SCIENCE IN THE INDUSTRIAL WORLD 



light gray. In such a smoky city as London this change 

 will naturally be much more rapid than in the open 

 country where the air is practically free from sulphides. 

 In such places as the vicinity of sewers, where the per- 

 centage of sulphides in the air is very high, white-lead 

 pigment turns gray very rapidly. Nevertheless, this 

 defect in white lead is so completely offset by its good 

 qualities, that it remains the most popular of pigments. 



The one quality above all others that endears it to 

 the practical house-painter is its remarkable "cover- 

 ing power" the property of covering a great space 

 with an opaque layer. But besides this, the lead paints 

 are very durable, and may be used in an endless variety 

 of combinations. 



If the question of permanency of color were the only 

 thing to be considered in selecting a white pigment, 

 however, the zinc, bismuth, and barium compounds 

 would have a better standing than white lead, as all of 

 them are less affected by atmospheric changes. In fact, 

 the only really permanent white familiar to the paint 

 trade is the sulphate of barium, known popularly as 

 "enamel white." This pigment will retain its pure 

 whiteness indefinitely, even a long exposure to London 

 smoke and fog having no effect upon it. Furthermore it 

 can be produced for something less than half the cost 

 of white lead. But when the relative covering powers 

 and mixing qualities of the two are considered, practised 

 painters regard white lead as the better pigment. 



In proof of this we find the painter always seeking a 

 pure white-lead paint, while the dishonest manufacturer 

 is forever trying to foist upon him a white-lead paint 



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