The magnesium is coated with hot wax or paraffin, which not only acts as a binder for the mixture when it is pressed into its container but also serves to prevent oxidation of the magnesium when the shells are stored. The barium and potassium nitrates supply the oxygen to the magnesium, which burns with a brilliant white flame. The potassium nitrate takes fire more readily than the barium nitrate, but it is more expensive than the latter.
Owing to the cost of magnesium, powdered aluminum has been used to some extent as a substitute. Aluminum does not have the illuminating value of magnesium and it is more difficult to ignite, but it is a good substitute in case of necessity. An English mixture containing these elements is,
| Barium nitrate | 58 per cent. |
| Magnesium | 29 per cent. |
| Aluminum | 13 per cent. |
Mixtures which are slow to ignite must be supplemented by a primary mixture which is readily ignited. For obtaining colored lights it is only necessary to add chemicals which will give the desired color. The mixtures can be proportioned by means of purely theoretical considerations; that is, just enough oxygen can be present to burn the fuel completely. However, usually more oxygen is supplied than called for by theory.
The illuminating shell is perhaps the most useful of these devices to the soldier. It has been constructed with and without parachutes, the former providing an intense light for a brief period because it falls rapidly. These shells of the larger calibers are equipped with time-fuses and are generally rather elaborate in construction. The shell is of steel, and has a time-fuse at the tip. This fuse ignites a charge of black powder in the nose of the shell and this explosion ejects the star-shell out of the rear of the steel casing. At the same time the black powder ignites the priming mixture next to it, which in turn ignites the slow-burning illuminating compound. The star-shell has a large parachute of strong material folded in the rear of the casing and the cardboard tube containing the illuminating mixture is attached to it. The time of burning varies, but is ordinarily less than a minute. Certain structural details must be such as to endure the stresses of a high muzzle velocity. Furthermore, a velocity of perhaps 1000 feet per second still obtains when the star-shell with its parachute is ejected at the desired point in the air.
The non-parachute illuminating shell is designed to give an intense light for a brief interval and is especially applicable to defense against air raids. Such a light aims to reveal the aircraft in order that the gunners may fire at it effectively. These shells are fitted with time-fuses which fire the charge of black powder at the desired interval after the discharge of the shell from the gun. The contents of the shell are thereby ejected and ignited. The container for the illuminating material is so designed that there is rapid combustion and consequently a brilliant light for about ten seconds. The enemy airman in this short time is unable to obtain any valuable knowledge pertaining to the earth below and furthermore he is likely to be temporarily blinded by the brilliant light if it is near him.
The rifle-light which resembles an ordinary rocket, is fired from a rifle and is designed for short-range use. It consists of a steel cylindrical shell a few inches long fastened to a steel rod. A parachute is attached to the cardboard container in which the illuminating mixture is packed and the whole is stowed away in the steel shell. Shore delay-fuses are used for starting the usual cycle of events after the rifle-light has been fired from the gun. The steel rod is injected into the barrel of a rifle and a blank cartridge is used for ejecting this rocket-like apparatus. Owing to inertia the firing-pin in the shell operates and the short delay-fuse is thus fired automatically an instant after the trigger of the rifle is pulled.
Illuminating "bombs" of the same general principles are used by airmen in search of a landing for himself or for a destructive bomb; in signaling to a gunner, and in many other ways. They are simple in construction because they need not withstand the stresses of being fired from a gun; they are merely dropped from the aircraft. The mechanism of ignition and the cycle of events which follow are similar to those of other illuminating shells.
The value of such artificial-lighting devices depends both upon luminous intensity and time of burning. Although long-burning is not generally required in warfare, it is obvious that more than a momentary light is usually needed. In general, high candle-power and long-burning are opposed to each other, so that the most intense lights of this character usually are of short duration. Typical performances of two flares of the same composition are as follows: