas was evolved in the usual way, from sodic carbonate * J. A. Newnham, No. 2801, 1870. decomposed by an acid; the air was expelled by boiling, and the milk was then preserved in bottles. A similar patent [No. 25, 1879] has been taken out by Riddell, and, without doubt, this method is scientifically correct, and, if properly done, would be effectual. Wanklyn and Eassie [patent No. 1861, 1871] have proposed the addition of two parts glycerin to every 100 of milk. All these methods of preserving milk have, it is obvious, no effect in destroying the germs of any disease possible to be communicated to man. Speaking generally, indeed, all additions to milk in the form of antiseptics, such as glycerin, salicylic acid, borax, and the like, should be looked upon with disfavour; for by their use cleanliness in the dairy would not be such an essential as it is now; and the addition of these antiseptics is somewhat analogous to the saturation of foul places with carbolic acid, when the more obvious and more effectual remedy would be to keep them free from filth. § 166. (3.) Action of Cold on Milk.—The simple action of cold on milk has been studied scientifically, and it has been conclusively proved that the artificial cooling of milk by ice (whether the milk be placed for the purpose in deep cans or in shallow pans) produces far better, sweeter cream than any other system. An analysis of cream by Voelcker* thrown up by the Swartz system,t gave the following: Tisseraud, from his experiments on the action of cold, concluded that 1. The rising of the cream is the more rapid the nearer zero the milk is kept; 2. The volume of the cream is greater; 3. The yield of butter more considerable; 4. The skim-milk, the butter, and the cheese, are in the latter case of better quality. Still, however valuable the use of cold may be to throw up the cream and to preserve milk in transit, it must not be forgotten that it in no way renders the milk safe, should it be contaminated by any specific poison of animal or human origin. * Journal Agricul. Soc., No. xxiv., 1879, p. 157. † Deep cans, (2 feet long, 20 deep, and 6 wide), in which the milk is artificially cooled, are the chief features of the Swartz system. Comptes Rendus, t. 82, 1876. § 167. (4.) Heating and then Cooling.-A very perfect process of preserving milk, if the temperature used had only been sufficient, was patented in 1857, by Joseph House, No. 15. He evaporated down to of its bulk, at a temperature not exceeding 150° Fahr., in a shallow circular dish. The concentrated milk was then put into tins, cooled artificially and soldered down. INFLUENCE OF FOOD ON THE QUALITY AND QUANTITY OF MILK. § 168. The influence of food on the lacteal secretion is great, a difference more especially appreciated if the produce of the whole milk be taken into account, and not the mere percentage composition. Indeed, in experiments on the influence of food, the mere composition of 100 parts of milk, without the knowledge of the total amount secreted, is not only useless, but misleading, and has caused many erroneous conclusions. It appears established that abundance of suitable food, with little exercise, increases the yield of milk in every animal, and therefore increases all the constituents; while the reverse decreases the yield, and therefore decreases all the constituents. The increase of the total solids, when a highly nitrogenous substance like flesh is given to a herbivorous animal, is remarkable. Thus, Weiske obtained daily 739 grms. of milk from a goat fed on potatoes and straw, but on the addition of a little powdered fibrin to the same weight of food, 1054 grms. Dumas considered it proved by his experiments* that, when bread was given to a bitch, her milk then contained milk-sugar, but when carbo-hydrates or starchy substances were withheld, and flesh given, then there was no milk-sugar, and that in all the carnivora lactic acid took the place of lactose (milk-sugar). It would, however, appear that this teaching is erroneous; for most certainly the milk of animals fed exclusively on flesh, does contain milk-sugar, and it is probably of albuminous origin. Beuscht has found milk-sugar in the milk of a bitch, the sample being drawn on the eighth, twelfth, and twentieth days of an exclusive flesh diet. Subbotin found the sugar in a bitch's milk to be 3:41 per cent. when fed on potatoes, and 2:49 when fed on meat; but since on the latter diet there was a far greater yield than on the former, the real fact was that the flesh increased, not diminished, the sugar. Comptes Rendus, t. xvii., p. 585, 1843. + Peusch: Ann. der Chemie u. Pharmacie, lxi., s. 221, 1874. |