the same time a wire which has been dipped in a solution of the substance supposed to be present, and to note whether the lines produced by it and the material under examination are identical.

SPECIFIC GRAVITY.

In the great majority of cases the determination of specific gravity is not essential. Ordinary river, spring and well waters contain such small proportions of solid matter that it is usually the practice to take a measured volume and to assume its weight to be that of an equal bulk of pure water. If the proportion of solids is high, a determination of the specific gravity may be desirable. For this purpose the specific gravity bottle may be used. This consists merely of a small flask provided with a finely perforated glass stopper. The bottle is weighed first alone, then filled with distilled water at 60°, and finally with the water under examination at the same temperature. In filling the bottle, the liquid is first brought to the proper temperature, the bottle completely filled, the stopper inserted, and the excess of water, forced out through the perforation and around the sides of the stopper, carefully removed by bibulous paper. The weight of the water examined divided by the weight of the equal bulk of distilled water at the same temperature gives the specific gravity.

Another method, and one which gives very satisfactory results, is by the use of a plummet. This may conveniently consist of a piece of thick glass rod of about 10 c. c. in bulk, or of a test-tube weighted with mercury and the open end sealed in the flame. The plummet is suspended to the hook of the balance by means of a fine platinum

wire and its weight ascertained. It is then immersed in distilled water at 60° and the loss in weight noted. The figure so obtained is the weight of a bulk of water equal to that of the plummet. This having been determined, the specific gravity of any water may be found by immersing in it the plummet and noting the loss in weight. This, divided by the loss suffered in pure water, gives the specific gravity.

INTERPRETATION OF RESULTS.

STATEMENT OF ANALYSIS.

The composition of water is generally expressed in terms of a unit of weight in a definite volume of liquid, but much difference exists as to the standard selected. The decimal system is very largely employed, the proportions being expressed in milligrams per liter, nominally parts per million; or in centigrams per liter, nominally parts per hundred thousand. Not infrequently the figures are given in grains per imperial gallon of 70,000 grains, or the U. S. gallon of 58,328 grains. Much more rarely grains per quart, parts per thousand, per cents or other inconvenient ratios are employed. In this work the composition is always expressed in parts per million. This ratio is practically equivalent to milligrams per liter, except in cases of waters very rich in solids, since in such waters a liter weighs notably more than one million milligrams. Factors for converting the different ratios are given at the end of the book.

From the analysis of a water it is rarely possible to ascertain the exact arrangement of the elements determined, but it is the custom to assume arrangements based upon the rule of associating in combination elements having the highest affinities, modifying this system by any inferences derived from the character or reactions of the water itself.

It has been demonstrated that, even in the case of mixtures of salts producing no insoluble substances, partial interchange of the basylous and acidulous radicles takes place. In a solution of sodium chloride and potassium sulphate, both sodium sulphate and potassium chloride will be found, as well as both of the original salts. And when the conditions are rendered more complex by the addition of other substances, it is obviously impossible to determine the exact arrangements. In view of these facts, it is preferable to express the composition of a water by the proportion of each element or radicle present. In this way a water, for instance, that might be supposed to contain K2SO, will be expressed in terms of K and SO,, respectively. In the case of bodies like CO, and SiO2, which may possibly exist free in the water, their proportion is directly expressed as such. It frequently occurs, however, that the characteristics of some of the compounds in a water are sufficiently marked to indicate their presence, and there can be no objection to suggesting in connection with the analytical statement the inferences which may thus be drawn.

The organic matter, or its derived products, are best stated in terms of the nitrogen which they contain, thus permitting a comparison between the different stages of decomposition. It is inadvisable to represent the amount of unchanged organic matter in terms of oxalic acid, as has been suggested, or to express the nitrogen in terms of albumin, or any other supposititious compound.

SANITARY APPLICATIONS.

Judgment upon the analytical results from a given sample of water depends upon the class to which it belongs, and to the particular influences to which it has been subjected.

A proportion of total solids which would be suspicious in a rain or river water, would be without significance in that from an artesian well. On the other hand, a subsoil water of unobjectionable character would contain a proportion of nitrates which would be inadmissible in the case of a river or deep water. Location has also much bearing in the case; subsoil waters near the sea will be found to contain, without invoking suspicion, proportions of chlorine which would be ample to condemn the same sample if derived from a point far inland. Hence the importance of recording at the time of collection all ascertainable information as to the surroundings and probable source of the water.

Color, Odor and Taste.-Water of the highest purity will be clear, colorless, odorless, and nearly tasteless. While in some cases a decided departure from this standard may give rise to suspicion, analytical observations are necessary to decide the point. Water highly charged with mineral matters will possess decided taste; vegetable matters may communicate distinct color, but on the other hand, it may be highly contaminated with dangerous substances and yet give no indications to the senses.

Total Solids.-Excessive proportions of mineral solids, especially of marked physiological action, are known to render water non-potable, but no absolute maximum or minimum can be assigned as the limit of safety. Distilled water and waters very highly charged with mineral matter have been used for long periods without ill effects. The popular notion that the so-called hard waters conduce to the formation of urinary calculi is not borne out by surgical experience or statistical inquiry. Most urinary calculi are composed of uric acid, and are the results of disorders of the general nutritive functions. Dr. D. Hayes Agnew,