When tinned iron serves for containing alimentary matters, it is essential that the tin employed should be free from lead. The latter metal is rapidly oxidized on the surface and is dissolved in this form in the neutral acids of vegetables, meat, etc. The most exact method of demonstrating the presence of lead consists in treating the alloy--so-called tin--with aqua regia containing relatively little nitric acid. The whole dissolves; the excess of acid is driven off by evaporation at a boiling heat, and the residue, diluted with water, is saturated with hydrogen sulphide. The iron remains in solution, while the mixed lead and tin sulphides precipitated are allowed to digest for a long time in an alkaline sulphide. The tin sulphide only dissolves; it is filtered off and converted into stannic acid, while the lead sulphide is transformed into sulphate and weighed as such.