Genuine coffee should not cake when pinched between the tingers. If a little be thrown on cold water it floats, and very slightly tinges the water. Adulterated coffee sinks and rapidly colours the water brown. Good coffee does not contain more than 1:3 per cent. of sugar, and affords less than 1 per cent. of ash. which latter ought to dissolve completely when digested for some time in strong hydrochloric acid. A residue indicates the presence of silica, which is not found in pure coffee, but is present in the ash of most of the bodies used to aduiterate coffee.

Ten grammes of the coffee are weighed out and placed in a glass task, and 100 c. e. of cold distilled water poured in. The mixture is then raised to the boiling temperature, and kept at this point for one minute. It is then rapidly filtered, the clear liquid allowed to cool down to 15.5° C., and the specific gravity taken by means of an accurate urinometer. Gennine coffee gives a decoction having a specific gravity not higher than 1·010; but if chicory (or most of the other adulterants) be present, the specific gravity exceeds 1·010, and sometimes reaches 120 when much chicory is present..

Cocoa. Adulterations.-Chicory, cocoa-husk, fats, starches, sugar, Venetian red, bole.

Genuine cocoa should not have a sweet taste, or a red colour. As much cocoa as can be piled up on a threepenny piece, when placed on a square of platinum foil, and strongly heated by a spirit lamp flame, should burn almost completely away, leaving less than 3 per cent. of a reddish coloured ash. The same remarks apply to CHOCOLATE.

VINEGAR.- Adulterations. Sulphuric acid and other mineral acids, water, "grains of paradise," chillies, arsenic, and copper, as accidental impurities.

Unadulterated vinegar is allowed by special enactment to contain one-thousandth of oil of vitriol. When paper moistened with vinegar containing this proportion of sulphuric acid is dried before the fire, no charring takes place intil the paper is rather strongly heated; but if the propor

tion of acid is much greater, blackening results before the paper seems quite dry. It must be remembered that this is but a very rough and indecisive test. When a piece of clean and bright copper wire is immersed in vinegar, diluted with a little water, and heated nearly to boiling in a glass vessel, the copper quickly loses its colour and assumes a leaden hue if arsenic is present. Copper may be detected in a fresh sample, much diluted with water, by means of the steel needle, as described under Preserved Fruits. Pungent substances-"grains of paradise," for example-may be detected by evaporating a quantity of the vinegar nearly to dryness in any convenient porcelain vessel. The residue should not have a fiery taste. For arsenic test, see "Water Analysis, "page 226.

MUSTARD.-Adulterants.-Ordinary "mustard" is rarely free from admixture with one or other of the varieties of flour, turmeric being added to improve the colour. The addition of flour in moderate proportion may be permitted on the score of convenience, but turmeric should not be added. For flour, china clay, plaster of Paris, or chalk have been substituted, the colouring material being yellow ochre, or even the poisonous chromate of lead.

Mustard should not become brown when moistened with a little "spirit of hartshorn," and when burnt on platinum foil should leave not more than 6 per cent. of nearly white ash.

CAYENNE PEPPER.-Adulterants.-Dense flours or starches, mustard, turmeric, ochre, vermilion (?), red lead.

Cayenne when shaken with cold water, the mixture allowed to stand for a minute, and the liquid poured off, should not leave any heavy red powder at the bottom of the vessel. It ought to leave but 5 per cent. of ash when burnt on platinum foil.

CHEESE.-Adulterants and Impurities.-Setting aside such colouring matters as annatto, saffron, etc., we find that the

mineral pigments, Venetian red, (red lead?), are used, and various flours or starches to increase weight.

Cheese should not be eaten when in a mouldy condition, or when containing "jumpers." It ought not to become blue when touched with dilute tincture of iodine, and it should leave but little ash when burnt on platinum.

ISINGLASS.—Adulterants.-Though the best, or Russian isinglass, is an unimportant article of food, it is well to mention that it is sometimes adulterated with gelatine and with inferior Brazilian isinglass.

