66 Chemical and Physical Notes 



slightly from exact proportionality in one sense or in the 

 other. 



If Tf be the freezing-point of water, and // be the freezing- 

 point of a solution which for a certain quantity of dissolved 

 salt contains a quantity Wj of water, then the value of the 

 product Wf(T f tj) is constant in the case of some salts ; in 

 the case of others it increases as Wf, or the amount of water, 

 increases ; in the case of others again it decreases as the 

 dilution increases. But the deviations from constancy are 

 never great even in nearly saturated solutions. 



If T b be the boiling-point of water, and t b be the boiling- 

 point of a solution which, for a certain quantity of dissolved 

 salt, contains a quantity W b of water, then the value of the 

 product W b (t b T b ) is constant for some salts ; in the case of 

 others it increases as W b , or the amount of water, increases ; 

 in the case of others again it decreases as dilution increases. 



The product of a mass into a temperature is a quantity of 

 heat. Therefore, the expressions W f (T f - //) and W b (t b - T b ) 

 represent quantities of heat 1 . The physical meaning or in- 

 terpretation of the statement that either of these products is 

 constant, when the proportion between the factors in it varies, 

 is that water can contain more heat without boiling, and less 

 heat without freezing, in proportion to the amount of salt 

 dissolved in it. 



This is a general law of which Blagden's law of freezing is 

 a particular case. 



The phenomena accompanying the freezing and the boiling 

 of saline solutions have always been perplexing. Salt-water 

 ice melts at a lower temperature than fresh-water ice, and it 

 is impossible to prepare pure water from it ; therefore it was 

 held by some that the salt must belong to the ice. But by 

 skilfully managing the melting, the temperature may approach 

 very near to o C., and the water prepared from it may 

 contain very little salt, and may be drinkable; therefore it 

 was held by others that the salt belonged to the brine. The 

 available experimental data did not conclusively prove either 

 that the salt belongs exclusively to the ice or to the brine. 



1 'Steam and Brines,' below, p. 173. 



