In photographic formulas, and many of the processes, the requirement of pure or distilled water is general. To secure good results, it is absolutely necessary that this condition be observed ; and it easily may be, if the following directions are observed.

The best water is that which has been distilled and bottled as soon as cool enough. Some druggists will supply this at a very moderate cost, but in purchasing such water make sure that it has not been exposed to the air, so that it has again absorbed oxygen. With some chemicals, the oxygen in the water will cause oxidization that will be harmful. Hence the necessity of keeping the water used in making up solutions in full bottles, well corked. A few bottles of various sizes will enable this to be done without excessive trouble.

If distilled water is not available, rain water collected in porcelain or glass vessels may be used. It should be collected in an open space, after sufficient rain has fallen to clean the air, and not from the roof of the building or waterspout, for such water contains dust and other impurities. It should then be boiled for at least ten minutes in an earthenware vessel, to expel all the air, and bottled when cool enough to avoid breaking the bottles.

At times the weather bureau may not provide an opportune rain, and resort must be had to other sources. If the regular water supply is not too " hard," that is, has in solution much lime or other mineral impurities, it may be used after boiling and bottling, as above directed.

Bottling should follow boiling as soon as possible, as the water, if left in an open vessel, soon absorbs oxygen in considerable quantity.