22 THE CULTURE OF FARM CROPS. 



previous periods, was the result of an accidental submission 

 of the red oxide of mercury to the rays of the sun concen- 

 trated by a lens or burning glass. It is an interesting coin- 

 cidence that the sun, the central focus of the chemical action 

 of the universe, should be the agent by which the most po- 

 tent of chemical agencies should have been brought to the 

 knowledge of mankind. 



This discovery may well be classed as the most important 

 in the history of human knowledge, rivaling the great dis- 

 covery of gravitation by Newton in the preceding century, 

 and throwing floods of light upon the investigations of the 

 mysteries of what we call nature. Of the discovery of this 

 potent substance, Prof. Liebig has observed that "it has 

 produced a revolution in the manners and customs of man- 

 kind. With it are linked, as results, our knowledge of the 

 composition of the atmosphere, of water, of the solid crust 

 of the earth, and of the influence of these upon the existence 

 and life of plants and animals. Every human industry has 

 been affected by it ; all trades and manufactures and by no 

 means least agriculture, have been aided and advanced im- 

 measurably by our knowledge of it." The study of its 

 properties may be made a profitable and most interesting 

 pursuit in the farmers household, in the leisure hours which 

 may be devoted to the acquisition of all useful knowledge 

 connected with his vocation, and no better subject could be 

 selected for the most pleasing and instructive experiments. 

 It is easily procured and managed by means of simple and 

 cheap apparatus. 



Oxygen is a transparent, colorless, tasteless, inodorous gas, 

 one-tenth heavier than the atmosphere of which it forms 23 

 per cent, of its weight. It has never been condensed into a 

 liquid. It exerts a weak magnetic force which is supposed 

 to cause, or to be concerned in, the daily fluctuations of the 

 magnetic needle ; and this property varies with its tempera- 

 ture. It is slightly soluble in water, 4? parts of it being ab- 

 sorbed by 100 parts of water. 



It is neutral, possessing neither acid nor alkaline qualities, 

 and although mild and bland, it exerts the most amazing 



