FEEEZIXG AND , BOIL1XG. 



or freezing, and the temperature 32 at which it takes place, is 

 called the freezing point of water. 



7. If water be exposed to any source of heat, such as a fire or 

 a lamp, it will, as may be naturally imagined, become continually 

 hotter and hotter, but this increase of heat will not be unlimited. 

 It will, on the contrary, after a certain continuance of the action 

 of the fire or lamp upon it, attain a degree of heat or temperature 

 which it will never exceed, however intense or long continued the 

 action of the fire may be. In the ordinary state of the atmos- 

 phere, this temperature is that marked 212 on the thermometer. 

 If a thermometer be immersed in the water, it will stand constantly 

 at this temperature, although the action of the fire upon the water 

 still continues. 



When the water attains this stationary point of temperature it 

 will be observed to be affected by a violent agitation throughout 

 every part of it. Bubbles of vapour are formed at the parts of 

 the vessel which are next the fire, and these rising with a certain 

 violence, escape continually from the surface and produce the 

 peculiar agitation of the liquid which has been just mentioned. 



This state of water is called EBULLITION or BOILING, and the 

 stationary temperature of 212, at which it takes place, is called 

 the BOILING POINT of the thermal scale. 



8. Until the water exposed to the action of fire has attained the 

 boiling-point, the heat imparted to it is employed in raising its 

 temperature, or, in familiar language, in rendering it hotter. But 

 after it has attained the limit of its temperature, and ceases to be 

 rendered hotter, the fire still continues to impart the same heat to 

 it, and it may be asked, What becomes of this heat ? How is it 

 absorbed, employed or disposed of? since it is certain that the 

 water does not receive it. 



This is easily explained. The water which the vessel contains 

 does not become hotter, and therefore can receive none of the heat 

 imparted by the fire, but it is rapidly converted into vapour, 

 and this vapour, escaping continually from the surface of the water, 

 rises into the air. The quantity of water in the vessel is con- 

 tinually diminished by the quantity thus escaping in the form of 

 vapour, and if the process be continued, the water will altogether 

 disappear from the vessel, being all converted into vapour. 



The heat, then, imparted by the fire, in this case, and which fails 

 to augment the temperature of the water in the vessel, is altogether 

 absorbed by the vapour into which the water is converted. This 

 vapour, it is true, is not hotter than the water in which it is 

 formed, its temperature, like that of the water, being 212 ; but 

 it is proved by experiments, made in the laboratories of chemists 

 and philosophers, that much more heat is required to impart to 

 n2 99 



