LEAD. 289 



and is capable of uniting with at least three proportions of oxygen, 

 forming three distinct oxides. At a high temperature, it absorbs 

 oxygen very rapidly, and, when fused in open vessels, a grey film 

 forms upon its surface, which is a protoxide of lead. By con- 

 tinuing the heat, the protoxide assumes a uniform yellow colour ; 

 it may also be formed by exposing the carbonate of lead to a red 

 heat, or it may be precipitated from a solution of the nitrate, by 

 potash, of a whitish colour, and in a state of purity. It is well 

 known in commerce by the name of massicot, and when exposed 

 to a strong heat it is partially melted into a semi-transparent glass, 

 which is called litharge. It is insoluble in water, but is dissolved 

 by the alkalies, and constitutes the basis of the salts of lead. The 

 deutoxide is obtained by exposing the protoxide, or massicot, to 

 heat, with a large surface, and free access of air. It is then con- 

 verted into the well-known pigment, red lead, or minium. At a 

 red heat it gives off this second dose of oxygen, and returns to the 

 state of protoxide. When digested in nitric acid it is decomposed ; 

 the greater part is reduced to the state of protoxide, which is dis- 

 solved by the acid, while the other portion combines with the 

 excess of oxygen and is converted into a peroxide. It is 

 of a dark brown colour, and, when strongly heated, it 

 gives off three or four per cent, of oxygen gas. The oxides 

 of lead are easily vitrified, and have the property of com - 

 bining with all the metals except gold, silver, and platina. Gold 

 and silver may thus be purified by melting them with lead ; the 

 process is called cupellatioji. The metal to be purified is wrapt up 

 in a sheet of lead, and laid upon a crucible made of some very 

 porous substance ; they melt together, the lead becomes first 

 oxidated, then vitrified, and sinks into the cupel, carrying along 

 with it all the baser metals, and leaving the gold or silver upon its 

 surface. Lead also combines with chlorine, iodine, sulphur, and 

 phosphorus, and with several of the metals. 



Lead, in its pure metallic state, does not appear to exert any 

 deleterious influence on the animal system ; but when oxidized, or 

 in combination with acids, it acts as a powerful poison. It is 

 probable that all the preparations of lead are poisonous, when ad- 

 ministered internally, or when applied externally for a length of 



