Water is an oxide of hydrogen, containing single atoms of each of the gases.
Distilled water is water which has been vaporized and again condensed: by this means it is freed from earthy and saline impurities, which, not being volatile, are left in the body of the retort. Pure distilled water leaves no residue on evaporation, and should remain perfectly clear on the addition of nitrate of silver, even when exposed to the light; it should also be neutral to test-paper.
The condensed water of steam-boilers sold as distilled water is apt to be contaminated with oily and empyreumatic matter, which discolors nitrate of silver, and is therefore injurious.
Rain-water, having undergone a natural process of distillation, is free from inorganic salts, but it usually contains a minute portion of ammonia, which gives it an alkaline reaction to test-paper. It is very good for photographic purposes if collected in clean vessels, but when taken from a common rain-water tank should always be examined, and if much organic matter be present, tinging it of a brown color and imparting an unpleasant smell, it must be rejected.
Spring or river water, commonly known as "hard water," usually contains sulphate of lime, and carbonate of lime dissolved in carbonic acid: also chloride of sodium in greater or less quantity. On boiling the water, the carbonic acid gas is evolved, and the greater part of the carbonate of lime (if any is present) deposits, forming an earthy incrustation on the boiler.
In testing water for sulphates and chlorides, acidify a portion with a few drops of pure nitric acid, free from chlorine (if this is not at hand, use pure acetic acid); then divide it into two parts, and add to the first a dilute solution of chloride of barium, and to the second nitrate of silver,—a milkiness indicates the presence of sulphates in the first case or of chlorides in the second. The photographic nitrate bath cannot be used as a test, since the iodide of silver it contains is precipitated on dilution, giving a milkiness which might be mistaken for chloride of silver.
Common hard water can often be used for making a nitrate bath when nothing better is at hand. The chlorides it contains are precipitated by the nitrate of silver, leaving soluble nitrates in solution, which are not injurious. The carbonate of lime, if any is present, neutralizes free nitric acid, rendering the bath alkaline in the same manner as carbonate of soda. Sulphate of lime, usually present in well water, is said to exercise a retarding action upon the sensitive silver salts, but on this point the writer is unable to give certain information.
Hard water is not often sufficiently pure for the developing fluids. The chloride of sodium it contains decomposes the nitrate of silver upon the film, and the image cannot be brought out perfectly. The New River water, however supplied to many parts of London, is almost free from chlorides and answers very well. In other cases a few drops of nitrate of silver solution may be added to separate the chlorine, taking care not to use a large excess.