FIXING, WASHING, AND DRYING 397 



public sources of supply may contain colloidal matter and dissolved salts which are not 

 removed by filtration, and certain municipalities intentionally add chlorine or other 

 substances as public health protective measures. These constituents of water may or 

 may not be objectionable from the photographic standpoint, although they are not 

 desirable. 



Wherever possible, and certainly when any doubt exists concerning the suitability 

 of the water supply for photographic purposes, photographic solutions and especially 

 developing solutions should be prepared with distilled water. If this is impracticable, 

 the solutions can be prepared with water which has been boiled. 



Solutions are usually made by adding chemicals to a quantity of water called for 

 by the formula. In some cases, however, water is added to a solution to obtain a 

 given concentration or volume of solution, and in this case the final volume should 

 be that called for in the formula. 



Chemical analj'sis of the water supply may be useful in determining the amount 

 of oxalate or lime which is to be added to coagulate slimes, to remove dissolved calcium 

 salts, or in determining which of several water supplies is most likely to be free from 

 dissolved or colloidal material. The greater the quantity of undesired products found 

 in the water supply, the greater may be the difficulty expected from drying marks, and 

 the less certain one can be of the purity of compounded solutions. Water supplies 

 containing iron, metallic sulphides, or hydrogen sulphide should be avoided. But 

 chemical analysis does not usually reveal much information as to the suitability of a 

 water supply for photographic purposes, and trial tests with sample solutions and 

 actual photographic materials are the only reliable means of determining the suit- 

 ability of a certain water supply for photographic purposes. Two solutions should 

 be prepared, one with distilled water and one with the water supply under considera- 

 tion. Samples of the film are then processed in the two solutions, the one made with 

 distilled water being used as a control. Comparisons of the processed films will then 

 show what, if any, undesirable effects may be expected from the water supply under 

 consideration. Another simple test which indicates the extent of the trouble which 

 might be anticipated due to residual scum may be made by permitting a large drop of 

 water to dry on a film; water suitable for photographic use should leave no scum or 

 stain. 



Water exists in various states of impurity, and while anj^ source of water may be 

 purified, purification is a time-consuming and expensive process which may not be 

 economically justified if the original source is reasonably satisfactory for photographic 

 purposes. Provided the distillation is carried out properly and with clean apparatus, 

 distilled water is the purest form of water supply. Next, in order of purity, may be 

 mentioned: (a) rain water collected on a clean surface and stored in clean containers, 

 (b) melted snow and hail, (c) spring and well water, (d) water from flowing rivers, 

 streams, and brooks, and (e) sea water. 



The impurities which may be present in the various kinds of water are dissolved 

 salts, such as bicarbonates, chlorides, and sulphates of calcium, magnesium, and 

 potassium. If iron is present to any considerable degree, the water should not be used 

 for photographic purposes. The water may also contain solid impurities, such as 

 silt, and, while these are not usually so injurious as iron or various salts, they may be 

 readily filtered out. Suspended matter, such as iron rust, dirt, or silt , should be removed 

 by filtration, as otherwise these particles may settle and cause spots on negatives or 

 prints processed in solutions made with such water. The water may also contain clay, 

 slime, or colloidal animal or vegetable matter which is not removed by filtration. If 

 such water is used in the preparation of photographic solutions, the undesired particles 

 may gradually coagulate and settle as a sludge. Dissolved gases are sometimes 

 encountered in water supplies. Hydrogen sulphide is objectionable in that it causes 



