abundant white precipitate will appear, which will be found to possess the properties of gum; when boiled with a little dilute sulphuric acid, this substance will be converted into grape sugar, which may be readily recognised by the sulphate of copper test (pp. 9 and 121).

5. Gum tragacanth is sometimes employed in the form of a mucilage, to increase the volume of the cream, or to simulate its presence in skimmed milk. It may be detected by boiling the milk, and leaving it at rest for some hours; a gelatinous translucid deposit is then formed, which, being washed with a small quantity of water, and tested with a few drops of solution of iodine, produces a blue colour, because gum tragacanth contains starch.

6. Chalk is seldom if ever employed to adulterate milk. From its separating readily and falling to the bottom of the vessel containing the fluid, its presence would be recognized by the formation of a white deposit in the milk, after its standing undisturbed for a few hours; the deposit will dissolve with the evolution of carbonic acid, when treated with dilute hydrochloric acid; this solution yielding, with ammonia and oxalate of ammonia, a precipitate, insoluble in acetic acid.

7. Turmeric is detected by mixing a solution of potash with a sample of the milk previously concentrated; if the milk apparently possesses a yellow colour, which is rendered brownish with the above reagent, turmeric has been added.

8. Sugar.-To determine the existence of this adulteration, a quantitative determination of sugar must be undertaken by means of the saccharometer (see page 122), the fermentation method (see page 125), or Bernard and Barreswill's normal solution (see page 121). According to Dr. Hassall's experiments, 1000 parts of milk contain 52.7

of sugar; so that if the proportion of sugar obtained be much above this number, the analyst may conclude that some has been added to the milk examined.

9. Cerebral matter.-It is very probable that the adulteration in question is not practised; at all events it would be easily found out with the microscope, cerebral matter consisting of large rounded vesicles or cells, and long narrow tubes or fibres.

10. Carbonate of soda is occasionally added to milk in order to prevent its becoming acid; if used in large quantity it must prove injurious to health, but a small amount may be harmless. To detect its presence a little acetic acid is to be added, which causes the evolution of carbonic acid gas, and, moreover, coagulates the casein; the fluid is next to be filtered, and evaporated nearly to dryness; by mixing it now with alcohol, and setting fire to the mixture, a flame will be noticed possessing a marked yellow colour, and thus showing the presence of soda.

11. Zinc.-It is hardly credible that zinc should occur in milk; still, milk is sometimes kept in zinc pans for the purpose of increasing the yield of cream; and a small quantity of the metal might be dissolved. Its presence is easily detected by evaporating a sample of the milk to dryness, incinerating the residue, and dissolving the ashes in dilute hydrochloric acid; to this, ammonia is added in excess, and, in the filtered fluid, sulphide of ammonium will produce a white precipitate of sulphuret of zinc if the metal be present.

The analysis of twenty-six samples of milk, by the Commission of the Lancet,' yielded the following results: 1. Twelve samples were genuine.

2. Of these, two showed a deficiency of cream.

3. Eleven were adulterated.

4. The adulteration consisted in all cases of water, which varied from 10 to 50 per cent., or one half of the article.

5. In no case was chalk, size, gum, or sheep's brains detected, or any other substance said to be occasionally used for the adulteration of milk.

CHAPTER VI.

ON MINERAL OR INORGANIC FOOD AND ITS ADULTERATIONS: SALT AND WATER.

FROM the circumstance that common salt is a very cheap article of food, it is seldom adulterated; I shall consequently allude but shortly to this substance.

I propose also in the accompanying chapter

1. To show what qualities water ought to possess in order to prove a wholesome article of food.

2. To give an account of the methods to be employed for the chemical and microscopical examination of pure and impure waters, and show how waters may be purified.

3. To inform the reader of the principal sources of impurities liable to pollute drink-water.

4. To add a few words on the water with which our metropolis is supplied.

SALT.

Salt, or chloride of sodium, is an important article of diet, as it forms a large proportion of the inorganic matters 1 For the chemical composition of salt, see page 25.

contained in the blood, and it is evident that salt must be supplied to the body in order to maintain health. Chloride of sodium occurs abundantly in nature, as the principal solid constituent of sea water, in the form of rock salt, or in the so-called brine springs. The salt-districts in this country are Northwich, Middlewich, and Nantwich, in Cheshire; Shirleywich in Staffordshire, and Droitwich in Worcestershire. Salt is also procured in Durham, at Lymington in Hampshire, and in some parts of Scotland; it is, moreover, obtained by the evaporation of sea-water.

Common salt, as found in commerce, is not absolutely pure, being contaminated with from 12 to 64 per thousand of foreign substances; they consist especially of sulphate and carbonate of lime, sulphate of magnesia, and chloride of magnesium. Sea-salt prepared by rapid evaporation, is not always entirely soluble in water, the insoluble residue being a mixture of carbonate of lime and carbonate of magnesia, with a fine siliceous sand. The salt prepared from Cheshire brine yields an insoluble residue consisting almost entirely of carbonate of lime. The insoluble part of the less pure pieces of rock-salt is chiefly a marly earth with some sulphate of lime, the quantity of this impurity in Dr. Henry's1 analysis varied from 10 to 45 parts in a thousand.

Extraction of salt.-Rock-salt or stone-salt is dug from the earth in salt-mines, and sold in the crude state; it forms compact masses of various sizes, translucent, colourless, or but slightly coloured, and somewhat deliquescent in moist air. For domestic use, it is purified by solution and crystallization. From mineral or brine springs, salt is obtained either by concentrating their water with

1 We are indebted to Dr. Henry for a very elaborate investigation into the different varieties of common salt.