116 



PRINCIPLES OF CHEMISTKV 



and it will easily be seen that two volumes of hydrogen are evolved for 

 every one volume of oxygen. This signifies that, in decomposing, water 

 gives two volumes of hydrogen and one volume 

 of oxygen. 



Water is also decomposed into its com- 

 ponent parts by the action of heat. At the 

 melting point of silver (960), and in its pre- 

 sence, water is decomposed and the oxygen 

 absorbed by the molten silver, which dissolves 

 it so long as it is liquid. But directly the 

 silver solidifies the oxygen is expelled from it. 

 However, this experiment is not entirely con- 

 vincing ; it might be thought that in this case 

 the decomposition of the water did not proceed 

 from the action of heat, but from the action 

 of the silver on water that silver decom- 



p ses water ' takin s U P the 



t s m - 



determining the relation be- possible to directly show the decomposition 



tween the volumes of hydrogen * . * L 



and oxygen. o f water by the action of heat, because the 



component parts of water, if they remain 



together, re-combine with a fall of temperature, and give water back 

 again. For instance, if steam be passed through a red-hot tube, 

 whose internal temperature attains 1,000, then a portion 5 of the water 

 decomposes into its component parts, forming detonating gas. But on 

 passing into the cooler portions of the apparatus this detonating gas 

 again reunites and forms water. The hydrogen and oxygen obtained 

 combine together at a lower temperature. 6 Apparently the problem 



5 As water is formed by the combination of oxygen and hydrogen, the reaction evolving 

 much heat, and as it can also be decomposed, therefore this reaction is a reversible 

 one (see Introduction), and consequently at a high temperature the decomposition of 

 water cannot be complete it is limited by the opposite reaction. Strictly speaking, it is 

 not known how much water is decomposed at a given temperature, although many efforts 

 (Bunsen, and others) have been made in various directions to solve this question. Not 

 knowing the coefficient of expansion, and the specific heat of gases at such high tem- 

 peratures, renders all calculations (from observations of the pressure on explosion) 

 doubtful. 



6 Grove, about 1840, observed that a platinum wire fused in the flame of detonating 

 gas that is, having acquired the temperature of the formation of water and having 

 formed a molten drop at its end which fell into water, evolved detonating gas that 

 is, decomposed water. It therefore follows that water already decomposes at the tem- 

 perature of its formation. At that time, this formed a scientific paradox ; this we shall 

 unravel only with the development of the conceptions of dissociation, introduced into 

 science by Henri Sainte-Claire Deville, about 1850. These conceptions form an im- 

 portant epoch in science, and their development is one of the problems of contemporary 

 chemistry. The essence of the matter is that, at high temperatures, water exists but also 

 decomposes, just as a volatile liquid, at a certain temperature, exists both as a liquid and 



