30 



HOW CROPS GEOW. 



to the air of the bottle wlU 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 predpUate, 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 

 carbonate of lime. These substances will be further 

 noticed in a subsequent chapter. 



Kg. 3. 



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, i 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 oxygen. Fig. 

 4. After the wood consumes, the iron Itself takes Are 

 and burns with vivid scintillations. It is converted into 

 two distinct oxides of iron, of which one, — ferric oxide, — 

 will be found as a yellowish-red coating on the sides of 

 the bottle ; the other,— magnetic oxide,— will fuse to 

 black, brittle globules, mhlch falling, often melt quite 

 into the glass. ^Ln^yvUl 7' l/t Qi t 'fiiii (jif Fig. 4. 



The only essential difference between these and ordi- ' 

 nary cases of combustion is the intensity with which the 

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

 to the combustible. 



Many bodies unite slowly with oxygen, — oxidize, as it 

 is bermed, — without these phenomena of light and intense 

 heat which accompany combustion. Thus iron rusts, lead 

 tarnishes, 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 

 matters at common temperatures appears to be analogous 



* 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 few minutes in a well-stoppered bottle. On standing, 

 the excess of lime will settle, and the perfectly clear liquid above it is 

 ready for use. 



