NOTES AND PROBLEMS 41By common consent of scientific men, standard conditions have
been defined as 760 mm. pressure, 0°C, and dry gas.
A remarkable uniformity has been found to exist in the behavior
of all gases under changing conditions. Three simple state-
ments, the so-called gas laws, suffice to define with a considerable
degree of accuracy the volume changes with changing pressure,
temperature, and water-vapor content.
Boyle's law states that at the same temperature the volume of a
definite amount of a gas is inversely proportional to the pressure.
The law is expressed by the equation
or piVi = P2V2
Charles' law defines the change of volume with changing
temperature: the volume of a definite amount of a gas under
constant pressure is directly proportional to the absolute tempera-
ture. The absolute temperature is 273° plus the centigrade
temperature; but really the determination of the absolute scale
of temperature depends entirely on the behavior of gases.
The original statement of Charles' law was made in this way:
For every degree rise or fall in temperature the volume of a gas
increases or decreases by an amount equal to -j^ its volume at
0°C. If this law held rigidly all the way down the scale of course
the volume of a gas would become zero at — 273°C. This point,
— 273°C, would be the absolute zero below which substances
could not be cooled. As different gases were studied it was found
that they obeyed this law quite exactly until they approached the
temperature at which they would condense to a liquid. The
more difficultly condensible a gas, the further down the scale it
would follow this law. Helium, which was the last gas to succumb
to efforts at liquefaction, follows the law with a good deal of accu-
racy to within a few degrees of — 273°C. Hence, since it was
found that the less condensible a gas the more nearly it approxi-
mated a certain ideal behavior, an imaginary " perfect gas " was
postulated which would have exactly the ideal behavior. The
absolute zero then is defined as the temperature at which the
volume of this perfect gas would become zero, that is — 273°C.
Charles' law is expressed in the equation