The Locomotive - Lighthouse Survival Blog

1901.] THE LOCOMOTIVE. Ql
this sort, made upon various gases, we could obtain a very good idea of the position of
the zero of the perfect gas thermometer.
So much for the temperature scale of the perfect gas thermometer, and for its abso-
lute zero. The question now arises whether we have any reason to suppose that the zero
so denned is really the point at which bodies are totally deprived of heat. It will be
remembered that we begged this question at the beginning of this article by first defin-
ing the absolute zero as the point at which bodies are totally deprived of heat, and then
discussing it as the point at which the pressure in a perfect gas thermometer would
entirely vanish. Is there any good reason for supposing that these two points coincide ?
This is a perfectly fair question, and its reasonableness will be the more apparent if we
remember that Rankine, before the science of heat was advanced as far as it is now,
recognized a possible difference between the two points, and that he distinguished them
carefully in his writings. Thus, in a paper published in the Philosophical Magazine, in
1851, he distinctly states that the "absolute zero of temperature is not the absolute zero
of heat"; and in another paper, published in 1853 or 1854, he actually calculates the
difference between the absolute zero of the perfect gas thermometer and the temperature
at which bodies are totally deprived of heat. His general re- It in this last-mentioned
paper is that " the temperature of absolute privation of heat j a real fixed point on the
scale, somewhat more than two Centigrade degrees (or say 3£ Fahrenheit degrees) above
the absolute zero of a perfect gas thermometer."
These citations from Rankine, who was one of the founders of the mechanical
theory of heat, and a very capable mathematician, afford another excellent illustration of
the exceeding difficulty of giving a correct popular account of the subject of absolute
temperature; for if Rankine, with his extensive knowledge of mathematics and physics,
fell into the error of separating two things that we now know to be identical, and even
calculating the difference between them when no real difference exists, how can one hope
to make the subject entirely clear, by the aid of simple language, to readers who presumably
have not given a hundredth part of the attention to the subject that Rankine
had!
It will be necessary to content ourselves with merely stating what has been proved
without attempting to go through with the proof.
Everybody knows that we can cool a body by taking away heat from it ; and there
is nothing in our everyday experience in life that would suggest that such a body could
ever be so cold that we could not cool it still more by a further abstraction of heat.
Yet a careful mathematical study of nature has demonstrated that there is a temperature
so low that no substance can ever be made cooler than that temperature, by any means
whatever; and that this temperature is in fact precisely the "absolute zero" of the per-
fect gas thermometer. This strange fact was solidly established by the joint labors of
numerous scientific men, among whom Carnot, Clausius, and Lord Kelvin may be promi-
nently mentioned. The proof by which it was established is just as sound as the proofs
that we find in geometry ; but it is unfortunately of such a nature that it cannot be prop-
erly explained without the use of the differential calculus. It may be said, however, that
the reasoning by which the proposition is established on a firm foundation is quite
different from that which we have advanced above. The "perfect gas " thermometer is
not an essential feature in the argument at all, and the absolute scale can be established
and the existence of the absolute zero proved without any reference to a perfect gas.
Lord Kelvin has shown all this in his article on " Heat," in the Encyclopaedia Britannica.
In following through an argument like that of Tyndall, one naturally inquires why we
must use a gas in our argument ; and the answer is, that in the rigorous treatment of the