86 ANSWERS TO PRACTICAL QUESTIONS 



but its tendency to attract moisture has frustrated the plan. It 

 is now extensively used as a fertilizer, and is said to be the 

 cheapest form in which N can be furnished the soil. 



4. Why is a potassium salt preferable to a sodium one 

 in glass-making ? 



Sodium salts give a greenish tint to the glass. 



5. What is the glassy slag so plentiful about a fur- 

 nace ? * 



A silicate of lime or some other base contained in the ore. 



Ordinary Slag from Blast Furnace (Bloxam). 



Silica 43.07 



Alumina 14.85 



Lime 28.92 



Magnesia 5.87 



Oxide of iron 2.53 



Oxide of manganese 1.37 



Potash. 1.84 



Sulphide of calcium 1.90 



Phosphoric acid trace 



100.35 



somewhat in different countries, as well as in different sorts of powder. 

 More charcoal adds to its power, but also causes it to attract moisture from 

 the air, which of course injures its quality. For blasting rocks, where a 

 sustained force, rather than an instantaneous one, is required, the powder 

 contains more sulphur, and is even then often mixed with sawdust to re- 

 tard the explosion. The niter, sulphur, and charcoal, having been ground 

 and sifted separately, are thoroughly mixed, and then made into a thick 

 paste with water. This is ground for some hours under edge-stones, after 

 which it is subjected to immense pressure between gun-metal plates, form- 

 ing what is known as press-cake. These cakes are then submitted to the 

 action of toothed rollers, whereby the granulation of the powder is effected. 

 The grains thus formed are sorted into different sizes by means of a series 

 of sieves, and thoroughly dried at a steam heat. The last operation, that 

 of polishing, is accomplished in revolving barrels, after which the powder 

 is ready for market. The heavier the powder, the greater is its explosive 

 power. Good powder should resist pressure between the fingers, giving no 

 dust when rubbed, and have a slightly glossy aspect. YOTJMANS. 



* The slag is commonly employed for road-making in the neighborhood 

 of the iron-works. Some attempts have been made to turn the slag to ac- 

 count by employing it as a manure for soils deficient in potash, of which 



