144 



Domestic Science 



length detach themselves from the interior of the 

 vessel and rise to the top of the liquid. The bubbling 

 thus produced must not be mistaken for boiling. 

 The small bubbles are not bubbles of steam but 

 of air, which was dissolved in the water and is driyen 

 out of solution by heating the liquid. At a later stage 

 somewhat larger bubbles may be seen to form at the 

 bottom of the flask and to rise a short distance, but 

 not to reach the top of the water. These are the first 

 steam-bubbles, which are formed at the hottest part 

 of the liquid but are condensed to water again on 

 coming in contact with the cooler water during their 

 upward passage. These bubbles of steam become larger 

 and more numerous as the heating proceeds and rise 

 to a greater height before collapsing. At length the 

 bubbles reach the top of the liquid and burst there. 

 As soon as steam is thus escaping freely at all parts of 

 the surface of the liquid, the true 

 " boiling-point " has been reached. 

 In order that the steam may thus 

 reach the surface of the water and 

 pass freely into the space above it, 

 the pressure of the steam must be 

 at least equal to that of the air which 

 fills this space, or the air will not be 

 pushed away by the steam. 



Boiling, or " ebullition ", must be 

 distinguished from "evaporation". 

 In the latter process the formation 

 and escape of vapour take place only 

 at the free surface of the liquid, while 



Fig. 45. 



during ebullition steam is formed in all parts of the 

 water. Ebullition occurs at a certain temperature, 

 dependent upon the exterior pressure on the surface of 



