376 



APPLIED SCIENCE 



Since white lead is expensive to prepare, various substi- 

 tutes, such as lead sulphate and chalk, have been used. In 

 addition, numerous more or less successful attempts have 

 been made to devise a method for making white lead quickly. 

 One method is to pass carbon dioxide through a ground 

 mass of litharge and salt in water; another is to pass a cur- 

 rent of electricity through a solution of sodium nitrate 

 (NaNO 3 ) in water in which a lead bar is suspended; and 

 a third is the wet process. The wet process is carried out in 

 the following manner. Metallic lead is melted and poured 



FIG. 187. 



from a height of 15 ft. into water, the result being that granu- 

 lated lead is produced. This is placed in cylinders and 

 treated with acetic acid, which is kept in motion by constant 

 pumping. The final product is filtered out, washed with 

 water, and ground in oil. It is not, however, so effective in 

 covering power as the Dutch white lead. 



420. Pigments and Colors of Paints. The color of a 

 paint is due to the pigment it contains, usually a metallic 

 salt. Sometimes aniline dyes (organic coloring substances 



