1. Prof. Angel gives the following directions for its detection : "Chlorine is a constituent of common salt, and is very widely distributed in nature. Good water on an average contains perhaps from 0.4 to 1.0 grain of chlorine per gallon. If a water contains more than this amount, it is a strong indication that it has received pollution from cesspools, sink-drains, or the excreta of animals, all of which are highly charged with salt. But some localities, especially those near the sea, contain more salt than others; so that a good water in those districts may contain five, or even ten, grains of chlorine per gallon, for that is the natural amount. Before one could pronounce with some confidence on the sanitary condition of a water from the determination of chlorine alone, it would be necessary to know the average amount 3 of it in the natural waters of the region; hence, if in a single instance a water contains more than the general average, and there are no other indications of impurity, it would be unwise to condemn it. On the other hand, it would be equally unwise to pronounce a water safe if it contains less than the average amount of chlorine; because waters very badly polluted with vegetable matter alone are deficient in chlorine. However, when chlorine is deficient, it is certain that there is no contamination from animal matter".

It is possible for waters to contain salt that has come from filth, without containing the filth itself. When this is the case, one of two condi-tions exists: it may be indicative of a past pollution, or a warning of coming danger. Filth that had previously found access to the well may have undergone complete decomposition, while the salt remains; or filth may be so far from the well that nothing but its salt is washed through the intervening earth into it. Both conditions render the well unsafe, for in the one case another inflow of filth is liable to occur; in the other, the soil may soon become too fully charged with it to retain it all.

To determine the approximate amount of chlorine, it is necessary to prepare a standard solution of salt. One ounce avoirdupois, 437.5 grains, of pure salt contains 265.5 grains of chlorine. If this be dissolved in 17.7 fluid-ounces of water, each drop of the solution, reckoning 480 drops to the ounce, ought to contain 1/32 grain of chlorine, since (265.5 X 32)480=17.7.

Weigh, as carefully as possible, one ounce avoirdupois of best table salt; dissolve it in eighteen ounces of clean rain-water. This solution will contain very nearly 1/3 grain of chlorine per drop. The greatest care should be exercised in dropping the fluid, since the size of a drop varies so much. It should be dropped from an ounce bottle, and the drop allowed to form slowly.

Prepare a very weak solution of nitrate of silver, by dissolving a crystal, not larger than half a pea, in about one ounce of pure rain-water. There will be hardly any risk of making this solution too weak. Also prepare a solution of chromate of potash; bichromate of potash will answer the purpose, if the chromate cannot be obtained. The solution should be made in rain-water. The strength of it is not important.

Pour four ounces of the water to be tested into a saucer, and add enough chromate of potash solution to impart a distinct yellow color; then add a drop of the silver solution: a red color is produced where the drop strikes, from the formation of chromate of silver, which is quickly destroyed if the water contains much salt; continue to add the solution drop by drop, counting the drops, and stirring the water after each additional drop, until it assumes a faint reddish tint, which will occur as soon as all the chlorine has been precipitated. Then pour four ounces of clean rain-water into another saucer, add one drop of the solution of salt, observing the precaution already given about the size of the drop, and proceed as before. If it takes a larger number of drops of the silver solution to produce a reddish tint in this than were required to produce it in the other case, the water tested contains less than one grain of chlorine per gallon, since 1/32 grain in four ounces of water is at the rate of one grain in 128 fluid ounces, or one gallon. If more drops of the silver solution were added to the water than to the fluid used for comparison, it is easy, from the number of drops added to the latter, to estimate the chlorine in the former. For example, suppose ten drops of silver solution represent one grain of chlorine per gallon, and the water in question requires thirteen drops, then it contains 1.3 grain of chlorine per gallon. From this it will be seen that if the solution of nitrate of silver is sufficiently weak, it is possible to estimate very small quantities of chlorine, providing the quantity of salt in the fluid used for comparison be known. But on account of the difficulties in the way of weighing, measuring, and dropping, nothing but an approximation can be expected from the process. We think that by careful working, the approximation may be made to exceed half a grain.

2. Another authority recommends the following volumetrical determination of chlorides: "Chlorine as chlorides may be readily determined volumetrically by means of a standard solution of silver nitrate. This solution is prepared by dissolving a quarter of an ounce of dry silver nitrate in a quart of distilled water. Each drachm of this solution will precipitate one grain of chlorine.

"A half pint of the water to be tested is placed in a beaker resting on a white plate, and a few drops of a solution of chromate of potash are added. The chromate acts as an indicator, the silver combining first with the chlorine until it has all been precipitated, and then forming red silver chromate. The red color develops the instant the silver nitrate is in the slightest excess. The silver solution is allowed to flow in drop by drop, the fluid in the beaker being constantly stirred, until the white precipitate assumes a faint reddish tinge, At this point each drachm of silver solution added represents one grain of chlorine in a quart of the water, and the corresponding number of grains per gallon. A white precipitate produced by silver nitrate in drinking water is indicative of the presence of chlorine, and suggests contamination with sewage".

The solution of nitrate of silver must be kept in a glass vial and must be stopped with a rubber or glass stopper. It must be carefully preserved from light and air, and must be renewed when much dark precipitate is visible.

The solution of chromate or bichromate of potash need not be made very exact, one ounce in nine fluid ounces of distilled water answering nicely. If the yellow chromate cannot be readily purchased, bichromate of potash will answer, or it may be prepared by neutralizing a solution of bichromate of potash with carbonate of potash and crystallizing.