This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
266. Good results can be produced with the ready prepared powders and solutions, and for the beginner's first experiments they are recommended, as he will, without doubt, secure better results with them. However, the experience gained from the preceding instruction will enable you to readily recognize the necessity of knowing the action of the different chemicals employed and the manner of preparing the various ingredients for the developing solutions. The following instruction is, therefore, given, not alone that you may learn how to prepare these solutions, but the experience you will gain thereby will prepare you for the more advanced instruction in Volume II.
267. Don't measure your sodas by weight; use the hydrometer for testing them and you will always have uniform results. A hydrometer can be procured from any dealer in photographic supplies. If sodas in crystal form should dry to a powder, by exposure to air, or vice versa, the weight would be altered, although the strength of the original quantity would remain the same. Consequently a solution made by dissolving 1 oz. of the dry powder would be stronger than that made by dissolving 1 oz. of crystals in the same quantity of water. Also sodas of different brands, and even of the same brand, but procured at different times, are seldom of uniform strength. So if used by weight instead of hydrometer you will be apt to meet with frequent failures.
Hydrometer. The hydrometer used in photography is an instrument for measuring the number of grains of a certain chemical in an ounce of water. The instrument consists of a glass tube, near the bottom of which are two bulbs. The upper bulb is filled with air, the amount of which is sufficient to make the whole instrument lighter than an equal volume of water. The lower and smaller bulb is loaded with mercury, in order that the instrument may remain in a vertical position when placed into the liquid to be tested. (See Illustration No. 30.)
Illustration No. 30.
269. The point to which the hydrometer sinks when placed in clear water is marked zero, the tube being graduated below this point in such a manner that the specific gravity of the liquid can be read. The specific gravity of a body is the proportion between its weight and the weight of a like volume of pure water.
How To Compound A Formula. First, be sure that your graduates, stirring rods, and any receptacles you may use are perfectly clean. (For methods of cleaning glassware we refer you to the Glossary, Volume X.) The water you use should not be taken fresh from the tap, but should be allowed to stand for some little while, to permit the air to escape. If scales are used for weighing out the chemicals, care should be taken that they are perfectly clean and work accurately. Always add the chemicals one to the other in the sequence given in the formula, and see that one chemical is thoroughly dissolved before adding another. Keep the chemicals you use always in tightly corked bottles or tins, and be careful to label all your bottles so that you should not, by accident, get hold of the wrong solutions.