of humidity in different places and in the same place with different weather.
The products of combustion, however, can hasten or retard the effects of the humidity of the air. For example, the most potent factor in hastening rusting is one of the products of combustion of the cap composition, and not of the powder, as is popularly supposed. Some types of caps are primed with a composition which contains potassium chlorate. On the cap being fired this substance gives up its oxygen very readily and becomes potassium chloride, and this potassium chloride is deposited in the bore.
Now potassium chloride is a salt very similar to ordinary table salt or sea salt (sodium chloride) and like common salt has an affinity for water. Everyone knows how iron and steel will rust much more readily at the seaside, the reason being the presence of the salt which attracts the moisture from the air on to any surface on which it is deposited. In exactly the same way the potassium chloride in the bore attracts moisture from the air, and thus gready helps the formation of rust in the bore.
But the proportion of potassium chlorate in the cap composition varies in different cartridges, and in some caps it is entirely omitted. Consequently the amount of potassium chloride deposited in the bore will depend on the type of cartridge used. Further, in recent years potassium chlorate has been replaced by barium nitrate and frequently the mercury fulminate by lead styphnate, while a very complicated substance known as "tetracene ' is added to ensure sensitivity.
Similarly the compositions of powders differ from each other and consequently their products of combustion. In many powders these products are neutral, that is neither acid nor alkaline. In a few powders they may be
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