LESSONS IN CHEMISTRY. 117 



atoms, represented by NH 4 , behave very much like some metals, 

 as potassium and sodium, and the name of ammonium has been 

 applied, but no such substance has ever been isolated. Ammo- 

 nia gas at a temperature of 40 F. becomes a liquid and at 

 103 it becomes a white crystalline solid. It can be reduced to 

 a solid by a pressure of about 10 atmospheres. When the pres- 

 sure is removed it expands into a gas by the action of heat de- 

 rived from surrounding objects. On this principle machines for 

 making ice are constructed. 



Ammonia and hydrochloric acid, whether as gases or in 

 solution, unite to form ammonium chloride, a white solid. 

 If bottles containing the gases in solution be uncorked and 

 brought near each other, a cloud of the ammonium chloride 

 will be formed by the volatile gases. The chemical equation is 

 2NH 3 -f HC1=2NH 4 C1. This compound, called salammoniac, is 

 found in volcanic regions in considerable quantities. 



Pure nitrogen may be prepared from the air, but the com- 

 pounds of nitrogen have never been prepared successfully from 

 the nitrogen of the air. It is difficult to cause nitrogen to 

 combine directly with either hydrogen or oxygen, but through 

 the action of myriads of minute organisms in some soils, atmos- 

 pheric nitrogen and decaying nitrogenous matters are trans- 

 formed into nitrates of lime or potassium. 



One of the most important compounds of nitrogen is nitric 

 acid, whose molecule is composed of one atom of hydrogen with 

 one atom of nitrogen and three atoms of oxygen. Its symbol is 

 HN0 3 . It is prepared by the action of sulphuric acid on potas- 

 sium nitrate or sodium nitrate. The action is as follows: 

 KN0 3 +H 2 S0 4 =HN0 3 +KHS0 4 . Nitric acid is a colorless, cor- 

 rosive liquid whose applications in the arts and manufacturing 

 industries are very extensive. 



Dilute nitric acid by adding an equal bulk of water, then add 

 ammonia water in small quantities from time to time, as long as 

 the mixture shows an acid reaction. If this mixture be evapo- 

 rated slowly, a solid substance with many transparent crystals, 

 called ammonium nitrate, will be formed. This experiment is a 



