very pure gas, or by the combustion of coal. This proc- 

 ess was invented by the chemist Thenard, and is used 

 extensively in France. 



For the English method, which is gradually dropping 

 out of use, a stiff paste of litharge (PbO) is made by 

 mixing it with a solution of lead acetate. This mass is 

 kneaded by means of grooved rollers, through which 

 carbonic acid is brought in contact with the paste. 

 By this method a good quality of white lead was pro- 

 duced only when the litharge was perfectly pure lead 

 oxide. But as commercial lead oxide is often con- 

 taminated with other oxides, such as copper and iron, 

 white lead manufactured by this process is likely to be 

 a mediocre product. 



Despite the fact that white lead has held first place 

 among the mineral pigments for so many centuries, 

 not only as a pure white pigment but as the basic sub- 

 stance for forming other shades and colors, it is not, 

 strictly speaking, a permanent white when used as a 

 pigment. It is affected by sulphuretted hydrogen and 

 the sulphides, and forms a black substance, lead sul- 

 phide, when brought into contact with them. As the 

 atmosphere everywhere contains more or less sulphur, 

 or sulphides, the air of some cities being strongly con- 

 taminated, the fate of every coating of white-lead pig- 

 ment is eventually to turn gray, or even dead-black in 

 time. This change is effected very gradually, of course 

 so gradually, indeed, that under ordinary circum- 

 stances the other causes for the wearing-out of the coat 

 of paint will make it necessary to repaint a building 

 before the white lead has turned to more than a very 



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