STEAM AND BRINES. 545 



diminishes as W increases ; at a height of 3000 metres it increases with W, and at a 

 height of 2000 metres it is sensibly constant. In the case of barium chloride, W(t — T) 

 diminishes with dilution ; the same is the case with strontium nitrate and ammonium sul- 

 phate. Potassium chlorate, barium nitrate, and lead nitrate show W(t — T) increasing with 

 W, while sodium nitrate and potassium sulphate show almost constant values of W(t — T) # 

 Mixtures of salts follow the rule of their components. There are several examples in the 

 table of pairs of salts which individually differ in the sense in which the values of 

 W(£-T) depart from constancy, and in mixture give constant values of W(t — T). 

 Examples are Nos. 63, 70, 71, 72, 73, 78, and 79. 



At the beginning of an experiment, when the steam reaches J:he salt, it condenses 

 very rapidly owing to abstraction of heat by the glass and by the salt, then it condenses 

 at a very regular rate, the salt dissolving in proportion as steam is condensed. After a 

 certain time the exact amount of steam has condensed which is necessary to form a boil- 

 ing saturated solution of the salt taken ; having observed (t — T) and W we have the 

 value of W (£ — T). If the elevation of the boiling point were proportional to the 

 concentration, this factor W (£ — T) would remain constant while the solution was 

 diluted by further condensation of steam. But if we deny thermal importance to the 

 salt and consider only the water, then W (£ — T) is the heat in the saturated water, 

 counting from the temperature of pure boiling water. If we prevent it from losing heat 

 externally, and provide for dilution by furnishing water of exactly the temperature of 

 pure steam condensing on pure water at the time and place, the saturated water will 

 mix with the free water, having a resultant temperature depending on the relative 

 quantities of the saturated and the free water. The case, then, of sodium nitrate, for 

 instance, in which the value of W(£ — T) is nearly constant, could be represented by 

 imagining the steam to condense at the temperature of the boiling saturated solution of 

 the salt so long as solid salt is present, and the condensation temperature of the steam 

 remains constantly at the maximum. When the solid salt has all disappeared, then the 

 steam condenses at the temperature at which it condenses in pure water. The two por- 

 tions of water, the saturated and the free, then mix, giving the resultant temperature, 

 depending on the relative quantities and on the assumption that the heat of the satu- 

 rated solution is that which the water present in it would have if it had the same tem- 

 perature. 



In the cases where W(t — T) is constant, we have Blagden's law of the lowering of 

 freezing point applied to the raising of the boiling point of saline solutions ; both vary 

 directly with the concentration or inversely with the dilution. But in the case of a 

 saline solution following Blagden's law, when ice is melted in the saturated solution 

 already cooled to its freezing point, the solution is diluted and its temperature rises. The 

 rise of temperature is thus the same as would have been produced if the quantity of ice 

 which has melted had been added as pure water of 0° C. to the saturated solution at the 

 initial temperature of its freezing point, and the two had been mixed. 



The same is the case, mutatis mutandis, in the condensation of steam by a saline 



VOL. XXXIX. PART III. (NO. 18 ). 4 N 



