ing as to watered milks, the author, in certain cases, uses in his certificate a particular form of words, which is calculated to represent the actual facts more accurately than the usual statement. An example will suffice:

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"This milk has been adulterated with water. The exact amount of water added can only be told when the original composition of this particular sample of milk is known. But if the original milk was a very rich milk, containing 11 per cent. of solids not fut,' then to every 100 parts of milk by weight 227 of water have been added. If the milk contained 10 per cent. of solids not fat,' then 15 parts of water to every 100 of milk have been added. Lastly, if the composition of the original milk was that of the poorest milk which a healthy, fairly-milked cow is known to yield as an average-viz., containing per cent. of solids not fat,' then to every 100 parts of such milk 5.6 of water have been added; or, to put it in another form, 1000 parts by weight of milk have been made into 1056. I therefore certify the milk to have been watered AT LEAST 5.6 per cent.”

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However verbose such a certificate may seem, yet when it is remembered that under the Act the analyst is seldom called upon to give verbal evidence before a magistrate, the more full and complete the written statement of facts the better, and the author has occasionally found it in practice extremely useful, more especially since few magistrates possess chemical knowledge, and, therefore, naturally expect and require a very full explanation of the principles on which the certificate is based.

§ 160. With regard to the removal of cream, the method of detection is, of course, to make a quantitative analysis of the milk, exhausting the dry solids, as before described, by suitable solvents. If the milk-fat so obtained falls below 2.5 per cent., the milk in all probability has had its cream removed; and the amount of fat abstracted is found, according to this standard, by the following formula, in which 8 = solids not fat, f = the fat found, and x the percentage of fat removed :

If the milk is both skimmed and watered, the following formula may be used, the letters having the same significance, with the exception that x means extraneous water :

Possibly one cow in a thousand, at one period of the year, and on a particular diet, may give such a milk.

In giving a certificate as to the skimming of milk, the author invariably states that it is calculated to a particular standard, which may not at all represent the actual amount of fat removed, for in rich milks the fat is considerably over 2.5 per cent.; and he is also careful never to use the word "skimming," but substitutes the phrase "the fat has been abstracted," a formula which includes all the indirect ways in which fat may be removed.

The "solids not fat" may be normal, and the specific gravity normal, and yet the milk may be much watered. This feat is accomplished by the addition of cane-sugar. Cane-sugar is cheap, its solution in water has a high specific gravity, and it, of course, raises the amount of "solids not fat." A little practice in tasting milk enables any one to suspect its presence, but a complete analysis alone establishes it. In a milk adulterated in this way with cane-sugar, if the milk-fat, the caseine, and albumen are separately estimated, the caseine, albumen, and probably the milk-fat will all be found low, while the ash will also be found less than the normal quantity, and the remaining organic constituents high.

Dr. Muter estimates the amount of cane-sugar added, by pouring 10 grms. of milk on to 4 of calcium sulphate hydrate, evaporating this mixture to dryness with constant stirring, exhausting the fat by ether, and then removing all the sugar by dilute alcohol. This alcoholic extract is made up to a known bulk, and divided into two parts; one portion is evaporated to dryness, weighed, and then burnt, and the weight of organic constituents found by difference; in the other portion an estimation by Fehling is made; if no cane-sugar is present, the difference between the two estimations will be merely due to galactine and other principles soluble, to some extent, in dilute alcohol, and to any experimental error. If, however, canesugar has been fraudulently added, there will be a most marked difference, which may be returned as cane-sugar. Dr. Muter considers that unless the sugar is sufficient to impart a taste to the milk, it is not likely to be with any certainty discovered by analysis.*

§ 161. The addition of common salt, carbonate of soda, or, speaking generally, mineral adulterants, if in large quantity, will be at once recognised by the abnormal weight of the ash. If in smaller, the relation existing between the amount of ash and the caseine will be destroyed, and render it necessary to submit the ash to a careful qualitative and quantitative analysis.

A normal milk-ash is white, or nearly so, contains scarcely a trace of sulphate, and does not effervesce on the addition of acids. Borax is difficult of detection, because so little is usually added. The best method would appear to be, to evaporate down as much of the milk as can be obtained, previously rendering it feebly alkaline. It is then burnt up at a low temperature to an ash, and a little glycerine added; the mixture stirred with a glass rod, and a portion on the loop of a platinum wire introduced into the Bunsen flame, and examined by the spectroscope, when the bands peculiar to boracic acid will be seen, and may be compared with the spectrum of pure boracic acid. Or the ash may be decomposed by pure sulphuric acid, the freed boric acid dissolved out by alcohol, the alcohol concentrated, and the platinum wire moistened as before, and the spectrum observed. The flame in all these cases will show a more or less marked green colour.

A process of preserving milk by glycerine exists, and occasionally it is found in milk. To detect glycerine the caseine, fat, and albumen must be separated by dilution, acetic acid, carbon dioxide, and heat, as described at p. 241. The sugar is then estimated in one portion of the yellow whey by copper solution; the remainder is first neutralised, and then evaporated to dryness, and freed from any trace of fat by exhaustion with pure ether. The glycerine is now dissolved out by a mixture of alcohol and ether, the alcohol-ether evaporated off, and the glycerine 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 glycerine sets free boracic acid from borax. A little borax, therefore, may be moistened with the syrupy drops supposed to be glycerine, 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.

* A. Senier and A. J. G. Lowe: Chem. Soc. Journal, clxxxix., Sept., 1878.

PRESERVATION OF MILK.

§ 162. 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 glycerine, or other antiferments.

(3) Application of Cold.

(4.) Application first of Heat, and then of Cold.

§ 163. (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:

In 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, and 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.

§ 164. (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.