CHAP. 49.—BLACK LEAD.
Black lead1724 is used in the form of pipes and sheets: it is extracted with great labour in Spain, and throughout all the Gallic provinces; but in Britannia1725 it is found in the upper stratum of the earth, in such abundance, that a law has been spontaneously made, prohibiting any one from working more than a certain quantity of it. The various kinds of black lead are known by the following names—the Ovetanian,1726 the Caprariensian,1727 and the Oleastrensian.1728 There is no difference whatever in them, when the scoria has been carefully removed by calcination. It is a marvellous fact, that these mines, and these only, when they have been abandoned for some time, become replenished, and are more prolific than before. This would appear to be effected by the air, infusing itself at liberty through the open orifices, just as some women become more prolific after abortion. This was lately found to be the case with the Santarensian mine in Bætica;1729 which, after being farmed at an annual rental of two hundred thousand denarii, and then abandoned, is now rented at two hundred and fifty-five thousand per annum. In the same manner, the Antonian mine in the same province has had the rent raised to four hundred thousand sesterces per annum.
It is a remarkable fact, that if we pour water into a vessel of lead, it will not melt; but that if we throw into the water a pebble or a copper quadrans,1730 the vessel will be penetrated by the fire.