244 WELLS'S NATURAL PHILOSOPHY. 



by heat, deposit their condensed vapors in a solid form. This process is 

 termed sublimation. 



What remark- 563. One of the most remarkable circum- 

 stance "nuend's stances which accompany the phenomena, 

 vaporSonT'^ ^oth of liqucfactioH and vaporization, is the 

 disappearance of the heat which has effected 

 the change. 



IIow may this Thus, ii' a thermometer be applied to a mass of snow, or ica 



principle be U- just upon tliG point of meltiug, it will be found to stand at 

 "'' '^''"^ 32° F. If the ice be placed in a vessel over a fire, and tho 



temperature tested at the moment it has entirely melted, the water produced 

 will have only the temperature of 32°, the same as that of the original ice. 

 Heat, heweper, during the whole process of melting, has been passing rajjidly 

 into the vessel from the fire, and if a quantity of mercury, or a solid of the 

 same size, had been exposed to the same amount of heat, it would have con- 

 stantly increased in temperature. It is clear, therefore, that the conversion of 

 ice, a solid, into water, a licjuid, has been attended with a disappearance of heat. 



Again : if one pound of water, having a temperature of 174°, be mixed with 

 one pound of snow at 32°, we shall obtain two pounds of water, having a 

 temperature of 32°. All the heat, therefore, which was contained in tho 

 hot water is no longer to be detected by the thermometer, it having been en- 

 tirely used up, or disposed of in converting snow at 32° into water at 32°. 

 Such disappearances always occur whenever a solid is converted into a liquid. 



If, however, a pound of water at 32°, instead of ice at the same tempera- 

 ture, had been mixed with a pound of water at 174°, we ehall obtain two 

 pounds at 103° a temperature exactly intermediate between the temperatures 

 of the components But if the pound at 32° had been soUd instead of liquid, 

 then the mixture, as before explained, would have had a temperature of 32°. 

 It is evident, therefore, that it is the process of liquefaction, and it alone, which 

 renders latent or insensible all that heat which is sensible when the pound 

 of water at 32° is liquid. 



„ „ In the same manner heat disappears when a liquid is eon- 



How may the , . m, , • /-, • 1 • • X 



absorption of verted mto a vapor. The absorption or heat, in this instance, 



heat la evapo- ^^^y j^^ easily rendered perceptible to the feelings by pouring 

 dcrud eviUent ? a few drops of some liquid which readily evaporates, such as 

 ether, alcohol, etc., upon the hand. A sensation of cold is immediately ex- 

 perienced, because the hand is deprived of heat, which is drawn away to effect 

 the evaporation of the liquid. On tho same principle, inflammation and fever- 

 ish heat in the head may be allayed by bathing the temples with Cologno 

 water, alcohol, vinegar, etc. 



If we surround the bulb of a thermometer loosely with cotton, and then 

 moisten the latter with ether, the thei mometer will speedily fall several degrees. 

 Why can not Water when placed in a vessel over a fire, gradually at- 



Vater impart tains the boiling temperature, or 212°; but afterward, how- 

 oftcr boUing f ever much we may increase the fire, it becomes no hotter, all 



