260 TOWERS AND TANKS FOR WATER- WORKS. 



can be traced directly to some agency foreign to the lead itself, 

 used either in its preparation or in the methods of its applica- 

 tion." 



A paper read by Prof. A. H. Sabin, before the Boston 

 Society of Civil Engineers, November, 1899, says of the red 

 oxide of lead : " There yet remains to be described one other 

 important pigment, red lead. This is entitled to a place in a 

 class by itself, because it is intermediate between the paints, 

 which it resembles in being used mixed with oil, and the 

 cements, which it resembles in its process of solidification. It 

 is, in fact, a powerful basic substance, and combines chemic- 

 ally with the oil, forming an insoluble, hard, tenacious mass, in 

 which the uncombined particles of the excess of red oxide are 

 imprisoned. This is what constitutes the protective film when 

 a red-lead paint is dry." 



By some authorities it is claimed that in the chemical com- 

 bination the glycerine, as well as the acid fats, is changed by 

 the lead oxide, volatilization of the glycerine being prevented, 

 but in oxidizing through the process common to all linseed- 

 oils, the mass is rendered insoluble, elastic, and adhesive; but 

 it seems very probable that the glycerine, not being a stable 

 product, soluble in water and volatilized by heat, acts as 

 described by Mueider, the film being rendered more or less 

 porous by the escape of the gases. 



Litharge mixed with commercial glycerine to a pasty 

 mass takes a most hard and tenaceous "set" when exposed 

 to the action of the atmosphere for twenty to thirty minutes. 



It is stated by Wood that, during the process of setting, 

 red lead and oil will oxidize the surface of clean iron or steel, 

 forming the black oxide of iron which is non-corrosive. It is 

 also believed to be a fact that where moisture exists upon the 

 metallic surface, the oil and lead rapidly absorbs this in the 

 chemical change requiring oxygen wherever found. 



These estimable qualities, however, are offset to a certain 



