The next process is to sort the good shot for size. This is done by sieves—one having holes a little larger than the size of shot required. This sieve passes through it all of the right size and smaller, and keeps the bigger ones. Those that have passed this examination are then put into another sieve, which has holes in it a little smaller than the size of shot wanted. This sieve retains the right shot, and lets the smaller sizes pass, and so on. The shot are sized and numbered, glazed by rolling them in a barrel with graphite, and then they are ready for use. Bullets are made by machinery by the thousand, and made up into cartridges with great speed.
Fig. 413.—Sieve for making shot.
The compounds of lead are also poisonous, and produce “colic,” to which painters are subject. Red lead, or minium, is a compound of the protoxide and the binoxide, and may be found native. The former oxide is litharge; white lead, or the carbonate of lead, is a paint, and is easily obtained by passing a stream of carbonic acid into a solution of acetate of lead. It is used as a basis of many paints.
Fig. 414.—Section of shot tower.
Tin is another well-known metal. It is mentioned by Moses. It possesses a silver-like lustre, and is not liable to be oxidised. The only really important ore is called Tinstone, from which the oxygen is separated, and the metal remains. Cornwall has extensive tin mines. Tin is malleable and ductile, and can be beaten into foil or “silver leaf,” or drawn into wire. It prevents oxidation of iron if the latter be covered with it, and for tinning copper vessels for culinary purposes. The Romans found tin in Cornwall, and the term “Stanneries” was applied to the courts of justice among the tin miners in Edward the First’s time. We have already mentioned the alloys of tin. The oxides of tin, “Stannous” and “Stannic,” are useful to dyers. The latter is the tinstone (SnO2). Sulphide of tin is called “Mosaic gold,” and is much used for decorative purposes.
Fig. 415.—Preparing lead for bullets.
Zinc is procured from calamine, or carbonate of zinc, and blende, or sulphide of zinc. It has for some years been used for many purposes for which lead was once employed, as it is cheap and light. Zinc is a hard metal of a greyish colour, not easily bent, and rather brittle; but when made nearly red-hot, it can be rolled out into sheets or beaten into form by the hammer. Zinc is about six-and-three-quarter times heavier than water. Like many other metals, it is volatile (when heated to a certain extent it passes off into vapour), and the probable reason that it was not known or used of old is that it was lost in the attempt to smelt its ores. Zinc is now obtained by a sort of distillation; the ores are mixed with the flux in a large earthen crucible or pot.