_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. Black Varnish. _Asphaltum, dissolved in Spirits or Oil of Turpentine._--The asphaltum may be coarsely pulverized and put into a bottle containing the turpentine, and in a few hours, if it be occasionally shaken, it will be dissolved and ready for use. It should be of about the consistency of thick paste. I use the above, but will now give two more compositions, for any who may wish to adopt them: