OBSERVATIONS.
The Phlogiston and the Vitriolic Acid have each an affinity with Oils. It is not therefore surprising that Sulphur, which is a compound of these two substances, should be soluble in oily matters. Yet it is remarkable, that Essential Oils, which are much thinner than the Fat Oils, dissolve Sulphur with much more difficulty; as will be shewn when we come to treat of those Oils; and that Spirit of Wine, which contains an exceeding subtile Oil, doth not act upon Sulphur at all.
Oil, by contracting an union, with Sulphur, produces a considerable alteration in that mineral: a phenomenon so much the more surprising, that we know it to be in some sort unalterable by any other solvent, of what kind soever, add, that its nature admits of no change but by burning. We shall say more on this subject under the head of Essential Oils.
PROCESS V.
To combine Fat Oils with Lead, and the Calces of Lead. The Basis of Plasters. The Decomposition of this Combination.
Into an earthen vessel put granulated Lead, Litharge, Ceruse, or Minium; and pour thereon twice its weight of any Fat Oil whatever. If you set the vessel over a brisk fire, the Lead at bottom will melt before the Oil begin to boil. When it boils, stir the matter with a stick: the Lead, or the Calx of Lead, will gradually disappear, and at last be totally dissolved by the Oil, to which it will give a very thick consistence.
OBSERVATIONS.
Fat Oils dissolve not only Lead, but its calces also: nay, they dissolve the latter more readily than Lead in substance; probably because they are more divided. The result of a combination of these matters is a thick, tenacious mass, that grows in some degree hard in the cold, and soft by heat. This composition is known in Pharmacy by the name of Plaster. It is made up with several drugs into plasters, which partake of the virtues of those drugs; so that it is the basis of almost all plasters.
Lead itself is seldom used to make plasters: Ceruse, Litharge, or Minium, are preferred to it; because these matters unite, as hath been said, more readily and more easily with Oils.