38 M. Deville on the Dissociation of Water. 



0° : it is the limit beyond which water is entirely decomposed. 

 But this decomposition is accompanied, as will be seen, by a 

 considerable absorption of latent heat, which is necessary to 

 maintain the molecules of hydrogen and oxygen at a greater 

 distance than the radius of the sphere of their affinity. The 

 phenomenon is analogous to that of the ebullition of liquids, in 

 taking place at the same temperature, whatever be that of the 

 source of heat. In short, water cannot resist the action of a 

 temperature which decuples its volume at 0°, and then it decom- 

 poses, while its elements absorb latent heat, which he calls 

 latent heat of decomposition, the amount and existence of which 

 may be readdy calculated. 



According to Clausius, the specific heat of a body does not vary 

 with the temperature. Thequantity of heat produced in the 

 formation of a gramme of water, from Favre and Silbermann's 

 determinations, is 3883 thermal units. Now the quantity of 

 heat absorbed by a gramme of water in passing from 0° to 2500° 

 is given by the formula 



637 + (.2500 - 100)0-475 = 1 680, 



in which 637 is the quantity of heat needed to transform 1 

 gramme of water into vapour at 100°, while (2500— 100)0*475 

 is the heat needed to raise this vapour from 100° to 2500°. The 

 difference between 3833 and 1680 = 2153 thermal units repre- 

 sents the latent heat of the decomposition of water, the heat 

 absorbed by its elements at the moment of their separation. 



The comparison between the effects of cohesion and affinity 

 is maintained in the inverse phenomena, volatilization and de- 

 composition. Assuming this relation, the decomposition of 

 bodies at a relatively low temperature, or the phenomenon of 

 dissociation, corresponds to the vaporization of a liquid at a 

 temperature below its boiling-point, and the quantity of the 

 body decomposed will be proportional to its tension of dissocia- 

 tion expressed in millimetres of mercury, as the quantity of 

 vapour formed above a liquid is proportional to the maximum 

 tension of its vapour. 



A liquid has no tension in its own vapour, and the quantity of 

 water vaporized in a closed space compared with the volume of 

 water is very small. In like manner the quantity of vapour 

 dissociated at 1200° in a porcelain balloon is so small that the 

 density of vapour is not affected by it. 



Of water placed in a closed vessel of small volume, the quan- 

 tity vaporized is very small, the tension of the liquid being 

 annulled as soon as the space is saturated; but if a fragment of 

 chloride of calcium is introduced, the water will evaporate until 

 this is liquefied, the tension always remaining the same. This 



