DISSOLVING CHEMICALS There is a right and a wrong way of dissolving and mixing most chemicals, and in some cases the method of dissolving affects the working powers of the solution. The slowest and possi bly the worst way of dissolving a chemical is to place it in a bottle of liquid and let it stand. The quickest way is to have the salt, particularly soda, near the surface of the water and suspended therein in a canvas bag. However, for most solutions, the simple addition of chemicals, when not in large quantities, and frequent shakings will be sufficient. Chemicals usually dissolve more quickly in hot water than in cold, though there are some notable exceptions to this rule. (See " Solubilities.") The order in which chemicals are dissolved or mixed has its influence on the working and keeping qualities of the solution. As a general rule, ingredients are added in the order given in the otherwise special instructions are given. The following general hints should be noted :—Metol should be dissolved in the water before any other chemicals are added. Hydro quinone should always be dissolved after sodium sulphite. When pyrogallic acid is used, the preservative—acid, metabisulphite, etc.—should be dissolved in the water before the pyro. When gold toning baths are made, the gold should always be added last, dissolved in water. Sul phuric acid should always be added gradually to the water and not water to the acid, otherwise combination takes place so energetically as almost to resemble an explosion, and some of the acid may be driven in the face or over the clothes. The list of solubilities presented in the table that is given under the heading "Solubilities " will assist in deciding whether much advantage will be gained by using hot or cold water. In some cases the temperature of the water is not of much importance. Developers such as pyro, metol, adurol, etc., are best dissolved in water that has been well boiled in order to get rid of the air, and then allowed to cool slowly.
These are obtained by the use of two lanterns, one above the other, or placed side by side. The discs of light projected by the two lanterns must coincide. Whilst a picture is being shown in the first lantern, the illuminant is quite low in the second one. A slide is placed in the second lantern, and then the light of the first one is gradually lowered whilst that of the second one is gradually turned up ; the first picture dis solves away during the blending of the lights, and at the same time the second picture makes its appearance on the screen, taking the place of the first. To facilitate the operation (especi ally when lime-light is in use) the rubber tubing from the two jets is connected to a duplex gas cock having four ways or channels, through which the gases may be made to pass. Thus, the oxygen and hydrogen of one illuminant is partially diverted or reduced, whilst the tap allows a full supply to pass to the other jet.
Dissolving effects are sometimes produced with bi-unial lanterns with their objectives furnished with iris-diaphragms opened and closed alternately. This method is very effec. tive, and saves having to interfere with the adjustment of the illuminants.
Dissolving effects of a kind may be produced by means of a single lantern. The special carriers for obtaining such effects are divided into two classes. In one, a translucent screen gradually obscures the light, and just at the moment when the obscurity is complete, the slide is quickly changed by means of a carrier of the " to and fro " type. In another class, a second slide is pushed into a carrier which already holds a slide that is being shown ; the second elide passes in front of the first, the latter is withdrawn, and then, by means of a spring, the out-of-focus slide is quickly pushed back into the place previously occupied by the first slide. This form of carrier was invented by R. R. Beard, and still enjoys popularity.