Heat as a Quantity 179 



In the case of the distilled water, it will probably 

 be found that the steam temperature differs but little 

 from that of the boiling water, while any variations 

 are temporary in character, no marked rise nor fall 

 being noted. It is likely, however, that the ther- 

 mometer, when plunged in the tap-water, will indicate 

 that it is slightly hotter than the steam. Small 

 variations of temperature may be observed, but, as 

 in the previous case, no indication of any definite rise 

 nor fall. The salt solutions will provide much more 

 strongly marked evidence of the difference of tempera- 

 ture between the liquid and the steam immediately 

 over it than that afforded by the tap-water, and the 

 water will, in each case, be the hotter body. As the 

 boiling is continued, the temperature registered will 

 slowly increase to a maximum of about 108, provided 

 the period of heating be long enough. On comparing 

 the four steam temperatures very little difference wi]l 

 be found between them. This observation may be 

 confirmed by treating other solutions of substances in 

 water similarly, provided that the solutions be not too 

 concentrated. With very strong solutions the tempera- 

 ture of the escaping steam is distinctly higher than 

 that of the steam from pure water. 



The general deduction from the above observations 

 is that 



The presence of a substance in solution in water 

 raises the temperature at which the liquid boils, while 

 the temperature of the steam produced is but little 

 affected, except with very strong solutions. 



Exercise for Student. 



Explain the slow rise of temperature produced when the salt 

 solutions are kept boiling for some time. 



122 



