EBULLITION. 



305 



pressure of the atmosphere. These experiments may be varied and repeated ; 

 and it will be always found, that as the pressure is diminished or increased, 

 the temperature at which the water will boil will be also diminished or in- 

 creased. 



The same effects may be exhibited in a striking manner without an air- 

 pump, by producing a vacuum by the condensation of steam. Let a small 

 quantity of water be placed in a thin glass flask, and let it be boiled by hold- 

 ing it over a spirit lamp. When the steam is observed to issue abundantly 

 from the mouth of the flask, let it be quickly corked and removed from the 

 lamp. The process of boiling will then cease, and the water will become 

 quiescent ; but if the flask be plunged in a vessel of cold water, the water it 

 contains will again pass into a state of violent ebullition, thus exhibiting the 

 singular fact of water being boiled by cooling it. This effect is produced by 

 the cold medium in which the flask is immersed causing the steam above 

 the surface of the water in it to be condensed, and therefore relieving the 

 water from its pressure. The water, under these circumstances, boils at a 

 lower temperature than when submitted to the pressure of the unconidensed 

 vapor. 



There is no limit to the temperature to which water may be raised, if it be 

 submitted to a sufficient pressure to resist its tendency to take the vaporous 

 form. If a strong metallic vessel be nearly filled with water, so as to prevent 

 the liquid from escaping by any force which it can exert, the water thus en- 

 closed may be heated to any temperature whatever without boiling ; in fact, it 

 may be made red hot, and the temperature to which it may be raised will have 

 no limit, except the strength of the vessel containing it, or the point at which 

 the metal of which it is formed may begin to soften or to be fused. 



The following table will show the temperature at which water will boil un- 

 der different pressures of the atmosphere corresponding to the altitudes of the 

 barometer between 26 and 31 inches : — 



Barometer. Boiling point. 



26 inches 204 G -9 1 



26-5 205°-79 



27 206°- 67 



27-5 207°- 55 



28 208°-43 



28-5 209°-31 



29 210°-19 



29-5 21 1°-07 



30 212° 



30.5 212°-88 



31 213°-76 



From this table it appears that for every tenth of an inch which the baro- 

 metric column varies between these limits, the boiling temperature changes 

 by the fraction of a degree expressed by the decimal -176, or nearly to the 

 vulgar fraction J. 



It is well known, that as we ascend in the atmosphere, the pressure is di- 

 minished in consequence of the quantity of air left below it, and consequently 

 | the barometer falls as it is elevated. It follows, therefore, that in stations at 

 i different heights in the atmosphere, water will boil at different temperatures; 

 J and the medium temperature of ebullition at any given place must, therefore, 

 i depend on the elevation of that place above the surface of the sea. Hence the 

 | temperature of boiling water, other things being the same, becomes an indica- 

 i tion of the height of the station at which the water is boiled, or in other words, 

 j becomes an indication of the atmospheric pressure ; and thus the thermometer 

 I serves in some degree the purpose of a barometer. 

 > VOU II.—30 



