140 
is neither saturated at room temperature nor perfectly dry, the change 
in the degree of moisthness of the air on becoming saturated is what the 
air originally lacked of being saturated. Jhe appropriate correction to 
introduce is that fraction of the tension of aqueous vapor for the room 
temperature which it lacked of saturation. Suppose the apparatus was 
originally filled with the air of the room, and that it was forty per cent. 
saturated at room temperature, sixty one-hundredths of the tension of 
aqueous vapor is the number to be subtracted from the observed barometer 
100 — H 
100 
barometer reading, H is the hygrometer reading in per cent., and w is 
reading; the corrected reading is B — w, in which B is the 
the tension of aqueous vapor for the room temperature. 
Nearly all works accessible to the author give such directions for the 
manipulation as involve the use of the air of the room in the inner tube, 
yet give for the calculation the correction B-w. The error introduced in 
this way would be greatest if the air were saturated with moisture, and 
would then amount at a room temperature of 20 deg. C. to 17 in approxi- 
mately 760, or 1 in about 45, and this condition is closely approached in 
damp, warm weather. Omitting the correction altogether when the air 
used is nearly dry gives an equal error in the opposite direction, approxi- 
mated in very cold weather. 
A quite appreciable error, then, may be avoided and the calculation 
made more nearly correct theoretically by using the correction given above. 
Of the works accessible to the author only H. Erdmann’s Anorganische 
Chemie discusses the correction, directing that if the apparatus is filled 
with a dried gas the tension of aqueous vapor should be deducted; if with 
ordinary air, no correction should be made. <All other works fail to con- 
sider the point, some deducting the tension, others not, without specifying 
the conditions. 
