220 WRITINGS OF JOSEPH HENRY. [1855- 



verted into a liquid, just in the same way that an increase 

 of pressure converts the vapor which fills the top of the 

 barometer tube into water. This supposition is the more 

 probable since several gases which were at one time consid- 

 ered permanently elastic have been reduced in this way to a 

 liquid by the application of a powerful pressure, combined 

 in some cases with a reduction of temperature. 



The foregoing table is limited to 100°, and is sufficient for 

 resolving problems relative to the hygrometrical condition 

 of the atmosphere. It is however important for the use of 

 the steam engineer that it should be extended to a much 

 higher degree, and accordingly experiments have been made 

 for this purpose by a number of persons, and particularly by 

 M. Regnault, at the expense of the French government. 

 From the table thus extended we may see that at the tem- 

 perature of 212° the elastic force of vapor balances 30 inches 

 of mercury, and is then just equal to the pressure of the 

 atmosphere. This fact gives the explanation of the phenom- 

 enon of boiling, since the vapor formed at the temperature 

 of 212° has just sufficient repulsive power to expand beneath 

 the pressure of the atmosphere, and to pass up in volumes 

 through the water, giving it the peculiar agitation known 

 as boiling. 



It is further evident from the same table that vapor is given 

 off from ice even at zero, or 32° below the freezing point. 

 If a lump of this substance on a cold day be placed under 

 the receiver of an air pump, even when the apparatus is 

 cooled down to zero, a portion of it will immediately spring 

 into vapor, sufficient to fill the whole capacity of the cylinder 

 when the air is withdrawn ; and if this vapor in its turn be 

 removed by working the pump another portion of the ice 

 will pass into the state of vapor, and if the pressure of this 

 be removed another quantity of ice will be evaporated ; and 

 if the pumping be continued sufficiently long all the ice will 

 be dissipated in vapor without passing through the inter- 

 mediate condition of water. Instead of continuing to work 

 the pump in order to evaporate the ice we may produce the 

 same effect by placing within the receiver a broad dish con- 



