PAINTS, DYES, AND VARNISHES 



kinds of protective and decorative purposes, is red lead 

 a lead oxide having lead and oxygen in the molecule in 

 proportion of three to four, Pb 3 O 4 . It is probable that 

 these proportions are not quite as exact as in the in- 

 dicated chemical formula, for, as we have seen, chemical 

 formulas and colors do not always correspond ; but this 

 formula represents the composition very closely. 



Red lead has good coloring and covering powers, and, 

 under ordinary atmospheric conditions, is permanent. 

 It has the defect of all the lead pigments of being 

 affected by sulphuretted hydrogen, but under ordinary 

 conditions this is so slight that it need not be considered. 

 It has a powerful drying effect upon linseed oil, and this 

 quality adds to its value for many purposes, such as 

 packing the joints of steam-pipes. 



Another form of red lead a pigment having the same 

 chemical composition as far as can be determined, 

 although of different color is orange lead. It is paler 

 in color, and lighter in weight, and is made by a dif- 

 ferent process, but what may be the difference in the 

 arrangement of the atoms in the molecules that makes 

 a different shade still remains a mystery. 



Among the other red mineral pigments which are of 

 little practical importance, although, in some instances, 

 of brilliant color, are the chromium salts of lead, mer- 

 cury, copper, and silver. Most of them are lacking in 

 permanency and are too costly for general use, al- 

 though chrome lead is an exception, and will be re- 

 ferred to again in a moment. 



The biniodide of mercury is also a brilliant color 

 "brilliant scarlet," it is called in the trade. It is very 



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