40 HOW CROPS GROW. 
oxygen and setting hydrogen free. This action is almost 
imperceptible, however, with pure water under ordinary 
circumstances, because the metals are soon coated with a 
film of oxide which prevents further contact. If to the 
water a strong acid be added, or, in case zine is used, an 
alkali, the production of hydrogen goes on very rapidly, 
because the oxide is dissolved as fast as it forms, and-»a 
perfectly pure metallic surface is constantly presented to 
the water. 
Exp. 12.—Into a bottle fitted with cork, funnel, and delivery tubes, 
fig. 6, an ounce of iron tacks 
or zine clippings is introduced, 
a gill of water is poured upon 
them, and lastly an ounce of 
oil of vitriol isadded. A brisk 
effervescence shortly com- 
mences, owing to the escape 
of nearly pure hydrogen gas, 
which may be collected in a 
bottle filled with water as 
directed for oxygen. The 
first portions that pass over 
are mixed with air, and should 
be rejected, as the mixture is 
dangerously explosive. 
One of the most strik- 
ing properties of free 
hydrogen is its levity. It is the lightest body in nature, 
being fourteen and a half times lighter than common air. 
: It is hence used in filling balloons. 
Another property is its combustibili- 
ty: it inflames on contact with a 
lighted taper, and burns with a flame 
which is intensely hot, though searee- 
ly luminous if the gas be pure. Final- 
ly, it is itself incapable of support- 
Fig. 7. ing the combustion of a taper. 
Exp. 13.—All these characters may be shown by the following single 
experiment. A bottle full of hydrogen is lifted from the water over 
which it has been collected, and a taper attached to a bent wire, fig. 7, is 
