Take what quantity of Lead you please; melt it in one or more unglazed earthen pans: a dark grey powder will be found on its surface. Keep stirring the metal incessantly till it be wholly converted into such a powder, which is the Calx of Lead.
OBSERVATIONS.
As Lead is a very fusible metal, and in that respect greatly resembles Tin, most of the observations we made on the calcination of Tin may be applied here.
In the calcination of all metals, and particularly in this of Lead, there appears a singular phenomenon which is not easily accounted for. It is this: though these matters lose a great deal of their substance, either by the dissipation of their phlogiston, or because some of the metal, perhaps, exhales in vapours, yet when the calcination is over their calces are found to be increased in weight, and this increase is very considerable. An hundred pounds of Lead, for example, converted into Minium, which is nothing but a calx of Lead brought to a red colour by continuing the calcination, are found to gain ten pounds weight; so that for an hundred pounds of Lead we have one hundred and ten pounds of Minium: a prodigious and almost incredible augmentation, if it be considered that, far from adding any thing to the Lead, we have on the contrary dissipated part of it.
To account for this phenomenon Natural Philosophers and Chymists have invented several ingenious hypotheses, but none of them entirely satisfactory. As we have no established theory to proceed upon, we shall not undertake to explain this extraordinary fact.
PROCESS IV.
To prepare Glass of Lead.
Take two parts of Litharge, and one part of pure crystalline Sand; mingle them together as exactly as possible, adding a little Nitre and Sea-salt: put this mixture into a crucible of the most solid and most compact earth. Shut the crucible with a cover that may perfectly close it.
Set the crucible thus prepared in a melting furnace; fill the furnace with coals; light the fire gradually, so that the whole may be slowly heated: then raise the fire so as to make the crucible very red, and bring the matter it contains into fusion; keep it thus melted for a quarter of an hour.