LECTURE XXIV.
Of Lead.
Lead is a metal of a bluish tinge, of no great tenacity, but very considerable specific gravity, being heavier than silver. It melts long before it is red hot, and is then calcined, if it be in contact with respirable air. When boiling it emits fumes, and calcines very rapidly. It may be granulated by being poured into a wooden box, and agitated. During congelation it is brittle, so that the parts will separate by the stroke of a hammer; and by this means the form of its crystals may be discovered.
In the progress of calcination it first becomes a dusky grey powder, then yellow, when it is called massicot; then, by imbibing pure air, it becomes red, and is called minium, or red lead. In a greater degree of heat it becomes massicot again, having parted with its pure air. If the heat be too great, and applied rapidly, it becomes a flaky substance, called litharge; and with more heat it becomes a glass, which readily unites with metallic calces and earths, and is a principal ingredient in the manufacture of flint glass, giving it its peculiar density and refractive power.
Though lead soon tarnishes, the imperfect calx thus made does not separate from the rest of the metal, and therefore protects it from any farther action of the air, by which means it is very useful for the covering of houses, and other similar purposes. All acids act upon lead, and form with it different saline substances. White-lead consists of its union with vinegar and pure air. Also dissolved in vinegar, and crystallized, it becomes sugar of lead, which, like all the other preparations of this metal, is a deadly poison.
Oils dissolve the calces of lead, which, by this means, is the basis of paints, plaisters, &c.
By means of heat litharge decomposes common salt, the lead uniting with the marine acid, and forming a yellow substance, used in painting, and by this means the fossil alkali is separated.
Lead unites with most metals, though not with iron. Two parts of lead and one of tin make a solder, which melts with less heat than either of the metals separately; but a composition of eight parts of bismuth, five of lead, and three of tin, makes a metal which melts in boiling water.
This metal will be dissolved by water if it contain any saline matter, and the drinking of it occasions a peculiar kind of cholic.