36 HOW CROPS GROW. 
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 
carbonate 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.—Proyide a thin knitting needle, heat one end red hot, and 
sharpen it by means of a file. Thrust the point thus 
made into asplinter of wood, (a bit of the stick ofa 
match, 14 inch long;) pass the other end of the needle 
through a wide, flat cork for a support, sect the wood on 
fire, and immerse the needle in a bottle of oxygen, 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 which 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. 
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 rusés, 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 
* 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 fora 
minute in a well stoppered bottle. On standing, the excess of lime will settle, 
and the perfectly clear liquid above it is ready for use. 
