36 



HOW CROPS GROW. 



Fig. 3. 



fig. 3. When the combustion has declined, a suitable test applied to the 

 air of the bottle -will demonstrate that another invisible gas has taken 

 the place of the oxygen. Such a test is lime-water* 

 On pouring some of this into the bottle and agitating 

 vigorously, the previously clear liquid becomes milky, 

 and on standing, a white deposit, or precipitate, as' the 

 chemist terms it, gathers at the bottom of the vessel. 

 Carbon, by thus uniting to oxygen, yields carbonic acid 

 gas, which in its turn combines with lime, producing 

 carionate of lime. These substances will be further 

 noticed in a subsequent chapter. 



Metallic iron is incombustible in the at- 

 mosphere under ordinary circumstances, but 

 if heated to redness and brought into pure 

 oxygen gas, it burns as readily as wood burns in the air. 

 Exp. 7. — Provide a thin knitting needle, heat one end red hot, and 

 sharpen it by means of a file. Thrust the point thus 

 made into a splinter of wood, (a bit of the stick of a 

 match, 3^ inch long;) pass the other end of the needle 

 through a wide, flat cork for a support, set the wood on 

 fire, and immerse the needle in a bottle of ojq'gen, fig. 

 4. After the wood consumes, the iron itself takes fire 

 and burns with vivid scintillations. It is converted 

 into oxide of iron, a part of whieli will be found as a 

 yellowish-red coating on the sides of the bottle; the 

 remainder will fuse to black, brittle globules, which 

 falling, often melt quite into the glass. j-jg. 4. 



The only essential difference between these and ordinary 

 cases of combustion is the intensity with which the pro- 

 cess goes on, due to the more rapid access of oxygen to the 

 combustible. 



Many bodies unite slowly with oxygen — oxidize, as it 

 is termed, — without these phenomena of light and intense 

 heat which accompany combustion. Thus iron rusts, lead 

 tar/iishes, wood decays. All these processes are cases of 

 oxidation, and cannot go on in the absence of oxygen. 



Since the action of oxygen on wood and other organic 



* To prepare lime-water, put a piece of unslaked lime, as large as a chestnut. 

 Into a pint of water, and after it has fallen to powder, agitate the whole for a 

 minute in a wi-ll stoppered bottle. On standing, the excess of lime will settle, 

 and the perfectly clear liquid above it is ready for use. 



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