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XXXIX.—On the Absolute Zero of the Perfect Gas Thermometer ; being a Note to a 
. Paper on the Mechanical Action of Heat. By Witt1aM Joun Macquorn RANKINE, 
C.E., F.R.S.E., F.R.S.S.A., &c. 
(Read January 4, 1853.) 
Temperature being measured by the pressure of a perfect gas at constant 
density, the absolute zero of temperature is that point on the thermometric scale 
at which, if it were possible to maintain a perfect gas at so low a temperature, 
the pressure would be null. 
The position of this point is of great importance, both theoretically and prac- 
tically; for by reckoning temperatures from it, the laws of phenomena depending 
on heat are reduced to a more simple form than they are when any other zero is 
adopted. 
As we cannot obtain any substance in the perfectly gaseous condition (that is 
to say, entirely devoid of cohesion), we cannot determine the position of the abso- 
lute thermometric zero by direct experiment, which furnishes us with approxi- 
mate positions only. Those approximate positions are always too high; because 
the effect of cohesion is to make the pressure of a gas diminish more rapidly with 
a diminution of temperature, than if it were devoid of cohesion. 
As a gas is rarefied, the cohesion of its particles diminishes, not only in absolute 
amount, but also in the proportion which it bears to the pressure due to heat. 
The gas, therefore, approaches more and more nearly to the stateo f a perfect gas 
as its density diminishes; and from a series of experiments on the rate of increase 
of its elasticity with temperature, at progressively diminishing densities, may be 
calculated the positions of a series of points on the thermometric scale, approach- 
ing more and more nearly to the true absolute zero. 
By observing the law which those successive approximations follow, the true 
position of the absolute zero can be determined. 
Having performed this operation by means of a graphic process, soon after the 
publication of the experiments of M. Reanavuut on the elasticity and expansion 
of gases, I stated the result in a paper on the Elasticity of Vapours (Edinburgh 
New Philosophical Journal, July 1849), and also in a paper on the Mechanical 
Action of Heat (Trans. Royal Soc. Hdin. vol. xx., Part 1), viz., that the absolute 
zero is 
274-6 centigrade degrees, 
or 494-28 degrees of FAHRENHEIT, 
or 462-28 degrees below the ordinary zero of FAHRENHEIT’S scale. 
VOL. XX. PART Iv. TN 
} below the temperature of melting ice ; 
