68 CHEMISTEY. 



is, it may be necessary in this experiment to heat it before 

 dropping the phosphorus upon it. In all these cases oxides 

 of the bodies named are formed. 



77. Nitric Acid in the Atmosphere. Nitric acid can not 

 be made by mixing together its ingredients, oxygen, hydro- 

 gen, and nitrogen. No degree of heat, however severe, will 

 make them unite to form the acid. Accordingly they ex- 

 ist together in the air without uniting, except under ex- 

 traordinary circumstances. If they could be made to unite 

 readily, producing every now and then nitric acid in con- 

 siderable amounts, the most destructive effects would result 

 from the corrosive acid as it descended in showers upon the 

 earth. As it is, there is only one agent that can cause them 

 thus to unite, and that is electricity. Even this does it, as 

 we may say, with difficulty. It is only when this agent 

 acts with violence that the effect is produced. Nitric acid 

 is, therefore, generated in the air only in small quantity, 

 and it is carried down by the rain into the earth, where 

 it answers a valuable purpose in vegetation, as you will 

 see in another part of this book. Its formation then, 

 small as the quantity is, is not a mere accident, but a 

 provision of Providence for a special purpose of a marked 

 character. 



78. Acids. Nitric acid being the first acid you have 

 studied, we can now tell you about the class of bodies called 

 acids. But first make a simple experiment. Purple cab- 

 bage, certain lichens, and other vegetables, when boiled 

 with water, furnish blue infusions. Paper steeped in this 

 strong blue solution, and dried, gives us a test-paper for 

 acids which is very useful. The substance usually em- 

 ployed is called litmus, and the paper, prepared as above, 

 litmus paper. Now this blue coloring matter is turned red 

 by the action of even a very small quantity of acid. Dip 

 some litmus paper in a very weak solution of acetic, sul- 



