that the tendency in pyrotechnic compositions has been in the direction of simplification. During the eighteenth century the useless ingredients had been in a great measure eliminated. The "burnables" had been reduced from a long list of alchemistic survivals to a mere half-dozen or so.
Gums had been reduced practically to shellac alone (the use of gum arabic as an adhesive is quite distinct), carbons to lampblack and charcoal, and these with sulphur and sulphides of antimony and arsenic practically completed the list.
Of the metals the use of pure zinc, copper, and brass has been discontinued, and the two almost revolutionary additions of magnesium and aluminium made, the former about 1865 and the latter in 1894.
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.
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
