91._] _Experiment_ .-- Ordinary lead is calcined in a cupel made of cinders or powdered bones; the lead is changed to a cinder which disappears into the cupel, and a button of silver remains.
_Conclusion_ .-- The lead has vanished; what more natural than the conclusion that it has been transformed into silver?
It was not known then that all specimens of lead contain more or less silver.
[Illustration: FIG.

X._See p.

92._] _Experiment._-The vapour of arsenic bleaches copper.

This fact gave rise to many allegories and enigmas concerning the means of transforming copper into silver.
Sulphur, which acts on metals and changes many of them into black substances, was looked on as a very mysterious thing.

It was with sulphur that the coagulation (solidification) of mercury was effected.
_Experiment_ .-- Mercury is allowed to fall, in a fine rain, on to melted sulphur; a black substance is produced; this black substance is heated in a closed vessel, it is volatilised and transformed into a beautiful red solid.
One could scarcely suppose that the black and the red substances are identical, if one did not know that they are composed of the same quantities of the same elements, sulphur and mercury.
How greatly must this phenomenon have affected the imagination of the chemists of ancient times, always so ready to be affected by everything that seemed supernatural! Black and red were the symbols of darkness and light, of the evil and the good principle; and the union of these two principles represented the moral order.