1. Prof. Angel says: "The pres-ence of these salts is a bad indication only so far as they have resulted from the oxidation of nitrogeneous organic, matter. Nitrates contain more oxygen than nitrites, and have required more time for their formation. Their occurrence, taken alone, teaches nothing positive; taken in connection with other evidence, it gives valuable information. But, as a rule, the presence of more than a trace of either salt is a strong indication of pollution from animal matter. However, some pure waters contain nitrates which they have dissolved from the earth and rocks of the locality. On the other hand, some very bad waters, especially those contaminated with vegetable matter, do not contain a trace.

"A little nitric acid exists in the atmosphere, coming probably from the oxidation of ammonia. Hence rain-water contains it, and surface-water receives an additional supply from the oxidation of nitrogenous matter on the ground. It is then absorbed largely by the rootlets of plants. Hence, shallow wells may receive it from surface-water. Other things being equal, they would naturally contain more of it when vegetation does not flourish.

"The importance that is to be attached to distinguishing whether the nitrogen compound is a nitrate or nitrite, is this generally: If nitrites occur, it would seem to show that the pollution is recent, or its source very near. If nitrates alone exist, it would be inferred that there has been time enough for complete oxidation, and hence the pollution is of longer standing, or its source far away. It sometimes happens that the occurrence of nitrates indicates the approach of pollution instead of showing actual or past pollution. This is especially the case when there is no other evidence of impurity, unless it is that of chlorine, for the soil about a well acts as a filter to retain deleterious matter, letting pass through it only the ultimate products of decomposition, which are in themselves harmless, until it becomes so saturated with filth that it can no longer accomplish this"

The following method for detecting nitrates and nitrites is delicate and easily applied:

Melt some zinc in a ladle, or iron spoon; stand in a chair and pour the melted metal in a fine stream into a pail of water standing on the floor. This granulates the zinc so it presents the greatest extent of bright surface. Prepare a little thin starch paste in the ordinary manner, dissolve a few grains of iodide of potash in water and mix it thoroughly with the paste. Have at hand a little sulphuric acid.

To test for nitrites, add half a teaspoonful of the iodide of starch solution to a tumbler of water, and allow it to mix. Then add a single drop of sulphuric acid. If any more than a trace of nitrous acid is present, a distinct blue color will result almost immediately. The test is so delicate that it gives, within a few seconds, a distinct reaction in water containing only the one hundred thousandth part of its weight of nitrous acid. And within a few minutes it will reveal less than one millionth part of it. If color does not appear at the end of a few minutes, it may be decided that no nitrous acid resulting from filth is present. After standing several hours the liquid usually assumes a blue color, from the infinitesi-mal amount of the acid that may naturally exist in the water.

If no nitrous acid, or but very little, is present, test for nitric acid as follows: Pour a pint of the water into a small nappy, add a spoonful of granulated zinc, and boil until about half of the water is driven off. This process reduces the nitric acid to nitrous acid. Let it cool and settle. Boiling is not absolutely necessary. Add the iodide of starch, acidify with sulphuric acid and add some zinc dust. Shake well in a bottle; this is sufficient to reduce the nitric acid to nitrous acid. Blue coloration indicates nitric acid. If boiled, carefully pour off the clear liquid, and test by the method given above. If nitrous acid has been found previously, it will be necessary to notice whether the reaction in this case is more prompt and marked. It is well to have two glasses in readiness at the same time - one containing the water as it came from the well, the other that which has been boiled with zinc - add a little of the iodide of starch solution, and then a drop of sulphuric acid to each, as nearly at the same time as possible, and notice whether the reaction occurs in one sooner than in the other, as well as whether the color varies in intensity. If much nitrous occurs, it will be impossible to detect nitric acid by this process. When this is the case the detection of nitric acid is not important. If a quite prompt and marked reaction for either nitrous or nitric acid takes place, the quantity is sufficient to render the water suspicious, and their presence forms a very valuable confirmatory indication of pollution in cases where a doubtful quantity of chlorine or ammonia occurs.

Any one desiring to do so can easily perform interesting and instructive experiments by operating on water in which a little nitrate of potash (saltpetre) has been dissolved.

2. Another sharp reaction on nitrix) acid is made in this way: a. Evaporate some water carefully and dissolve the residue in sulphuric acid. b. Make a concentrated solution of sulphate of iron (green vitriol), add some sulphuric acid and let this solution cool. Pour solution a carefully on solution b. Where both liquids meet, a yellow to dark brown zone will be visible when nitric acid is present.

3. Hager recommends to evaporate the water to be tested to the twentieth part of its volume, then to acidify strongly with sulphuric acid and add some brucine. When nitrates are present a purple red will occur.

4. Reichardt and Boettger recommend: Mix 3 drops of water, 2 drops solution of brucia, and 3 to 4 drops sulphuric acid. If nitric acid is present, a red to brownish color occurs. (Wilder's test book.)

5. Mashke (nitrous acid in potable water). Add 6 to 10 drops diluted acetic acid, and then 1 to 2 drops of blue molybdic acid solution. If nitrous acid be present the bluish color disappears within one hour (Wil-der's test book). 6. Schcenbein (nitrous acid in potable water). 1. Add a solution of pyrogallic acid and a little dilute sulphuric acid. Brown color by presence of nitrous acid. 2. Add to water sufficient indigo solution to color it a deep blue; add a little muriatic acid, and while stirring, sufficient potassium pentasulphide till the blue color just disappears; filter, and add the suspected water; blue color if nitrous acid be present. (Wilder's test book).

7. Howard (nitrous acid in water). Into a test glass place some of the water (not more than 50 ccm.), and add a drop of hydrochloric acid, then a drop of sulphuric acid, and one of a solution of naphthylamine hydrochloride. If the water does not contain more than one in 100,000,-000, after standing for ten minutes, it should not show more than the faintest tint of pink color. - (Chemist and Druggist.)