246 WELLS'S NATURAL PHILOSOPHY. 



How great a crater may be obtained. The application of this experiment 



degree of cold to tlie freezing of ice-creams is familiar to all. 

 can be obtain- _ . . _,,.., , . 



ed by freezing By mix'ng snow and sulphuric acid together m proper pro- 

 mixtures ? portions, iii temperature of 90° below zero can be obtained 

 without difficulty. 



y^ . , . The air in the spring of the year, when the ice and snow 



In spring cold are thawing, is always peculiarly cold and chilly. This is due 

 and chilly ? ^ ^j^g constant absorption of heat from the air by the ice and 



snow in their transition from a soUd to a liquid state. 



„^ , A shower of rain cools the air in summer, because the earth, 



why does a , , . , , . , , . , 



Bhowcrinsum- and the air both part with their heat to promote evaporation. 



mer cool the jy g, like manner, the spriukUng of a hot room with water 



cools it. 



Why is the The draining of a country increases its warmth, since by 



warmth of a withdrawing the water, evaporation is diminished, and less 



country pro- •,,.,, 



moted by heat IS subtracted from the earth. 



draining ? rjj-^g danger arising from wet feet and clothes is owing to 



Why do wet the absorption of heat from the body by the evaporation from 



feet or clothes the surfaces of the wet materials ; the temperature of the body 

 tend to impair .... ■,,,,. , , -i i , 



the health of 13 ID this way reduced below its natural standard, and the 

 the body ? proper circulation of the blood interrupted. 



566. The absorption of heat in the process by which liquids 

 tin vessel°con- ^^^ converted into vapor, will explain why a vessel containing 

 tainlng water a liquid that is constantly exposed to the action of fire, can 

 fire dMtroyed? never receive such a degree of heat as would destroy it. A 

 tin kettle containing water may be expo.sed to the action of 

 the most fierce furnace, and remain uninjured ; but if it bo exposed, without 

 containing water, to the most moderate fire, it will soon be destroyed. The 

 heat which the fire imparts to the kettle containing water is immediately ab- 

 sorbed by the steam into which the water is converted. So long as water is 

 contained in the vessel, this absorption of heat will continue ; but if any part 

 of the vessel not containing water be exposed to the fire, the metal will be 

 fused, and the vessel destroyed. 



567. When vapors are condensed into liq- 



TJnder what ., ^ ^• • ^ i i* ti i 



circumstances uids, and liquids are changed into solids, the 

 become sensi- latent hcat containcd in them is set free, or 

 made sensible. 



If water be taken into an apartment whose temperature is several degrees 

 below the freezing point, and allowed to congeal, it will render the room sen- 

 sibly warmer. It is, therefore, in accordance with this principle that tubs of 

 water are allowed to freeze in cellars in order to prevent excessive cold. 



It is from this cause that oceans, seas, and other large collections of water 

 ere most powerful agents in equahzing the temperature of the inhabited parts 

 of the globe. In the colder regions, every ton of water converted into ice 

 «»ve3 out and diffuses in the surrounding region as much heat as would 



