AQUEOUS VAPOUR, WATER, AND ICE 27 



as figured. Continue the experiment till a constant reading is 

 obtained. 



The temperature of boiling water depends upon the purity of the water, 

 the temperature of the steam is constant ; hence the latter is determined. 

 A small flame without wire gauze is best for heating the water, otherwise 

 the sides of the upper part of the flask are apt to become superheated. 

 Since the recording of the temperature by the thermometer depends upon 

 the expansion of the mercury by heat, the mercury in both bulb and stem 

 must be in the steam. Water only boils when it is so hot that the pressure 

 of the water vapour is able to overcome the pressure of the atmosphere. 

 The boiling point, therefore, varies with the atmospheric pressure ; and 

 hence, when a boiling point is determined, the reading of the barometer 

 must also be noted. 



The boiling point of water is also the condensing point of 

 steam; likewise, the freezing point of water is the melting 

 point of ice. Determine the latter by immersing the end of a 

 thermometer in a beaker filled with melting snow, or with 

 finely powdered ice saturated with water. Constantly stirring 

 the mixture with the thermometer, repeatedly note the tem- 

 perature without removing the thermometer from the beaker 

 till a constant reading is obtained. 



The graduation of a thermometer is seldom quite correct, 

 and future readings should always be corrected for any error 

 thus found. On the centigrade scale, the freezing point is the 

 zero (" 0° C"), and the distance between that and the boiling 

 point is divided into 100 degrees. On the Fahrenheit scale, 

 the freezing point is 32 degrees above the zero, and the distance 

 between this and the boiling point is 180 degrees, the boiling 

 point being "212° F." Each degree Fahrenheit is therefore 

 \%% of a degree centigrade. Compare the reading of a centi- 

 grade and Fahrenheit thermometer. Do they record the same 

 temperature ? 



