112 PRACTICAL LESSONS IN SCIENCE. 



becomes too weak. Care should be taken to see that the bottle 

 is closely corked so that the gas shall be free from air, as hy- 

 drogen with a little air makes an explosive mixture of great 

 vigor. 



The chemical equation in the preparation of hydrogen by this 

 method is as follows : Zn, Zinc-f-H 2 S0 4 , sulphuric acid ZnS0 4 , zinc 

 sulphate +2H, that is one atom of zinc has taken the place of two 

 atoms of hydrogen, forming zinc sulphate and free hydrogen. 

 Hydrogen may be kept in an inverted jar for some time, but 

 quickly escapes from one that is upright. It can be poured up 

 but not down. Hydrogen or some of its compounds is used for 

 filling balloons. It is combustible, but must be heated up to the 

 kindling temperature when it unites vigorously with oxygen. The 

 temperature of the hydrogen flame is higher than that of any 

 other flame known. The oxyhydrogen flame, while giving very 

 intense heat gives but little light. But if a cylinder of lime be 

 put in the focus of this flame, it is soon heated to incandes- 

 cence and gives out a light second only to that developed by 

 electricity. 



Oxygen and hydrogen combined form water, whose molecule 

 is composed of one atom of oxygen and two of hydrogen ; for- 

 mula H 2 0. Two volumes of hydrogen combined with one volume 

 of oxygen forming two volumes of water vapor or steam. 



Water can be produced from a mixture of oxygen and hydro- 

 gen. If we introduce into a stout test tube a mixture of two 

 volumes of hydrogen and one of oxygen, on touching a lighted 

 match to the mixed gas it explodes, the hydrogen burning rap- 

 idly, so that a flash of flame fills the tube. After the explosion 

 water will be found deposited as dew on the sides of the tube. In 

 this experiment there is no attempt to measure the waterformed, 

 but careful experiments show that all the oxygen unites with all 

 of the hydrogen, forming the corresponding amount of water 

 vapor or steam. 



In the decomposition of water by the voltaic current the 

 volume of the hydrogen is double that of the oxygen, and that 

 experiment seems to show that water contains only oxygen and 



