MOLECULES AND ATOMS 805 



The law of combining volumes and the law of multiple proportion 

 were discovered independently of each other the one in France by 

 Gay-Lussac, the other in England by Dalton almost simultaneously. 

 In the language of the atomic hypothesis it may be said that atomic 

 quantities of elements occupy equal or multiple volumes. 



The first law of Gay-Lussac expresses the relation between the 

 volumes of the component parts of a compound. Let us now consider 

 the relation existing between the volumes of the component parts and 

 of the compounds which proceed from them. This may sometimes be 

 determined by direct observation. Thus the volume occupied by water^ 

 formed by two volumes of hydrogen and one volume of oxygen, may be 

 determined by the aid of the apparatus shown in fig. 56. The long 

 glass tube is closed at the top and open at the bottom, which is 

 immersed in a cylinder containing mercury. The closed end is 

 furnished with wires like a eudiometer. The tube is filled with 

 mercury, and then a certain volume of detonating gas is introduced. 

 This gas is obtained from the decomposition of water, and therefore in 

 every three volumes contains two volumes of hydrogen and one volume 

 of oxygen. The tube is surrounded by a second and wider glass tube, 

 and the vapour of a substance boiling above 100 that is, whose boiling, 

 point is higher than that of water is passed through the annular space-* 

 between them. Amyl alcohol, whose boiling point is 132, may be 

 taken for this purpose. The amyl alcohol is boiled in the vessel to the> 

 right hand and its vapour passed between the walls of the two tubes. 

 In the case of amyl alcohol the outer glass tube should be connected with 

 a condenser to prevent the escape into the air of the unpleasant smelling 

 vapour. The detonating gas is thus heated up to a temperature o 

 132 When its volume becomes constant it is measured, the height of 

 the column of mercury in the tube above the level of the mercury in the 

 cylinder being noted. Let this volume equal v ; it will therefore con- 

 tain ^ v of oxygen and 'v of hydrogen. The current of vapour is then 

 stopped, and the gas exploded ; water is formed, which condenses into 

 a liquid. The volume occupied by the vapour of the water formed has; 

 now to be determined. For this purpose the vapour of the amyl alcohol 

 is again passed between the tubes, and thus the whole of the water, 

 formed is converted into vapour at the same temperature as that atj 

 which the detonating gas was measured , and the cylinder of mercury 

 being raised until the column of mercury in the tube stands at,the same 

 height above the surface of the mercury in the cylinder as it did before 

 the explosion, it is found that the volume of the water formed is equal! 

 to v that is, it is equal to the volume of the hydrogen contained 



