10 CHEMISTRY OF FARM PRACTICE 



upon the nature of the combining substances and upon 

 how intimately the combustible and supporting gases may 

 be mixed at the point of ignition. Hence a blast is used 

 to provide a large quantity of oxygen which is introduced 

 into the interior of the combustible gas, which is then 

 forced to combine internally with the air of the blast and 

 externally with the oxygen of the at- 

 mosphere. The hole near the base of 

 the Bunsen burner, Fig. 3, is required 

 to supply air for combustion. The 

 blacksmith's forge, Fig. 4, must be 

 equipped with a " blower " to furnish 

 enough air for the complete combustion 

 of the large amount of gas. 

 e j n man y case s the oxidation is a 

 slow process. This may be seen in the 



rusting of iron, the tarnishing of copper. 



FIG. 3. Bunsen B ' . ** .' 



burner ^ ne dulling * zmc - When ink that is 



pale when first applied becomes dark 

 upon exposure, the ink has undergone oxidation. When 

 paper turns yellow with age, it is being slowly oxidized. 

 Almost everything we see about us has been oxidized. 

 Besides the noble metals, such as gold and platinum, it 

 is only those substances that have been artificially deprived 

 of their oxidizing constituent that exist in any other than 

 an oxidized condition. 



The rapidity with which oxidation takes place does not 

 affect the total quantity of heat produced. An iron wire 

 may be burned in a jar of oxygen and the combustion may 

 last but a few seconds, or the wire may be rusted by exposure 

 to moist air, and the oxidation may take a month to complete 

 itself, but when the two actions are complete, if the same 

 iron oxide is finally produced and in the same quantity, 

 the total amount of heat will be identical in the two cases. 

 Although combustion cannot take place without light, 

 yet light may be produced by other means than com- 



