198 HYDROGEN. 



also, largely into the composition of all animal and 



vegetable matter, and forms the basis of most liquids. 



489. PREPARATION. Introduce a few 



Describe the 



method of pre- bits of iron or zinc 



paring iff ^ ft yial one _ t hird 



filled with water. Add a tea- 



spoon-full or more of common 



sulphuric acid, and attach to the 



vial a bent tube or a clay pipe, 



as represented in the figure. The evolution of the gas 



immediately commences. The first portions, which 



contain an admixture of air, are allowed to escape ; the 



pipe-stem is then brought under the mouth of the vial, 



and the gas collected.* 



590. EXPLANATION. Water is compos- 



Explain the 



formation of ed of oxygen and hydrogen gases. Each 



hydrogen? WQuld be & ag but for 



holds it in the liquid form. In the above process for 

 preparing hydrogen, the zinc is, as it were, the ransom 

 paid for its liberation. The oxygen combines with 

 the zinc and the hydrogen escapes. 



491. Pure water will not suffice in the 



What purpose 



is served by process. It must contain acid, to unite 



the acid? with the Qxide of zinc? as fagt ag f ormecl> 



The presence of an acid, for which the oxide has great 

 affinity, seems to stimulate its formation. It may, 



* When a taper can be applied at the mouth of the pipe-stem without 

 explosion, it may be certainly known that an unmixed gas is in pro- 

 cess of evolution. A cloth should be thrown over the vial and this test 

 made before commencing the collection. The connection of the ap- 

 paratus in the above experiment is made with a paper stopper, formed 

 on a bit of pipe-stem or glass tube. 



