The Optical Magic Lantern Journal (May 1895)

The Optical Magic Lantern Journal and Photographic Enlarger. 87 water is gathered. It is always best in collecting rain-water from any source to allow the first of the shower to pass away into the drains before reserving any for photographic use. If this is done, and the water well filtered before use, it will be found tolerably pure, and good enough for most photographic operations. The most important photographic use, necessitating pure water, is making solutions of silver nitrate, in which case any contamination of organic, or saline matter would cause decomposition. If it so happens that there is no choice of water, and a known impure sample is to be used, the best plan is to first boil it in order to precipitate any lime, afterwards add a few grains of silver nitrate and expose to the brightest sunlight available for a day or two, the organic matter will by this means be precipitated as a black mud, and, although the water thus treated will not be absolutely pure, still it will be sufficiently go for most purposes, after filtering, in which silver nitrate has to be dissolved. Spring water, and river water, are next in order of purity, but (there is a great deal in this ‘‘but”’) it altogether depends on the soil through which the spring flows. Springs are caused by rain fall on higher grounds through which they filter, and in their course dissolve any soluble matter in their passage. In chalky soil, spring water is saturated with lime, but is tolerably free from organic impurities. If we take such water, clear and sparkling as it may appear, for development in which oxalates are used, we immediately get into difficulties by a precipitation of oxalate of lime in our films, which causes a granular muddy appearance, unfitting the , ' curdle a solution of soap. Hard water, which transparency for lantern work; in fact, any development in which lime-salts are decomposed, or precipitated, indicates that such water should be deprived of its lime by boiling for a considerable time. Lime, however, is not, the only substance likely to be found in spring water, chlorides of various kinds, metallic impurities, more especially iron, magnesium, and soda, as we see by our medicinal and chalybeate springs. The earth surrounding and over which the spring flows, when iron is contained in solution, proclaims its presence by the rusty colour of the precipitated oxides. Sulphates and chlorides are, of course, not noticeable in this manner, by taste when they occurin important quantity. Hydrates are frequently present ; in fact, almost always, their quantity or character depending mostly on the soil through which the water flows. Water is one of the most powerful solvents known, and at various temperatures few things are not more or less soluble in it; even but detectable | ; tinually goes glass is, by its long continued action at a high temperature, corroded away. It is, therefore, easily understood how it becomes almost impossible for such a fluid to be anything like pure in a natural state. Water proceeding from glaciers and snow on lofty mountains is its purest form. River water is a mixture of drainage and spring water, and of necessity much more impure, being additionally contaminated with organic matter in all forms. Putrefactive and excretive matter is being continually added, but by a fortunate dispensation of Nature they soou become oxidised and deposited as mud, forming a nutritious soil for plants and vegetation of various kinds, the plants in their turn further purifying the water in which they grow by the absorption of various earthy salts that are contained in solution. So the process conon — addition, decomposition, absorption, and purification, until the main bulk of the stream becomes bright and useable. The pollution of the water by manufactories is sometimes carried to such an extent that a river becomes little better than a sewer. Water from such’ a source is evidently useless for photographic purposes, aS no amount of filtering or subsidence will remove the obnoxious matter, distillation being the only possible means of making it suitable. Rivers flowing through sandy and gravelly soils are generally purer than those where the bed is of a clayey or chalky character. River water is usually termed soft; that is, the sulphates contained in it are insufficient to contains a superabundance of these salts, has an opposite effect. Well water varies in its composition more perhaps than that from any other source; the depth of the well and kind of soil in which it is sunk, and the various sources of contamination that are in the vicinity which percolate into it. As a general rule, well water obtained from diluvial gravel above the clay is more impure than from the green sand at a lower level, and water from wells sunk in the London clay are most impure of any, and contain on an average three times as much solid matter as deep Thames water. A good, pure, natural water is without odour, although the finer senses of some animals can detect it at a distance. Muddy water may be cleared by adding two or three grains of alum to a quart of it, then allowing it to subside. This effect is produced by the alum combining with the lime in solution, and forming an insoluble precipitate, which carries down other