14 ELEMENTARY AGRICULTURAL CHEMISTRY 



and very slight or no elevation of temperature is apparent. 

 In some cases, however, of slow oxidation, when the escape 

 of heat is hindered from any cause, the temperature rises so 

 as to be perceptible, or even dangerous. Under particularly 

 favourable conditions the rise of temperature may be sufficient 

 to start rapid combination with oxygen, and flame then results. 

 Such cases of *' spontaneous combustion," as they are called, 

 not infrequently occur. Among the chief causes may be 

 mentioned absorption of oxygen by drying oils, e.g., linseed or 

 cotton-seed oil, especially when spread on cotton waste, as in 

 mills; fermentative changes in vegetable matter, e.^,, hay, 

 Wbacco ; slow oxidation of certain minerals, e.g., iron pyrites 

 m coal. 



Hydrogfen is also very abundant in nature, though, because 

 of its low atomic weight (1-008), the proportion by weight 

 present on the earth's surface is small. Its most abundant 

 compound is water, H,0, It can readily be obtained from 

 water by the removal of the oxygen by the aid of metals. 

 Some will set hydrogen free at the ordinary temperature on 

 contact with water, e.g., potassium or sodium, the reaction 

 being 



Na, + 2H,0 = 2NaOH + H,, 



only half the hydrogen being thus evolved. Other metals 

 liberate hydrogen from the v/ater at about the boiling-point, 

 e.g., magnesium : 



Mg+2H,0 = MgH,0,+ H,; 



while others require a red heat, e.g., iron. The reaction in the 

 last case is 



3Fe + 4H,0 = ^efi^ + 4H,. 



A more convenient method of preparing hydrogen is by the 



action of a dilute acid upon a metal, e.^., dilute sulphuric acid 



upon zinc : 



Zn + H,St), = ZnSO, + H,. 

 Zinc sulphate. 



