218 WRITINGS OF JOSEPH HENRY. [1855- 



The first column of the above table gives the temperature 

 of the water and vapor in the Torricellian vacuum for every 

 ten degrees; the second, the depression of the mercury, or 

 the elastic force of the vapor, corresponding to the several 

 degrees of temperature of the first column. The remaining 

 columns give the depression of the mercury for the inter- 

 mediate degrees, this arrangement being adopted to save 

 space. 



For example, if we wish to know the elastic pressure of 

 vapor at the temperature of 70°; by looking opposite to 70°, 

 in the second column, we find 0*733 or nearly seven-tenths 

 and a third inches of mercury. Again, if we wish the amount 

 of repulsive force of the atoms of vapor at the temperature 

 of 86°, we cast our eye along the line of 80° until it comes 

 under the 6°, which is at the top of the table, and find 1'242 

 or very nearly an inch and a quarter as the height of a 

 column of mercury which vapor of water will balance with- 

 out being condensed into a liquid at the temperature of 86°. 



By looking along the foregoing table it will be seen that 

 equal increments of heat are attended with more than equal 

 Increments of elastic pressure. Thus while the elastic force 

 of vapor at 20° is sufficient to depress the mercurial column a 

 little more than one-tenth of an inch, at 40° it depresses it 

 nearly two and a half times as much, at 60° five times, at 80° 

 ten times, and at 100° nineteen times. The reason of this is 

 not difficult to understand, since it is evident that the elastic 

 pressure of the vapor must be increased by the action of two 

 causes : First, by increasing the temperature the vapor tends 

 to expand just as air would do under the same circumstances ; 

 and second, by the same increase of temperature a new por- 

 tion of water is converted into vapor, which being forced 

 into the same space, increases the density, and consequently 

 the elasticity of the vapor which existed there before. 



Dalton also showed that there is a remarkable dif- 

 ference between vapor which exists over water and vapor 

 separated from the liquid from which it is produced. In 

 the first case, as we have seen, every increase of temperature 

 causes the formation of a new quantity of vapor which 



