188 



Dr. C. V. Burton on Endless Availability. 



Experiment III. — The apparatus was arranged as follows: — 

 A beaker, A (fig. 2), contained saturated solution and crystals 



Fig. 2. 



of sodium sulphate, in which some 

 parchment-paper, C, was partly im- 

 mersed, so as to increase the surface 

 for evaporation ; A was placed within 

 a larger beaker, B, which was closed 

 by a glass plate, D, luted on with 

 soft wax so as to be air-tight. The 

 apparatus was left in a dark corner, 

 and after a day or two, dew was seen 

 to have collected on the sides of 

 the outer beaker, B. After six weeks, about half a gram of 

 water had collected in the beaker. 



Now, undoubtedly the apparatus underwent changes of 

 temperature ; but these would be essentially very slow, so 

 that the difference of temperature between one part of the 

 apparatus and another would be extremely small, and this 

 must have been especially the case between the surface of the 

 solution and the adjacent walls of the beaker A, where dew 

 had also formed and gradually increased in amount. 



From the continued distillation it seemed probable that the 

 vapour-pressure over thoroughly saturated sodium-sulphate 

 solution is slightly greater than over pure water. It is evident 

 that the saturated solution has the smaller latent heat of 

 vaporization, since evaporation is then always accompanied 

 by . crystallization. Hence a water-molecule expends less 

 energy in attaining the gaseous state than would be the case 

 at the surface of pure water. 



Here, again, we can perform a complete cycle of operations. 

 The water which has distilled may be collected, and the 

 deposited crystal dissolved in it. A fall of temperature will 

 result, which will render some of the heat of surrounding 

 objects available for mechanical work. When the resulting 

 solution has been raised to its previous temperature, it will 

 just be saturated ; and on pouring it back into the beaker A, 

 the initial conditions are exactly restored. 



A direct measurement was also made of the vapour-pressure 

 over saturated sodium-sulphate solution (without parchment- 

 paper). A flask containing a thick paste of sodium-sulphate 

 solution and powdered crystals was immersed in a bath of 

 water, and was connected by a rubber tube to a Sprengel 

 pump and a barometer-tube. The vapour-pressure at 12°*72 

 was found to be 10-7 mm., which differs from the vapour- 

 pressure over pure water at the same temperature by only 



