THE COMPOSITION OF WATKK, II VJ>1;< >< i F.N 



117 



to show the decomposability of water at high temperatures is un- 

 attainable. It was considered as such before Henri Sainte- Claire 

 Deville (in the fifties) introduced the conception of dissociation into 

 chemistry, as of a change of chemical state resembling evaporation, if 

 decomposition be likened to boiling, and before he had demonstrated 

 the decomposability of water by the action of heat in an experiment 

 which will presently be described. In order to demonstrate clearly the 

 dissociation of water, or its decomposability by heat, at a temperature 

 approaching that at which it is formed (as a volatile liquid, at a given 

 temperature, can be either in a liquid or vaporous condition) it was 

 necessary to separate the hydrogen from the oxygen at a high tempe- 

 rature, without allowing the mixture to cool. Deville took advantage 

 of the difference between the densities of hydrogen and oxygen. 



A wide porcelain tube p (fig. 19) is placed in a furnace giving a 



FK;. 19. Decomposition of water by the action of heat, and the separation of the hydrogen formed by 

 its permeating through a porous tube. 



strong heat (it should be heated with small pieces of good coke). In 

 this tube there is inserted a second tube T, of less diameter, and made 

 of unglazed earthenware and therefore porous. The ends of the tube 

 are luted to the wide tube, and two tubes, c and c', are inserted into 

 the ends, as shown in the drawing. With this arrangement it is 

 possible for a gas to pass into the annular space between the walls 

 of the two tubes, from whence it can be collected. Steam from 



as a vapour. Similarly as a volatile liquid saturates a space, attaining its maximum 

 tension, so also the products of dissociation have their maximum tension, and once that is 

 attained decomposition ceases, just as evaporation ceases. Under like conditions, if 

 the vapour be allowed to escape (and therefore its partial pressure be diminished), evapora- 

 tion recommences, so also if the products of decomposition be removed, decomposition 

 again continues. These simple conceptions of dissociation introduce infinitely varied 

 consequences into the mechanism of chemical reactions, and therefore we shall have 

 occasion to return to them very often. 



