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 zinc 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 zinc clippings is introduced, 

 a gill of water is poured upon 

 them, and lastly an ounce of 

 oil of vitriol is added. 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 scarce- 

 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 waiter over 

 which it has been collected, and a taper attached to a bent wire, fig. 7, is 