Genuine Russian isinglass occurs in opaque white filaments, which do not become transparent when placed in water, nor do they swell to a material extent. Gelatine, on the contrary, becomes transparent, and swells considerably Russian isinglass affords a firm, translucent jelly; the Brazilian variety, for corresponding weights of material and water, does not afford nearly so firm a jelly, and it is much more milky.

ARROWROOT (WEST INDIAN OR MARANTA).-Adulterants.Potato starch, sago meal, rice, gypsum, china clay, chalk.

Genuine Maranta arrowroot is a dull white powder, which crackles strongly and in a peculiar manner when pressed between the fingers. When mixed with twice its weight of strong hydrochloric acid it yields an opaque jelly. Potato starch, under similar circumstances, affords a transparent jelly. When burnt on platinum foil, arrowroot will leave not more than 1 per cent. of ash if unadulterated with mineral powders. A fragment of iodine, placed on a warm plate near to the sample, colours Maranta arrowroot a chocolate brown, sago starch yellowish, wheaten starch violet, and potato starch a dull lilac tint.

CHAPTER XVIII.

WATER SUPPLY.

Division of the Subject-Allowance per Head-Probable Permanence of Supply-Quality of the Water-Influence of Soil and Rock StrataSurface and Spring Water-Spas-Contamination of Wells by Animal Excreta Church's Mode of Detecting Source of Impurity-Purification-Filtration; Boiling; Clarke's Process -Storage of Water in Reservoirs or Cisterns.

THE questions relating to water supply which the Medical Officer of Health may be fairly called upon to consider are those affecting—(a.) the allowance to be provided for each inhabitant per diem; (b.) the probable permanence of the supply; (c.) the quality of the water; (d.) its purification prior to distribution; and (e.) its storage in house cisterns. Engineering questions may be left out of consideration, as they are best dealt with by specially skilled advisers. Hence, in dealing with water supply, we shall consider, in the above order, the matters of interest to the sanitarian, without treading upon the ground properly occupied by the engineer. (a.) Allowance per head per diem.—The average daily supply required for each inhabitant is estimated by Mr. Rankine at 22 gallons. This is, on the whole, a fair proportion, though, when possible, it is well to aim at a supply of 35 gallons per head, in order to admit of the free use of water for public and private baths and for general cleansing. Other conditions being equal, the largest proportion is undoubtedly the best for the sanitary condition of a town. If, however, an abundant but polluted supply, incapable of proper purification, and one more scant in quantity but excellent in quality, are compared, we should give the preference to the latter, as a comparatively small supply of good water, amounting to five or six gallons per head, may be made to suffice by strict economy in use.

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(b.) The probable Permanence of Supply.-In the consideration of this point, the source of the water is an element of considerable importance. A few general remarks on the sources of drinkable water may, therefore, most suitably be made in this place.

The prime source of all water supply is, undoubtedly, the ocean, since in nature there is a continuous circulation from the sea to the air, then from air to rivers, and, finally, to sea again. The air in contact with the ocean becomes quickly saturated with the vapour of water, and then being carried by currents over the earth and suddenly cooled, deposits much of the aqueous vapour in the form of rain. If the soil be not porous, very small streams are formed, and these flowing into a common channel produce a river. If the soil be porous, the water percolates through it, and may drain away again at a lower level, and form rivulets and rivers, or it sinks into the subjacent permeable strata, thus serving to maintain the supply of wells and of natural springs, often situated at a great distance from the place of rainfall. If the permeable strata are not overlaid by those only slightly pervious, land-springs not rising above the surface are obtained over the district; but if the strata dip between two impermeable beds, an Artesian spring is obtained on boring at a lower level through the upper bed to the water-bearing strata. Land springs cannot be much relied upon for a permanent supply. Artesian springs, on the other hand, can be depended upon to a much greater extent if the outcrop of the pervious strata be considerable and the rainfall of the district high in which the strata reach the surface. The best waterbearing strata are the chalk, limestone, green sand, and the older metamorphic rocks; water can almost always be obtained with facility on boring sufficiently far into them, and, as might be anticipated, springs occurring in them or similar rocks, and situated at the base of a hill, usually yield an abundant and steady stream of water.

The usual source of supply to towns is the rain falling