LEAD. 



to be oxyded, it' respirable air be present. 

 In a strong- heat it boils, and emits fumes ; 

 during which time, if exposed to the air, 

 its oxydation proceeds with considerable 

 rapidity. If melted lead be poured into 

 a box previously rubbed with chalk, to 

 prevent its action on the wood, and be 

 continually agitated, it will concrete into 

 separate grains, of considerable use in 

 various mechanical operations, particu- 

 larly that of weighing. Lead is brittle at 

 the time of congelation. In this slate it 

 may be broken to pieces with a hammer, 

 and the crystallization of its internal parts 

 will exhibit an arrangement in parallel 

 lines. 



This metal, during the progress of heat, 

 first becomes converted into a dusky pow- 

 der, which by a continuation of the heat 

 becomes white, yellow, and afterwards of 

 a bright red, inclining to orange colour, 

 called minium, or red lead. The process 

 requires considerable management with 

 regard to the heat and access of air, in 

 the making of red lend. Many days are 

 required for this purpose. If the heat be 

 too great or rapid, the lead becomes con- 

 verted into a flaky substance, called 

 litharge ; and a still greater heat converts 

 it into a clear, transparent, yellow glass, 

 which powerfully dissolves and corrodes 

 metallic oxides or earths ; and on this ac- 

 count it usually finds its way through the 

 crucibles in a short time. It acts more 

 difficultly on argillaceous than on siliceous 

 earths ; w r hence it is found that vessels 

 made of clay mixed with broken pottery 

 are preferable to those that are composed 

 of clay and sand. The oxide of lead is a 

 principal ingredient in most of the modern 

 fine white glasses. It is more particularly 

 calculated to form the dense glass, vised to 

 correct the aberration arising from colour 

 in those telescopes which are known by 

 the name of achromatic, because it com- 

 municates the property of separating the 

 coloured rays from each other in greater 

 angles than obtain in alkaline glasses at 

 equal angles of mean refraction. The 

 imperfection which most considerably af- 

 fects this kind of glass is, that its density 

 is seldom uniform throughout. The ir- 

 regularities show themselves in the forms 

 of veins, which greatly disturb the regu- 

 lar refraction. 



Lead is not much altered by exposure 

 to air or water, though the brightness of 

 its surface, when cut or scraped, very soon 

 goes off. It is probable that a thin stratum 

 of oxide is formed on the surface, which 

 defends the rest of the metal from corro- 

 sion. 



All the oxides of lead are veiy easily re- 



duced. Minium, when exposed to a strong 

 heat, gives out part of the oxygen it ab- 

 sorbed during its oxidation ; but, like the 

 other oxides of this metal, it requires the 

 addition of some combustible substance 

 for its complete revival : a familiar in- 

 stance of this revival is seen by exposing' 

 the common wafers to the flame of a can- 

 dle. The wafers are coloured with 

 minium, which is revived by the heat and 

 inflammable substance of the wafer, so 

 that it falls down in metallic globules. 



Lead is found native, though seldom ; 

 and also in the form of an oxide, called 

 native ceruse, or lead ochre, or lead spar, 

 of various colours, red, brown, yellow, 

 green, bluish, and black. These ores, 

 when freed as much as possible from 

 earthy matter, may be dissolved in diluted 

 nitrous acid. Oxide of iron is usually 

 thrown down from the solution by boiling. 

 If the lead be then precipitated by the 

 carbonate of soda, and weighed, 132 grains 

 of the dry precipitate will correspond 

 with 100 grains of lead in the metallic 

 state. If the precipitate be suspected to 

 contain copper, it may be separated by 

 digesting in ammonia. If it be supposed 

 to contain silver and copper, the precipi- 

 tate may again be dissolved in nitric acid, 

 and separated by the addition of muriatic 

 acid ; which, combining 1 with the metal, 

 produces the muriates of silver and of 

 lead ; the latter of which, being soluble in 

 thirty times its weight of boiling water, 

 may be washed off, while the silver re- 

 mains undissolved ; or the silver, if alone 

 in the precipitate, may be taken up by 

 ammonia, which will leave the oxide of 

 lead of the same value, with regard to 

 weight, as the foregoing. 



Lead is also found mineralized by the 

 sulphuric and the phosphoric acids; this 

 last is of a greenish colour, arising from a 

 mixture of iron. The sulphate of lead is 

 soluble in about eighteen times its weight 

 of water. One hundred and forty-three 

 grains of the dried salt represent 100 

 grains of lead. The phosphate of lead 

 ore may be dissolved in nitric acid by 

 means of heat, except a few particles of 

 iron, which remain at the bottom. By 

 the addition of sulphuric acid, the lead is 

 thrown down in the form of white flakes 

 of sulphate ; which, when washed and 

 dried, discover the quantity of lead they 

 contain, by the same allowance of 143 

 grains of the salt to 100 grains of metallic 

 lead. The remaining solution being 

 evaporated to dry ness, affords phosphoric 

 acid. Lead is abundantly found in com- 

 bination with sulphur, in the form of 

 heavy, shining, black, or bluish, lead- 



