PYRONOMICS. 97 
lower ; until this time the vesicles become condensed before they reach the 
top, giving out their latent heat to the upper strata. 
Substances only mechanically united with water do not change the 
temperature at which boiling takes place ; the case is different, however, if 
solution takes place, the boiling point being elevated. The steam formed 
is, nevertheless, pure watery vapor, and its temperature is precisely the 
same as if generated from pure water. 
The generation of steam, both in respect to quantity and rapidity, depends 
entirely upon external circumstances, particularly upon the more or less 
suitable application of fuel, upon the material and form of the boiler, and 
upon the amount of surface coming in contact with the flame. 
As boiling is a formation of vapor, taking place throughout the entire 
liquid, so there is still another formation of vapor, which takes place only at 
the surface, namely, exhalation or evaporation. This phenomenon occurs 
over the whole surface of the earth at all temperatures. The vapor thus 
formed has a certain tension, which, however, is not sufficient to overcome 
the pressure of the atmosphere. A chemical mixture here takes place, as 
between two gases, and the principal condition is, that the air be not 
saturated with vapor, else the exhalation ceases. For this reason 
evaporation does not take place so readily in a calm as during windy 
weather. As to the rest, evaporation is constantly in proportion to the 
amount of surface exposed to the air. In the section devoted to meteoro- 
logy, we shall have occasion to refer more particularly to this phenomenon, 
and its influence in organic nature. 
When a liquid evaporates, heat combines with the vapor, or becomes 
latent, as is shown by the fact, that whatever be the amount of heat applied, 
the temperature of the water never rises above the boiling point. The 
vapor must therefore take up the heat, even although its own temperature 
does not rise above the boiling point. This phenomenon may be illustrated 
by pouring upon the hand a few drops of ether or other quickly vaporizing 
liquid. A sensation of cold will be experienced, which is owing to the 
abstraction of heat from the hand during the production of vapor, this heat 
becoming latent in the vapor. The amount of heat latent in the vapor may 
be ascertained by allowing the vapor of a known amount of water to pass 
into a quantity of water, also known, and determining the temperature to 
which this water is elevated. Now, knowing how many units of heat, that 
is, how many times the temperature necessary to raise one pound of water, 
one degree in temperature, are required to raise the water to as many 
degrees as has been done by the steam, we can calculate the amount of 
heat which was rendered latent. 
In the process of distillation, the steam raised from the liquid is con 
ducted through a tube lying in cold water, and there condensed by becoming 
cooled. The heat given out in this process elevates the temperature of the 
circumambient water very considerably.. The small apparatus of distil- 
lation (pl. 19, fig. 47) exhibits this very clearly. The steam generated in 
the small balloon passes through the Straight tube into the wide one, 
provided with a funnel and an escape tube. The water poured in through 
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