These combinations, in the small quantities in which they exist, in no degree injure the diluent properties of rain water. It is indeed to the presence of the two elastic gases, that rain water owes the taste which renders it palatable to animals and useful to vegetables. Ice water, being destitute of these gases is extremely vapid; fish cannot live in it; and it does not seem either to quench thirst or to be so complete a solvent in the stomach as rain water. Carbonate of ammonia is also another ingredient. It is derived from the putrefaction of nitrogenous substances. When several hundred pounds of rain water were distilled by Liebig, in a copper still, and the first two or three pounds evaporated with the addition of a little muriatic acid, he found a very distinct crystallization of sal-ammoniac, the crystals having a brown or yellow color. “It is worthy of observation,” says Liebig, “that the ammonia contained in rain and snow water possesses an offensive smell of perspiration and animal excrements, a fact which leaves no doubt respecting its origin.” It is owing to the presence of carbonate of ammonia that rain water owes its _softer_ feel than pure distilled water. According to Liebig, it is the atmospheric ammonia which furnishes the nitrogen of plants. The traces of nitric acid which have been detected in the air, are referable to the oxidation of the constituents of ammonia; and not to the direct union of the oxygen and free nitrogen of the atmosphere. Dr. Pereira states that a carbonaceous (sooty) substance, and traces of sulphates, chlorides, and calcareous matter, are the usual impurities of the first rain water of a shower. Zimmerman found oxide of iron and chloride potassium in rain water; other chemists have been able to detect no iron in it, but have found meteoric iron and nickel in dew. Brande detected in it, chloride of sodium, chloride of magnesium, sulphate and carbonate of magnesium, sulphate of lime, and oxide of manganese. The putrefaction to which rain water is subject, shows that some organic matter is present. The term _pyrrhin_ (from πυρρος red) has been applied by Zimmerman to an atmospheric organic substance which reddens solutions of silver. Whenever rain water is collected near large towns, it should be boiled and strained before use, as it contains less saline impregnation than other kinds of natural waters, it is more apt to become contaminated with lead from roofs, gutters, cisterns, and water pipes. To purify rain water and render it useful, for the delicate purposes of chemical experiment, Morveau recommends dropping into it a little barytic water and then exposing it for some time to the atmospheric air. This combines with the carbonic acid, which being the solvent of the carbonate of lime, both it and the carbonate of baryta are precipitated as insoluble salts. Instead of exposing it to the atmosphere, it may be poured from one vessel to another; by which means not only the minute