134 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



was not an element, as had been supposed previously, but a 

 compound of inflammable air with oxygen. Lavoisier and 

 his colleagues in 1787 therefore gave to the inflammable air 

 the name "hydrogen'' or water-producer (compare German 

 ivasserstoff}. Lavoisier's "Decomposition and Recomposition 

 of Water" may be represented by the equations : 



3Fe + 4H 2 O-> Fe 3 O 4 + 4H 2 

 Iron + steam -> oxide of iron + hydrogen 

 2H 2 + O 2 -> 2H 2 O 

 Hydrogen + oxygen -> water-vapour. 



Priestley, in 1785, showed that many metallic oxides could 

 be reduced to metals by heating them in hydrogen ; Lavoisier 

 showed that water is also produced. The action may be 

 represented by equations such as the following : 



PbO + H 2 -> Pb 4- OH 2 

 Litharge + hydrogen > lead + water-vapour 

 CuO + H 2 -> Cu + OH 2 

 Copper oxide 4- hydrogen -> copper 4- water- vapour. 



Iron oxide is also reduced by hydrogen ; this action is 

 therefore reversible : 



the action proceeding in one direction or the other according 

 as hydrogen or steam predominates in the gas. 



B. The Composition of Water. 



Cavendish, in 1781, showed that water was formed by the 

 combination of two volumes of hydrogen with one volume of 

 oxygen. This proportion was confirmed by Lavoisier, Monge, 

 and others, who burnt or exploded together large volumes of 

 the two gases, as well as by the careful experiments on the 

 explosion of hydrogen and oxygen which Gay-Lussac and 

 Humboldt carried out in 1805 with the help of Volta's 

 " eudiometer." Monge used the measured densities of the two 

 gases to calculate the composition of water by weight, but 

 obtained very erroneous results as the hydrogen was not dried. 



Berzelius and Dulong, in 1820, made the first successful 

 determination of the composition of water by using copper 



