610 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



that will not take dust. This drying property is dependent upon the 

 chemical nature of the oil; and if it is an unsaturated compound 

 like linseed oil, rapid absorption of oxygen will cause the film to 

 dry rapidly and become hard. A saturated oil like mineral oil will 

 not take up oxygen to any great extent, and drying will therefore 

 not take place. Linseed oil is the oil obtained by crushing the seed 

 of the flax plant and because of its rapid drying properties and 

 general adaptability for all classes of paints has never been sup- 

 planted by any other oil. 



Many salts of lead other than the oxides are used, and for a variety 

 of purposes. The most important of these are lead acetate, lead 

 nitrate, and lead arsenate. Lead acetate, commonly known as sugar 

 of lead, has certain medicinal values, especially in skin diseases. It 

 is used also in textile dyeing and calico printing and in the manu- 

 facture of lead chromate or chrome yellow, a much-used pigment. 

 Lead nitrate, too, is used in dyeing and in the manufacture of the 

 paint pigments chrome yellow and chrome green. Lead arsenate is 

 used largely as an insecticide, especially for the protection of fruit 

 trees against the larvae of the gypsy moth. Other commercially 

 used lead salts are basic lead sulphate, basic lead chromate (a red 

 pigment) , and lead fluosilicate. Two grades of basic lead sulphate 

 are in extensive use, namely, white and blue, the latter being essen- 

 tially white basic lead sulphate colored with lampblack accidentally 

 in the process of manufacture. They closely resemble white lead in 

 appearance, but differ in certain physical characteristics as well as 

 in chemical composition, the white analyzing about 80 per cent lead 

 sulphate, 15 per cent lead oxide, and 5 per cent zinc oxide, and the 

 blue analyzing lower in lead sulphate, higher in lead oxide, and 

 from 2 to 3 per cent carbon. A large part of the production of the 

 white sulphate is used in combination with some other white pig- 

 ment for paint, and practically all of the remainder is used in the 

 rubber industry for tubing, hose, automobile tires, and molded 

 articles. The blue sulphate likewise is largely used in the manu- 

 facture of rubber products, and a portion is utilized for metal paint. 



THE MANUFACTURE OF SHEET LEAD, PIPE, AND SHOT. 



The manufacture of sheet lead involves the recasting of "pig" 

 lead into flat plates and passing these through a series of rolls until 

 the desired thickness of the sheet is attained. 



Lead pipe is without seams, for it is made by forcing the semi- 

 molten metal through a circular die. 



Lead shot is made by alloying the metal with arsenic, which im- 

 parts a greater fluidity to the metal and increases its tendency to 

 assume a spherical form. The molten metal is caused to run through 

 collenders and to fall through a considerable height into water. 



