THE ELEMENTS OF PLANT FOOD 59 



taining zinc white from its ore, zinc blend, ZnS. It 

 involves forming SO 2 , which, uniting with water vapor, 



H 2 O + SO 2 = H 2 SO 3 



produces sulphurous acid, H 2 SO 3 . Adding one atom 

 of O produces sulphuric acid. This oxidation is pro- 

 duced in lead-lined chambers by the aid of nitric acid. 

 The process is complicated, but it is given fully in all 

 the standard encyclopedias. 



A later and more simple method of manufacture of 

 H 2 SO 4 is called the contact process. It depends on the 

 fact that certain finely divided metals, more particularly 

 platinum, have the power of hastening some chemical 

 reactions. By this process SO 2 is made to pass through 

 platinized asbestos, during which process it takes O and 

 becomes SO 3 . The reaction. is 



2 SO 2 + O 2 = 2 S0 3 



Sulphur trioxide (SO 3 ) combines directly with H 2 O 

 to make H 2 SO 4 . 



IRON (Fe) 



Description and Occurrence. Iron is the most 

 important of all the metals, but is seldom found free 

 in nature. In its pure state it is of little consequence. 

 The iron that is manufactured into machinery, hard- 

 ware, and building material is a combination of iron and 

 carbon. The proportion of carbon in the combination 

 determines the quality of the metal and the use to 

 which it may be applied. Wrought iron has less than 

 two tenths per cent of carbon and cannot be tempered 

 by sudden cooling, while steel has from two tenths per 

 cent to about two per cent of carbon and may be 

 tempered by sudden cooling. The hardness of the steel 



