The Locomotive - Lighthouse Survival Blog

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1901.] THE LOCOMOTIVE. 57
in degrees of such a size as to make the boiling point read 100 on the scale. In the
original Fahrenheit scale, the temperature of the human body was taken as zero and
the scale was so arranged that lower temperatures were denoted by larger numbers so
that the numbers on the scale increased from the top towards the bottom. The present
zero of the Fahrenheit scale was probably selected so as to avoid the use of " negative"
temperatures so far as possible; and if this appears to be absurd, it should be remem-
bered that Fahrenheit had never visited North Dakota in the winter, and that the only
means he had of producing artificial cold was by the use of some such simple freezing
mixture as pounded i«e and salt. Every "zero" of this sort, which is selected merely
because it happens to correspond to the freezing point of water, or some other fluid, is
purely arbitrary. We might have as many such zeros as we wanted to, and no one of
them would have the least advantage over any other one, except on the score of con-
venience. The absolute zei-o, or point from which we measure the absolute tempera-
tures that we are discussing, was selected as a zero point for scientific purposes, because
it differs from every other zero point that has ever been proposed in the one respect that
it is not in the least arbitrary. The absolute zero, in fact, is the temperature of absolute
cold. "We shall explain this point a little more fully presently, but just for the time
being we desire to consider the absolute zero from a slightly different point of view.
We wish to regard it, namely, as the zero of the " perfect gas" thermometer.
Every reader of The Locomotive is doubtless aware that for accurate scientific
purposes the ordinary mercury thermometer is no longer taken as the standard. Some
form of gas thermometer is used in its place, in which the temperature is measured by
the variation in pressure of a gas whose volume is kept constant. The gas used may be
air, or hydrogen, or any other of the familiar ones that we used to call "permanent,"
before we knew better. Nitrogen is the particular gas that is used by the International
Bureau of Weights and Measures, and the temperatures that such a thermometer gives
are said to be given " on the nitrogen scale." There are slight differences between the
readings of thermometers filled with different kinds of gases, and for this reason it is
desirable to fix on one particular kind of gas as a standard. The choice of this standard
gas is arbitrary, and is determined, not by any general fact of nature, but by the con-
venience with which the selected gas may be procured and purified, and by certain
other practical considerations that we do not need to dwell upon in this place. The
ideal gas to use for thermometric purposes would be one in which the pressure is pre-
cisely doubled when we compress the gas,. at constant temperature, into half its original
volume; and so on. In other words, one which precisely obeys Boyle's law, as explained
in the issue of The Locomotive for January, 1900. Such a gas is called a "perfect
gas." No perfect gas exists in nature, but those gases that can be liquified only with
great difficulty approximate very closely to the state of a perfect gas, under ordinary
conditions of temperature and pressure. Temperatures read from a thermometer filled
with a perfect gas would be absolute temperatures. If we neglect, for the moment, the
slight deviation of air from the ideal that we have in mind when we speak of a perfect
gas, we may explain the existence of the absolute zero by the following quotation from
Tyndall's " Heat a Mode of Motion "
"We have seen," he says, "that the pressure of air is augmented by an increase of
temperature. It has been shown that if the volume of the air is not allowed to change,
we have, for every degree of temperature, a certain definite increase of pressure. Reck-
oning from 0° Fahr. upwards, we find that every degree added to the temperature pro-
duces an increase of pressure equal to 1/460 of that which the air possesses at 0° Fahr.,
and hence, that by raising the temperature to 460° Fahr. we double the pressure. An