METHOD OF WASHING There are several excellent washing machines on the market, but at this stage the prints must not be left un attended, and washing by hand is prefer able. For twelve prints, half-plate size, take two porcelain dishes, which must be chemically cleaned, filling each with clean water. There is a distinction be tween ordinary cleanliness and chemical cleanliness. To ensure the latter, it is a good plan to make up a solution of potas sium bichromate 1 part, sulphuric acid 1 part, and water 20 parts. This may be used repeatedly. Pour it into the dish or measure, and rinse round. Then wash freely in water. This mixture must not be allowed to get on the clothes or fingers. The dishes should be deep, and at least a size or two larger than the prints, say 8 n. by a in. Place six prints, one at a time, in the first dish, keeping the water steadily moving. The latter will speedily be come like milk, in consequence of the rapid formation of silver chloride by the combination of the soluble chlorides and silver nitrate. _After a minute, transfer the six prints, separately, into the second dish, pour off the milky water, and refill. When another two minutes have elapsed, transfer the prints back to the first bath. Pour off the second milky water, and refill, placing the second six prints in the clean water. The water in the first dish will be
now found to be much less affected than before, which shows that most of the soluble salts have been eliminated. These first prints must be removed to another dish, and the whole so interchanged that they have each at least six separate changes of water. The prints should be left in the last. three changes for a longer time than in the early ones, as while the free chemicals are liberated quickly at first, the action is not thorough, and the final removal takes place gradu ally. The prints should be placed in the water face upwards, although if they are constantly moved about, and care taken that they do not touch one another, this is not very important. The last water should remain quite clear. The time taken with the six washings is about ten to fifteen minutes. Some workers prefer to use rain-water in which has been dis solved a small quantity of common salt. This certainly has the advantage that the printer knows exactly what he is using—a state of things not existing when ordinary water is used, in which a variety of earthy salts are dissolved. It is, how ever, an unnecessary refinement.