118 PRACTICAL LESSONS IN SCIENCE. 



good illustration of what is meant by chemical combinations. 

 Two liquids of strongly marked qualities, one an acid, one an 

 alkali, united forming a solid as unlike its constituents as could 

 well be imagined. Neither acid nor alkaline, neither sour nor 

 bitter, and with but slight odor. The chemical equation is 

 NH 3 +HN0 3 =NH 4 N0 3 . 



If the ammonium nitrate obtained above be heated moder- 

 ately, it melts and a colorless gas is given off which supports 

 combustion almost as vigorously as oxygen. The equation is, 

 NH 4 N0 3 ==2H 2 0-f N 2 0. This gas, the sub-oxide of nitrogen or ni- 

 trous oxide, is the laughing gas used by dentists as an anaesthetic. 



The several compounds of oxygen and nitrogen that occur are 

 of little value beyond illustrating the law of multiple propor- 

 tions. They are the nitrous oxide, N 2 0; nitric oxide, NO; 

 nitrous anhydride, N 2 3 ; nitrogen peroxide, N0 2 ; and nitric 

 anhydride, N 2 5 . An anhydride is an oxide which combined with 

 water, forms an acid, as nitrous anhydride N 2 3 fH 2 2HN0 2 , 

 nitrous acid; and nitric anhydride N 2 5 4-H 2 O=2HN0 3 nitric 

 acid. 



Acids are composed of hydrogen and some other non-metallic 

 element, and in most cases they contain oxygen, as hydrochloric 

 acid, composed of hydrogen and chlorine, formula HC1, nitric 

 acid, composed of hydrogen, nitrogen and three atoms of oxy- 

 gen, formula HN0 3 , and sulphuric acid composed of hydrogen, 

 sulphur and oxygen, formula H 2 S0 4 . Acids usually have a sour 

 taste, turn vegetable blue colors to red and are often poisonous. 



The combination of a metal with hydrogen and oxygen, non- 

 metals, forms a compound called a base, as sodium hydroxide 

 composed of sodium, hydrogen and oxygen, formula NaHO, called 

 caustic soda, as potassium hydroxide, composed of potassium, 

 hydrogen and oxygen, formula KHO, called caustic potash, and 

 as calcium hydroxide, formula Ca0 2 H 2 , called caustic lime. The 

 bases are sometimes bitter corrosive poisons, turning red colors 

 to blue, but generally they are mild compounds. 



The customary method of testing a substance for acid or 

 basic reaction is by the use of paper colored by the dye litmus. 



