124. HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



Lavoisier called attention to the fact that the calx was 

 diminished in weight by its combination with inflammable 

 air, and concluded that some other product, namely water, 

 must be formed in addition to lead, thus : 



red lead '+ hydrogen = lead+ water. 



Water was also produced in small quantities when red lead 

 was revivified by means of charcoal, showing that the 

 charcoal contained hydrogen as well as carbon (Lavoisier's 

 Works, II. 344348). 



Oxidation and reduction. The oxides of copper and iron 

 can be reduced like red lead by heating them in a current 

 of hydrogen, but the refractory oxide of zinc is not acted 

 on by the gas. The case of iron is of interest because 

 red-hot iron is oxidised by heating in a current of steam 

 (p. 1 1 8) but the oxide is reduced when a current of hydrogen 

 is passed over it. The action is therefore reversible, pro- 

 ceeding in one direction or the other, as shown by the 

 two arrows, according as hydrogen or steam is present in 

 excess. 



iron + steam^iron oxide -f hydrogen. 



It should be noticed that the term OXIDATION is often 

 used to describe the removal of hydrogen as well as the 

 addition of oxygen, whilst the converse process of REDUCTION 

 may result either in the removal of oxygen or the addition 

 of hydrogen. 



Dumas's experiments on the composition of water, 

 (1842). The reduction of copper oxide by hydrogen was 

 used by Berzelius and Dulong in 1820 (Ann. de Chimie, 15, 

 386-395) to determine the composition of water. Instead 

 of weighing the two gases of which it is composed, they 

 weighed the water and calculated the weight of the oxygen 

 from the loss in weight of the copper oxide ; the difference 

 between these weights gave the weight of the hydrogen. 



