Till: COMPOSITION OF WATER, HYDROGEN 147 



although they may take place with the aid of energy (electrical, ther- 

 mal, &c.) borrowed from some foreign source." 1 



The reduction of metals by hydrogen is taken advantage of for 

 determining the exact composition of water by weight. Copper oxide is 

 usually chosen for this purpose. It is heated to redness in hydrogen, 

 and the quantity of water thus formed is determined, then the quantity 

 of oxygen which occurs in it is found from the loss in weight of the 

 copper oxide. This loss will depend on the fact that the oxygen has 

 entered into the water. The copper oxide must be weighed immediately 

 before and after the experiment. The difference shows the weight of 

 the oxygen which entered into the composition of the water formed. 

 In this manner only solids have to be weighed, which is a very great 

 gain in the accuracy of the results obtained. 47 Dulong and Berzelius 

 (1819) were the first to determine the composition of water by this 

 method, and they found that water contains 88'91 of oxygen and 11*09 

 of hydrogen in 100 parts, or 8-008 parts of oxygen per one part of 

 hydrogen. Dumas (1842) improved on this method, 48 and found that 



46 Several numerical data and reflections bearing on this matter are enumerated in 

 Notes 7, 9, and 11. It must be observed that the action of iron or zinc on water, or, con- 

 versely, of hydrogen on the oxides of iron or zinc, forms a reversible reaction, which 

 proceeds in one or the other direction, according to which is removed from the sphere of 

 action ; the hydrogen or the water act according to which is present in a predominating 

 mass. The influence of mass is clearly evinced in this case. . But the reaction 

 CuO + H. 2 = Cu + HoO is not reversible ; the difference between the degrees of affinity is 

 very great in this case, and, therefore, as far as is at present known, no hydrogen is 

 evolved even in the presence of a large excess of water. It is to be further remarked, 

 that under the conditions of the dissociation of water, copper is not oxidised by water, most 

 probably because the oxide of copper itself is decomposable by heat. 



47 This determination may be carried on in an apparatus like that mentioned in Note 

 13 of Chapter I. 



48 We will proceed to describe Dumas' method and results. For this determination 

 pure and dry copper oxide is necessary. Dumas took a sufficient quantity of copper 

 oxide for the formation of 50 grams of water in each determination. As the oxide of 

 copper was weighed before and after the experiment, and as the amount of oxygen con- 

 tained in water was determined by the difference between these weights, it was essential 

 that no other substance besides the oxygen forming the water should be evolved from 

 the oxide of copper during its ignition in hydrogen. It was necessary, also, that the 

 hydrogen should be perfectly pure, and free not only from traces of moisture, but from 

 any other impurities which might dissolve in the water or combine with the copper and 

 form some other compound with it. The bulb containing the oxide of copper (fig. 26), 

 and which was heated to redness, should be quite free from air, as otherwise the oxygen 

 in the air might, in combining with the hydrogen passing through the vessel, form water 

 in addition to the oxygen of the oxide of copper. The water formed should be entirely 

 absorbed in order to accurately determine the quantity of the resultant water. The 

 hydrogen was evolved in the three-necked bottle. The sulphuric acid, for acting on the zinc, 

 is poured through funnels into the middle neck. The hydrogen evolved in the Woulfe's 

 bottle passes through U tubes, in which it is purified, to the bulb, where it comes into 

 contact with the copper oxide, forms water, and reduces the oxide to metallic copper; 

 the water formed is condensed in the second bulb, and any passing off is absorbed in the 

 second set of U tubes. This is the general arrangement of the apparatus. The bulb 



L 2 



