96 
HAROLD’S DISCUSSIONS. 
We have learned that when liquids change to 
vapor they take up heat. The water from the cloth 
changing to vapor drew olf heat from the bulb, or, 
in other words, it cooled the bulb. Consequently 
the mercury stood 
lower in the 
wrapped ther¬ 
mometer than it 
did in the other. 
The more rapid 
the evaporation 
the greater the 
difference in the 
two thermome¬ 
ters. 
I soon discov¬ 
ered that cold air 
can hold but little 
moisture. Just as 
it is with a sponge: 
the drier it is the 
more moisture it 
will absorb, so 
with the air, the 
warmer it is the 
more vapor it will 
hold. When air 
contains all the water it will hold we say it is satu¬ 
rated ; if it should grow cooler, its vapor would turn 
to rain, snow, dew, or frost. If the air above is 
cooled by a cold current, the vapor falls as rain; if 
Fig. 52.—Simple hygrometer. Dry- and 
wet-bulb thermometers. 
