=Intensification and Reduction.=—These processes have been little employed in air work. Reduction is rarely necessary, for obvious reasons. Intensification would often be of value, but the common practice, which saves some time, is to use printing paper of strong contrast for those negatives which are deficient in density and contrast. When intensification is desirable or permissible, either the ordinary mercury or uranium intensifier may be used. =Water.=—In the field it is found necessary in many cases to purify the water that is to be used in mixing up chemicals. Water may contain suspended matter or dirt, dissolved salts, and slime. It is important to remove the suspended matter, as it may cause spots on the plates and papers, while any slime would coagulate, forming a sludge in the developer which would also tend to settle on the plates and cause marks during development. The dissolved salts may or may not cause trouble. Two methods of purification are possible: (_a_) Filter the water through a cloth into a barrel, add about one gram of alum for every four litres of water, and allow to settle over night. Draw off the clear liquid from a plug in the side as required. (_b_) Boil the water and allow it to cool over night. If the water contains dissolved lime, boiling will often cause this to come out of solution. V METHODS OF HANDLING PLATES, FILMS AND PAPERS CHAPTER XXIII THE DEVELOPING AND DRYING OF PLATES AND FILMS =Field Requirements.=—Developing, fixing, drying and printing in the field demand simple and convenient apparatus that may be carried about and installed with the least amount of labor. On top of these requirements military needs impose others that are more difficult. _Speed_ is, on occasion, imperative. A print may be required within a few minutes after landing, and many thousands within a few hours. Quantity production must be achieved under the most primitive conditions. Nothing, in fact, shows the calibre of the photographic officer better than his choice of workplaces as the army moves forward. Ingenuity and practical judgment are at a premium. Cellars, stables, dog kennels, or huts hastily built from packing cases, must be equipped and in working order over night. All the facilities offered by a great city are urgently needed—water, electric light, power for driving fans—but must be dispensed with if the photographic section is to be convenient to the airdrome, whose portable hangars are most apt to be pitched in the open country. Water must be carried, electricity generated, and to the photographic problem is added the military one of concealment and protection. Dugouts and bomb proofs must be built for supplies, and “funk holes” for the men. Entire underground emergency extensions have sometimes been built in stations occupied for extended periods, for airdromes are a favorite bombing target.