This is the purest kind of natural water, although it is by no means free from foreign ingredients, as an analysis of a sample collected in a clean vessel, upon which it is incapable of acting, shows. The gases to be found are mainly oxygen, nitrogen and carbonic acid, as well as, occasionally, traces of certain sulphur compounds, such as sulphurous anhydride and sulphuretted hydrogen, the former derived principally from the combustion of coal, and the latter from the decomposition of organic bodies. In addition to these, sometimes there is present animal and vegetable matter, as well as definite inorganic compounds that may chance to be floating in the atmosphere.

Carbonators are not wanting who favor rain-water as the best and purest water obtainable, and its gas-taking quality recommends it at all times, but the difficulty of obtaining it in sufficient quantity is a drawback to its universal use. Of all natural waters rain-water contains the smallest proportion by weight of dissolved substances, averaging from two to three grains per gallon. The first portions of rain which fall after dry weather contain, in districts remote from towns, the dust of the district raised by wind, or dust and saline matter brought from a distance by wind. The first collections of rain near a town may contain particles of soot or ashes. Collected from the roofs of houses rain may contain also twigs, moss, leaves, and products of the woody tissues, as well as the dust of mortar and all kinds of impurities. Most of these substances will be in suspension in the rain-water; but in true solution, besides the saline matters from the lighter ashes discharged from chimneys, traces of hydrochloric acid and sulphuric acid may be present, products of chemical decompositions in factories, or, in the case of sulphuric acid, products of the combustion of sulphur, etc., in coals. After a thunder-storm minute amounts of nitric acid may be found in rain, a product, probably, of the combination of the nitrogen and oxygen of the air under the influence of the electric current. Ammonia appears to be a constant constituent of the air, and therefore is a constant constituent of rain-water; usually in chemical union with one of the acids mentioned. Besides these solid matters rain-water contains the gases of the air, ten gallons holding in solution usually about a pint and a quarter of nitrogen, less than a pint of oxygen and little more than an eighth of a pint of carbonic acid gas.

When rain is to be used for drinking and carbonating purposes great care should be observed in its collection, storage, etc. Usually it will be collected from roofs. Trees should not overhang the roofs. The presence of birds should be discouraged. The roofs should be kept free from collections of moss, etc. Gutters should be periodically brushed out. Means should be provided for preventing the collection of the first runnings after dry weather. If arrangements can be adopted for filtering the supply through a cubic yard or two of clean gravel, and afterwards through a cubic foot or two of charcoal, good well-carbonated water may be obtained. Other filters may be used. Tanks should be above ground and covered, or, if below, be of brickwork set in cement and plastered over with cement. The use of lead-lined tanks and leaden pumps should be avoided, for soft water is liable to attack lead and dissolve enough to render the water harmful; an iron pump may be employed. In some sections of the country the bottlers are largely dependent on stored rain for their supplies of water, and when the reservoirs are small, crude, and underground tanks, the water often becomes impure to a revolting degree.