122 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP- 



tion ; this product must then be regarded as pure water 

 charged with the small quantity of vitriolic acid which in- 

 flammable air carries when prepared by the dissolution of 

 iron." 



" It follows from this experiment, that when inflammable 

 air and dephlogisticated air, both pure, are exploded, there 

 is no other product but pure water, heat, and light " (loc. cit., 

 pp. 86 and 87). 



Monge's experiments agreed with those of Cavendish and 

 with the later experiments of Humboldt and Gay Lussac 

 (Journ. de Physique, 1805, 60, 129 168), in showing that the 

 volume of hydrogen used in the explosion is about twice as 

 great as that of the oxygen. But, owing to the use of moist 

 gas, the density of the hydrogen and its proportion by weight 

 were greatly over-estimated. It is, however, noteworthy that 

 the method used by Monge in 1781 was essentially the same 

 as that employed 115 years later in the classical experiments 

 of Scott and of Morley, who determined the composition of 

 water with very great accuracy by measuring the densities 

 of the two gases and the ratio of their combining volumes. 



Volta's eudiometer. The fact that oxygen and hydrogen 

 (unlike nitrous air and oxygen, Chapter X, p. 201) combine 

 together in a definite ratio, was used by the Italian physicist 

 Alexandro Volta (1745 1827) (Annali di Chimtca, 1790, 

 I. 171 232; 1791, II. 261 286; 1791, III. 36 45) to 

 estimate these gases. In order to explode the gases together, 

 Volta introduced a metal cap, b, carrying an insulated wire, 

 c, into the upper part of the graduated tube which constituted 

 Priestley's " eudiometer." The instrument thus modified 

 was known as VOLTA'S EUDIOMETER (Fig. 27). In order to 

 estimate oxygen, for instance in atmospheric air, an excess 

 of hydrogen was added and the mixture exploded ; as 

 two volumes of hydrogen combined with one of oxygen to 

 form water, one-third of the decrease of volume was due to 

 oxygen, the volume of which could thus be estimated very 



