304 Introduction to the Study of Science 



application of heat, is equal to the pressure upon its free sur- 

 face. When equal, the water vapor as bubbles will rise to, and 

 escape from, the surface of the liquid. This is " boiling." 



Fifth. The temperature of steam remains the same as that 

 of boiling water as long as it is in contact with it. 



132. Amount of heat required to produce steam. After 

 water has been brought to the boiling point by heat, it requires 

 a great deal more heat to change it into steam. You can make 

 a rough judgment of the additional heat needed by experi- 

 menting with a given quantity of water, as fifty grams by 

 weight, and comparing the time required to raise its tempera- 

 ture from 32 F. to 212 F. with the time required to convert a 

 certain fraction of the water into steam. If, for instance, ten 

 grams of water were evaporated in a given time, the total 

 quantity or fifty grams would be vaporized probably in five 

 times the time required for the ten grams. The result is of 

 course only approximate, but good enough for practical pur- 

 poses. 



The quantity of heat absorbed by water in being raised from 

 freezing to boiling temperature is usually referred to as the 

 heat of water, or as applied to all liquids as the heat of the liquid. 



The heat of steam is the other factor of importance. The 

 additional heat necessary to change water into steam does not 

 register sensibly or by the thermometer. The general belief is, 

 however, that somehow the extra heat is not lost or wasted if 

 the steam generated is utilized for heating or for power. This 

 implies that steam may contain the heat of the liquid and also 

 heat supplied in changing the liquid into steam. The facts 

 may be made clear by experiments. 



Exercise : Heat of steam. Balance a beaker containing a ther- 

 mometer on a pair of trip scales, and pour into it enough water at 

 32 F. to balance a hundred gram weight. Arrange a boiling-flask 

 with about fifty grams by weight of water (the apparatus used in 

 showing relation of high pressure and boiling temperature will be 

 excellent) ; connect the escape valve by rubber tubing, tipped with 



