VAPORIZATION. 321 



of the overlying water, and the pressure of the atmosphere 

 above the water. Then the water boils. 



(a.) When the first bubbles of steam are formed at the bottom of 

 the water, they rise through the water, condense in the cooler layers 

 above, and disappear before reaching the surface. The formation 

 and condensation of these bubbles produce the peculiar sound known 

 as singing or simmering, the well-known herald of ebullition. 

 Finally, the water becomes heated throughout, the bubbles increase 

 in number, grow larger as they ascend, burst at the surface, and 

 disappear in the atmosphere. The whole liquid mass is agitated 

 with considerable vehemence, there is a characteristic noisy accom- 

 paniment, the quantity of water in the flask diminishes with every 

 bubble, and finally it all disappears . as steam. The water has 

 "boiled away." 



502. Laws of Ebullition. It has been found by 

 experiment that the following statements are true : 



(1.) Every liquid begins to boil at a certain temperature, 

 which is invariable for the given substance if the pressure 

 be constant. When cooling, the substance will liquefy at 

 the temperature of ebullition, or at the boiling point. 



(2.) The temperature of the liquid, or vapor, remains 

 at the boiling point from the moment that it begins to 

 boil or liquefy. 



(3.) An increase of pressure raises the boiling point; a 

 decrease of pressure lowers the boiling point. 



503. Effect of Pressure upon Boiling Point. 



We saw in 501 that when a liquid is boiled, the heat 

 has three tasks or three kinds of work to perform, viz., 

 overcoming cohesion, liquid and atmospheric pressures. 

 Nothing can be more evident than the propositions that 

 increasing the work to be done involves an increase in the 

 energy needed to do the work ; that decreasing the work 

 to be done involves a decrease in the energy needed to do 

 the work. In the case of boiling any given liquid, the first 



