The active material in most of the self-lighting incandescent inirners is the metal platinum in some form or other and the reason for its employment depends on the fact that it is capable of condensing either on its surface or in its pores a large amount of oxygen, the latter being derived from the air; the result of this is thai when a gas such as hydrogen is brought in contact with the metal the two gases unite and in time chemical action ensues. Now coal gas contains, roughly speaking,about 50 per cent, of hydrogen by volume, so in allowing a stream of coal gas to impinge on a pellet of specially prepared platinum the heat generated is sufficient to ignite the gas. The form in which the platinum is usually employed is that known as spongy platinum, and is obtained by dissolving metallic platinum in aqua regia (nitric and hydrochloric acids), which converts it into perchloride of platinum (PtCl) the solution is then mixed with chloride of ammonia, which combines with the perchloride of platinum to form a yellow insoluble salt (ammonio-chloride of platinum). This precipitate is collected on a filter, washed, and then heated very gently in a stream of coal gas as long as any fumes of hydrochloric acid are evolved.

The spongy platinum thus obtained can then be used in the form of pellets, either alone or mixed with other substances.