The metals (a) called imperfect or destruc-tible, as tin, lead, bismuth, zinc, regulus of antimony, copper, and iron, on being exposed to the joint action of fire and air, gradually lose their metallic form, and change into a friable or powdery calx; which, urged with a stronger heat, either does not melt, or runs into a vitreous mass, not mifcible with metals in their entire state any more than earths and earthy glasses. Some emit flames in their calcination: zinc in particular burns strongly and vividly: from whence it is presumed, that an inflammable substance is one of the constituent principles of these metals, and that the loss of their metallic form and qualities in calcination is owing to the avolation of this principle.

(a) The term semimetal is throughout this work avoided, as being liable to ambiguity. All the pure metallic bodies I have called by the general appellation of metals: such of them as want malleability, are, I presume, as properly distinguished by the epithet brittle or unmalleable, as by a name which may be imagined to imply that one half of their substance is of an unmetallic nature, and which, in effect, has been often understood in this sense, and accordingly applied to ores, vitriols, and metallic recrements.

The calcination is greatly promoted by the addition of nitre; which, with molt of these metals, visibly deflagrates, and is by all of them alkalized in the same manner as by charcoal or other inflammable substances. The calx is freed from the saline matter by ablution with water: a part of it commonly dissolves in the water along with the alkalized salt, but either separates spontaneoufly on standing, or may be precipitated by adding any acid.

A little powdered charcoal, or any other inflammable matter not partaking of a mineral acid, added to the destructible metals in fusion, prevents their calcination: and the calces and glasses, melted with the like additions, recover the principle which they had loft, and are revived or reduced-, that is, they resume their metallic appearance, and all their former qualities. But, in order to this reduction, as calces in general melt much more difficultly than the metals themselves, and some of them scarce at all when the fire acts on them through the sides of a vessel; an addition of fixt alkaline salt, borax, or fusible glass, is generally requisite, for bringing them into fusion; as well as of inflammable substances, for restoring their metalleity, All the metals dissolve in acids; some in one acid, and others in another: the dissolution, like that of absorbent earths and- alkaline salts, is generally accompanied with an effervescence, heat, and discharge of vapours. In this process, the phlogiston or inflammable principle of the imperfect metals, is absorbed or expelled: hence the vapour, which arises during the dissolution in the vitriolic acid of the metals which abound with this principle, is inflammable and truly ful-phureous: and hence the metal, precipitated from the acid by alkaline salts, or by other bodies void of inflammable matter, is found to be a true calx, which, like the calces made by fire, cannot be revived without the introduction of fresh phlogiston.

The perfect metals, gold, silver and mercury, suffer no resolution, or dissipation of any of their principles, from any known power. If changed into the appearance of a calx, by fire or by additions, they are recoverable without loss, either by the simple action of a stronger heat, or by the addition of such substances as may barely absorb from them the matter by which their form had been concealed.

All the weights and measures mentioned in this book, are those of the London Pharmacopoeia: - the troy pound, divided into twelve ounces, the ounce into eight drams, the dram into three scruples, and the scruple into twenty grains: - the wine gallon, divided into eight pints, the pint into sixteen ounces, and the ounce into eight drams or two spoonfuls.

It is, however, to be observed, that the college of Edinburgh, in order to avoid the con-fusion arising from the promiscuous use of terms signifying both weight and measure, have thought proper entirely to abolish liquid measure, and to reduce every thing, as well fluid as solid, to troy weight.