152 



ELEMENTARY GENERAL SCIENCE 



CHAP. 



water become heated and rise, their place being taken by the 

 colder layers from above. After this heating has been continued 

 for a certain length of time, the bulk of the water gets so hot that 

 the bubbles of vapour which are formed near the source of heat 

 are not condensed again in their upward passage through the 

 liquid, and coming to the surface they escape as steam. The 

 temperature at which bubbles of this sort get formed throughout 

 the mass of the liquid is quite definite for a given pressure of the 

 atmosphere and is known as the boiling point. 



EXPT. 152. Fit a cork, with two holes through it, into a flask 

 or test-tube (Fig. 69). Half fill the 

 flask or test-tube with water, and 

 push the thermometer through one 

 of the holes in the cork until the bulb 

 is wholly immersed. Into the other 

 hole fit a piece of glass tubing bent 

 at right angles. Place the flask or 

 test-tube upon a retort stand, and 

 observe the division of the thermo- 

 meter scale level with the top of the 

 mercury. Gently heat the water until 

 it boils, noticing how the mercury of 

 the thermometer is affected through- 

 out the process. See where the 

 mercury stands when the water is boil- 

 ing and find whether any difference 

 is produced when the water is boiling 

 furiously and when it is boiling gently. 

 Raise the thermometer until the bulb is just above the boiling 

 water, and observe the temperature which it then indicates. 



Effect of Pressure on the Boiling Point. A word or two 

 must be said with respect to the reservation which has been 

 made about the pressure of the air. It has been seen that 

 the weight of the atmosphere is very considerable. It presses 

 upon the surfaces of all bodies with a force dependent upon 

 its weight, which, like all other forms of matter, is pro- 

 portional to its mass, and the mass of the atmosphere will depend 

 upon the extent of the air above the body, which will clearly be 

 less at the top of a mountain than at the bottom of a mine. 



If we wish to boil a liquid, therefore, in those cases where the 

 pressure of the atmosphere is great we shall have to heat the 



FIG. 69. Determining a 

 boiling point. 



