Thus, smoke has already begun to complicate, and in the future will complicate still more, every phase of fighting. It will be used for deception, for concealment, for obscuring vision, for signalling and to hide deadly gases. The signal rocket will be used to start battles, change fronts, order up reserves, and finally to stop fighting. The signal smokes by day will be displaced at night by brilliantly colored lights which will have the same meaning as similarly colored smokes during the day. Thus, literally, smoke in the future will be the cloud by day and the pillar of fire by night to guide the bewildered soldier on the field of battle with all its terrors and amidst the confusion, gas, smoke and dust that will never be absent while battles last. CHAPTER XX INCENDIARY MATERIALS Since it is generally known that white phosphorus, when exposed to the air, takes fire spontaneously, it logically follows that numerous suggestions should have been made for using this material in incendiary devices. Practice, however, has shown that, while phosphorus is undoubtedly of value against very easily ignitable materials, such as hydrogen in Zeppelins, or the gasoline tanks of aeroplanes and dry brush or grass, it is of much less value when wood and other materials are considered. This is partly because of the low temperature of burning, and partly because the product of combustion (phosphoric anhydride) is really an excellent fireproofing substance. In view of this, phosphorus was used primarily for smoke production. A superior incendiary material is found in thermit, a mixture of aluminum and iron oxide. When ignited, it produces an enormous amount of heat very quickly, and the molten slag that results from the reaction will prolong the incendiary action upon inflammable materials. When used alone, however, it has the disadvantages that the incendiary action is confined to a small area and that the heat energy is wasted because of the fact that it is so rapid in its action. For this reason it is customary to add a highly inflammable material, which will become ignited by the thermit and will continue the conflagration. Petroleum oils, carbon disulfide, wood distillation products and other inflammable liquids were thoroughly tested for this purpose. The final conclusion was reached that oil, solidified with soap (sodium salts of the higher fatty acids) by a special method developed by the Chemical Warfare Service, was by far the best material to be used. In certain tests, using a combination of thermit and solid oil, flames fifteen feet high were obtained, which would be very useful against walls, ceilings, etc.