H4 ANSWERS TO PRACTICAL QUESTIONS 



2. If, in making a solution of Na 2 SO 4 , we use the salt 

 which has effloresced, and so become anhydrous, the tempera- 

 ture will rise instead of falling as before. Explain. 



This is because a solid hydrate is formed before the 

 salt dissolves in the H 2 O. The same holds true of other 

 anhydrous bodies, as the chlorides of Zn, Fe, and Cu. 



3. Why is KNO 3 used instead of NaNO 3 for making 

 gunpowder ? 



Sodium nitrate is imported from Chili in large quanti- 

 ties, and attempts have been made to use it for making 

 gunpowder,*' but its tendency to attract moisture has 

 frustrated the plan. It is now extensively used as a fer- 

 tilizer, 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. 



* Gunpowder is an intimate mechanical mixture of about i part nitre, i part 

 sulphur, and 3 parts charcoal. These proportions, however, vary 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 retard the explosion. The 

 nitre, 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 im- 

 mense pressure between gun-metal plates, forming what is known as press- 

 cake. These cakes are then submitted to the action of toothed rollers, where- 

 by 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 re- 

 volving 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. YOUMANS. 



