116 CHEMISTRY. 



separated from each other, as just described, so, on the other 

 hand, water may be formed by uniting these gases. But 

 they will not unite by merely being mixed together. Some 

 force must be brought to bear on them to effect their union. 

 Heat will do it when sufficient to produce combustion. Ac- 

 cordingly, when combustion takes place where there is hy- 

 drogen, the oxygen unites with the hydrogen, forming wa- 

 ter; and this occurs very generally in most cases of what 

 we call combustion, as you will see in the next chapter. 

 Electricity, also, will do it. If a charge of electricity be 

 passed through a mixture of the two gases, they will unite, 

 and water will be formed. 



To show this experiment an apparatus called a Eudiometer is employed. 

 It consists of a strong- glass vessel containing two platinum wires soldered 

 into the glass and nearly touching at their points. The glass vessel is filled 

 with two volumes of hydrogen and one volume of oxygen, and closed tightly 

 with a well-fitting stopper. An electric spark is then passed between the 

 wires, so that as it jumps from one end of a wire through the mixed gases to 

 the other wire, the gases are intensely heated and unite with explosive vio- 

 lence. If the eudiometer is cooled and opened under water, water will rush 

 in to fill the space left by the condensation of the gases. If, however, the 

 eudiometer is placed in a vessel heated by steam to 100 C., the water-gas 

 will be found to occupy two thirds of the volume of the mixed gases, pro- 

 vided they were measured at the same temperature and pressure. This 

 confirms the statement already made that two volumes of hydrogen com- 

 bine with one volume of oxygen to form two volumes of water-gas. (See 



= H 2 O or H 2 +O = H 2 O. 



145. Specific Gravity of Hydrogen. In Fig. 39 is repre- 

 sented the weight or gravity of hydrogen as compared 

 with the gravity of some other substances. Platinum is the 

 heaviest of all substances ; hydrogen, on the other hand, is 

 the lightest substance known. In the figure are represented 



