548 MR J. Y. BUCHANAN ON 



kilogr. per square centimetre, or a total weight of WA kilogr. In order to confine the 

 water so as to admit of its temperature being raised from T to t, we must add to WA 

 a certain additional load, b kilogrammes, which is found by consulting Regnault's tables 

 connecting the temperature and tension of saturated steam. In these tables we find 

 directly the tension, p , of saturated steam of temperature t , in millimetres of mercury, 

 which is converted into a in k/c 2 . The excess of this above the atmospheric pressure 

 multiplied by the surface, W, gives the extra load required : 



6=W(a -A), 



and this quantity b kilogrammes is the mechanical equivalent of the fifth of a molecule 

 of salt in so far as the raising of the boiling temperature of water, or the resistance to 

 steam pressure, is concerned. 



In the case of the boiling saturated solution of salt when steam is continued to be 

 passed through and heat is lost, the solution is diluted by the addition of the condensed steam 

 to the original quantity, W, of saturated water. This dilution, or addition of pure water 

 to the saturated water, is accompanied by a fall of the temperature of ebullition, which 

 is very rapid at first, but becomes slower as the quantity of condensed steam increases, 

 tending ultimately towards the boiling temperature of pure water at atmospheric pressure. 



The more concentrated the solution is, the more accentuated are the specific proper- 

 ties of the dissolved salt, and they are most pronounced in the saturated solution which 

 approximates to the condition of the liquefied salt, as the dilute solution approximates to 

 that of pure water. The specific nature of the dissolved salt shows itself first in the 

 maximum temperature to which the solvent water can be raised under a given pressure, 

 and then in the rate of fall of boiling temperature with dilution. Different salts behave 

 differently in these respects. 



The uniformity observed in the physical properties of very dilute solutions is due 

 in part to our limited powers of perception, and to arithmetical necessity. In propor- 

 tion as the number expressing the dilution becomes very great it tends to occupy the 

 whole field of view, and, consequently, to obscure or obliterate the specific properties 

 of the substance dissolved. 



Similarly, in considering the trigonometrical functions of angles, if we limit our con- 

 templation to very small angles, we can perceive no difference between the sine, 

 the arc, and the tangent, 3'et the difference is none the less real on that account. 



In our mechanical experiment the excess of pressure of the water at any moment 



above that of the atmosphere is =^-, and when this is multiplied by the quantity of water 



present, W n , the product is constantly b. Now b is the equivalent of the salt dissolved, 

 therefore our mechanical experiment represents the case where the increase of steam 

 tension, neutralised by the salt, is proportional to the quantity of the salt. 



For convenience in reference, the values of the tension of saturated steam, at ten) 



