224 WRITINGS OP JOSEPH HENRY. [1855- 



this point; and since the weight of the steam which fills the 

 globe is equal to the weight of the water originally contained 

 in the small bulb, we have the weight of the vapor, and 

 knowing the number of cubic inches of the capacity of the 

 globe, we can easily determine the weight of a cubic foot of 

 vapor at the temperature at which the experiment was 

 made. 



In this experiment care must always be taken to determine 

 the exact temperature at which the water disappears; for if 

 a portion of water remains in the liquid state we shall not 

 have the true weight of the vapor; and we are assisted in 

 determining this point by the fact that in gradually increas- 

 ing the temperature of the apparatus we shall find that at 

 the moment when all the water is evaporated the vapor will 

 change its rate of expansion, and be governed by the same 

 law as that of the expansion of dry air. 



After having determined the weight of a given quantity 

 of vapor, for example a cubic foot, by direct experiment ac- 

 cording to the method we have described, the weight of an 

 equal quantity of vapor at other temperatures may be deter- 

 mined by calculation. For example, the density of the vapor 

 (as in the case of air) will be in proportion to its elastic force 

 or the pressure to which it is subjected, if the temperature 

 remained the same; hence from the table of elastic force 

 already given, we may calculate the corresponding weights of 

 a foot of vapor. The numbers thus obtained however must 

 be corrected for the diminution of weight on account of the 

 expansion due to increased temperature. In this way table 

 B was constructed, in which the first column indicates the 

 temperature of every ten degrees of Fahrenheit's scale; 

 the second column gives the weight of vapor in Troy grains 

 contained in a cubic foot of space; the remaining columns 

 give the weight of vapor at intermediate degrees. 



