In addition to this type of incendiary material, it was desirable to have a spontaneously inflammable mixture of oils, which could be used in Livens’ shell, Stokes’ shell or aeroplane bombs. The basis of these mixtures is fuel oil and phosphorus. By varying the proportions of the constituents it is possible to obtain a mixture that will ignite immediately upon exposure to the air, or one that will have a delayed action of from 30 seconds to two minutes. The incorporation of metallic sodium gives a mixture that will ignite when spread upon water surfaces. INCENDIARY DEVICES The incendiary devices used during the late war included: bombs, shell, tracer shell and bullets, grenades, and flame throwers. BOMBS Incendiary bombs were used almost exclusively by aircraft. The value of bombs which would cause destruction by starting conflagrations was early recognized but their development was rather slow. While the designs were constantly changing, two stand out as the most favored: a small unit, such as the Baby Incendiary Bomb of the English, and a large bomb, such as the French Chenard bomb or the American Mark II bomb. In general bombs which, when they function upon impact, scatter small burning units over a considerable area, are not favored. Small unit bombs can be more effectively used because the scatter can be better regulated and the incendiary units can be more advantageously placed. =German Bombs.= Incendiary bombs were used by the Germans in their airplane raids, usually in connection with high explosive bombs. A typical armament of the later series of German naval airships consisted of the following: 2 660-pound bombs 10 220-pound bombs 15 110-pound bombs 20 Incendiary bombs making a total weight of about 2½ tons. [Illustration: FIG. 107.—Incendiary Devices. (From Left to Right). Mark II Bomb, B. I. Bomb, Mark I Dart, Mark II Dart, Mark I Dart, Grenade, Mark I Bomb.] A typical German bomb is shown in Fig. 108. It consists essentially of a receiver of white iron (R) composed of a casing and a central tube of zinc, joined together in such a fashion that, when the whole was complete, it had the appearance of an elongated vessel with a hollow center. Within this central hollow is placed a priming tube (T) of thin sheet iron, pierced by a number of circular openings. The receiver is about 445 mm. (17.5 in.) high and 110 mm. (4.3 in.) at its maximum diameter. It is wrapped with strands of tarred cord over nearly its entire length. The empennage (270 mm. or 10.6 in.—in height) consisted of three inclined balancing fins, which assured the rotation of the projectile during its fall. In the body of the bomb was a viscous mass of benzine hydrocarbons, while the lower part of the receiver contained a mixture of potassium perchlorate and paraffin. The central tube apparently contained a mixture of aluminum and sulfur. [Illustration: FIG. 108.—Aerial Incendiary Bomb, November, 1916.]