The Locomotive - Lighthouse Survival Blog

58 THE LOCOMOTIVE. [April,
image will fix all this forever in your minds, and enable you to see clearly certain con-
sequences that flow from it. Supposing you have a purse containing at the outset 460
pennies, and that you have a volume of gas at the temperature of 0° Fahr. Let the air
be gradually warmed, and for every degree of temperature imparted to it let a penny
be added to the store already in your purse. A single degree would then raise your
money to 461 pennies; 10° to 470 pennies; 100° to 560 pennies; while 460° would augment
your store to 920 pennies. You have thus, at the end, twice the sum you possessed
at starting, and you have also twice the pressure in the air whose temperature you have
been increasing at the same time. Now let us invert the whole proceeding. Starting
with a temperature of 460° Fahr., and with 920 pennies in the purse, let us gradually
cool the air, removing a penny for every degree of temperature taken away from it. On
reaching 0° Fahr., we should obviously have 460 pennies in our purse. But there is no
magic in the temperature 0° Fahr. that could cause the value of a degree of chilling to
change at that particular point. Below it, as above, the value will be still a penny.
Let us, then, continue the cooling process, throwing away a penny per degree as before.
One degree of chilling would lessen our cash one penny; 10° ten pennies; 100° one
hundred pennies; while 460° of chilling would entirely empty our purse. The diminu-
tion of pressure would here follow the same rule as the diminution of cash, were it not
that the molecular forces which are insensible at higher temperatures come into play.
At 460° below the Fahrenheit zero we should empty, at the same time, our purse of
pennies and our air of pressure. The air would then have sunk to tltc absolute zero of
temperature. The absolute zero has never been attained, and long before reaching it all
actual gases would become liquids, ceasing to follow the law of diminution of pressure
which we have here applied." (We have modified Tyndall's phraseology somewhat,
and reduced his figures to the Fahrenheit scale; but the sense of what he says has been
carefully preserved.)
The foregoing explanation of the absolute zero serves fairly well to fix the main
idea of the thing in the mind; but we admit that reasoning of this sort affords but a
poor foundation upon which to build a general theory of absolute temperature. We
have quoted it at some length, because Tyudall is admitted to be a competent authority,
whose writings are usually very logical and simple and clear; and if a writer as well informed
and experienced as he was finds it hard to give a really satisfying exposition of
the subject, it is reasonable to suppose that the proper presentation of the theory of ab-
solute temperature, without the use of mathematics, is indeed a difficult thing.
If the foregoing extract represents the best that so competent a man as Tyndall
could do in the way of making the subject of absolute temperature intelligible in simple
language, it is not to be wondered at, that many writers and many mechanical engineers
of good standing should treat the absolute zero, and the absolute scale of temperature,
as though it were merely a convenient artifice, useful in simplifying calculations, but
not corresponding to any real thing in nature. Inventors, in particular, appear to be
afflicted with this delusion, and every once in a while motors are devised which receive
a great share of public attention, and sometimes a considerable amount of financial
backing, even when they are opposed to sound thermodynamical principles; and when
these motors refuse to work, there is great disappointment among those who had in-
sisted upon opening their pocket-books, instead of their books on the mechanical theory
of heat.
Let us examine Tyndall's exposition somewhat critically. In the first place, he uses
air instead of the ideal perfect gas, upon which the absolute scale is supposed to be
based. It is true that air behaves, under ordinary conditions, very much as a perfect