8 Agricultural Chemistry. 



the earth, eight-ninths of the water, and about one-fifth of the 

 weight of the air. In the first and second instances the oxygen 

 is in a combined state. That which is held in chemical combina- 

 tion in the soil takes no part in the formation of plant tissue. 

 In the atmosphere it exists as a free element, merely mixed with 

 the other constituents. Oxygen in the interstices of the soil is 

 an active agent in bringing about many chemical changes, as 

 oxidation of the organic matter and disintegration of the soil 

 particles. It also forms about fifty per cent of the compounds 

 found in plants and animals. 



Oxygen is a colorless, odorless gas and very slightly soluble in 

 water. It shows great tendency to combine with other substances 

 and the act of union is usually attended by the production of 

 much heat. Burning or combustion is nearly always due to the 

 heat produced by the combination of the substance burned with 

 the oxygen of the air. Any substance, which will burn in air 

 (containing about twenty-one per cent of free oxygen) will burn 

 with increased brilliancy in pure oxygen. 



It is possible, with suitable apparatus, to measure the quantity 

 of heat a substance will produce when burned. The unit of heat 

 here employed is the " calorie, " which represents the quantity 

 of heat required to raise one gram (about 1-28 of an ounce) of 

 water from to 1 on the scale of the centigrade thermometer. 

 A large Calorie, one thousand times larger than the above, is 

 employed for the expression of large quantities of heat and will 

 be employed here. 



When one gram of the following dry substances is burned in 

 oxygen, the quantity of heat produced, expressed in large Cal- 

 ories, is as follows : 



Charcoal 8.0 Fat of sheep !>.4 



Hydrogen 34.4 



Wood 2.8 



Coal 7.5 



Coke 7.0 



Casein ., 6.8 



Fat of butter 9.2 



Cane sugar 4.0 



Cellulose 4.1 



Starch . 4.1 



