HEAT. 



325 



ature. The temperature at which different liquids thus boil is called their 

 boiling point. 



The melting or freezing point and the boiling point constitute important 

 physical characters, by which different substances are distinguished from each 

 other. 



When heat continues to be supplied to a liquid which is in the state of ebul- 

 lition the liquid is gradually converted into vapor or steam, which is a form of 

 body possessing the same physical characters as atmospheric air. The steam 

 or vapor thus produced has the same temperature as the water from which it 

 was raised, notwithstanding the great quantity of heat imparted to the water in 

 its transition from the one state to the other. This quantity of heat is therefore 

 latent. 



The abstraction of heat produces a series of effects contrary to those just 

 described. If heat be withdrawn from a liquid, its temperature will first be 

 gradually lowered until it attain a certain point, at which it will pass into the 

 solid state. This point is the same as that at which, being solid, it would pass 

 into the liquid state. Thus water, gradually cooled from sixty degrees down- 

 ward, will fall in its temperature until it attains the limit of thirty-two degrees ; 

 there it passes into the solid state and forms ice ; and during this transition a 

 large quantity of heat is dismissed, while the temperature is maintained at 

 thirty-two degrees. 



In like manner, if heat be withdrawn from steam or vapor, it no longer re- 

 mains in the aeriform state, but resumes the liquid form. In this case it un- 

 dergoes a very great diminution of bulk, a large volume of steam forming only 

 a few drops of liquid. Hence the process by which vapor passes from the 

 aeriform to the liquid state has been called condensation. 



When a liquid boils vapor is generated in every part of its dimensions, and 

 more abundantly in those parts which are nearest the source of heat ; but li- 

 quids generate vapor from their surfaces at all temperatures. Thus, a vessel 

 of water at the temperature of eighty degrees will dismiss from its surface a 

 quantity of vapor, and if its temperature be retained at eighty degrees, it will 

 continue to dismiss vapor from its surface at the same rate, until all the water 

 in the vessel has disappeared. This process, by which vapor is produced at 

 the surface of liquids at temperatures below their boiling point, is called vapor- 

 ization. 



The process of vaporization is generally going on at the surface of all collec- 

 tions of water, great or small, on every part of the globe ; but it is in still more 

 powerful operation when liquid juices are distributed through the pores, fibres, 

 and interstices of animal and vegetable structures. In all these cases, the rate 

 at which the liquid is converted into vapor is greatly modified by the pres- 

 sure of the atmosphere. The pressure of that fluid retards vaporization, if its 

 effects be compared with that which would take place in a vacuum ; but, 

 on the other hand, the current of air, continually carrying away the vapor, as 

 fast as it is formed, in the space above the surface, gives room for the formation 

 of fresh vapor, and accelerates the transition of the liquids to the vaporous state. 

 The process of vaporization, thus modified by the atmosphere and its currents, 

 so far as it affects the collections of water and liquids generally in various parts 

 of the earth, is denominated evaporation. 



The condensation of the vapor, thus drawn up and suspended in the atmo- 

 sphere by various causes, tending to extricate the latent heat which gives to it 

 the form of air, produces all the phenomena of dew, rain, hail, snow, &c., &c. 

 A slight degree of cold converts the vapor suspended in the atmosphere into a 

 liquid, and by the natural cohesion of its molecules it collects into spherules or 

 drops, and falls in the form of rain. A greater degree of cold solidifies or con- 



