The date of the introduction of these metals marks almost as great advances in the art as did the introduction of chlorate of potash. Not only are they used as spark-producing metals in the same way as are steel and iron, but they are also used as “burnables,” that is, they are consumed inside the case; and many of the present-day firework compositions owe their brilliance to one or other of these metals. It is, however, in colour compositions that the tendency towards simplification is most strongly exhibited. In Kentish’s book colour compositions containing as many as seven or eight ingredients are common, whereas to-day formulæ containing over four are the exception rather than the rule. [Illustration: Roman Candles. This untouched photograph illustrates the extraordinary brilliance of aluminium compositions. Each “plume” (200 ft. high) is produced by a single star about ¾ in. diameter and ¾ in. long. Each line is a microscopic particle of aluminium.] The reason for this complexity is not easy to follow, but it may have been in some measure due to the difficulty of obtaining sufficiently finely ground chemicals before the days of machine grinding; in some cases it was found that by melting two of the ingredients together and allowing the mass to cool they could be ground with greater ease. Chertier went so far as to melt shellac and salt together, grind them and remove the salt by dissolving in water. Also by adding a finely ground chemical of similar action to one only coarsely ground a better result was obtained. Whatever may have been the reason, there can be no doubt that, except for secondary shades, the fewer the chemicals used the more brilliant will be the resulting fire. CHAPTER IX MILITARY PYROTECHNY The use of pyrotechnic mixtures for military purposes is the basis of artillery, and one might almost say the foundation of chemistry. Before the age of the alchemist men were at work endeavouring to produce some weapon which would give them an advantage over their enemies. Of the natural phenomena none made so strong an appeal as fire, which from earliest times had been a mysterious and therefore terrible element. The early use of fire or pyrotechnic mixtures gave the users so decided an advantage over their enemies that their use was chronicled by historians of the day either on the side of the victors as a pæan of praise for their invincible weapon, or as an excuse for defeat on the side of the vanquished. Such reports are necessarily vague and exaggerated, vague because the writer had no technical knowledge of the subject, and the users naturally wished for secrecy and exaggerated because exaggeration increased the value of the weapon.