
ON THE MECHANICAL ACTION OF HEAT. 563 
The approximate positions of the absolutezero contained in this table were 
laid down on the diagram, in which they are marked by crosses. The longitudinal 
divisions represent centigrade degrees divided into tenths; the transverse divisions, 
atmospheres of pressure at 0° centigrade, also divided into tenths. The positions 
of the crosses indicate at once the pressures in the second column of the table, and 
the approximate zeros in the fourth; and the numbers affixed to them correspond 
with those in the first column. 
As the effect of cohesion is greater, and more easily eliminated, in carbonic 
acid gas than in atmospheric air, the determination of the true absolute zero was 
made from the experiments on the former gas. It will be observed that the ap- 
proximate positions of the absolute zero for carbonic acid lie nearly in a straight 
line. A straight line (dotted in the diagram) having been drawn so that it should 
as nearly as possible traverse them, was found to intersect the line corresponding 
to the zero of pressure, that is, to the state of perfect gas, at a point on the scale 
of temperatures 274°6 centigrade degrees below the temperature of melting ice ; 
which point was accordingly taken as the true absolute zero of the perfect gas 
thermometer. 
So far as their irregularity permits, the experiments on atmospheric air con- 
firm this result, for the approximate positions of the absolute zero deduced from 
them, evidently tend towards the very same point on the diagram with those 
deduced from the experiments on carbonic acid. 
The values of the coefficient of dilatation and of increase of pressure, of a per- 
fect gas, per degree, in fractions of its volume and pressure, at the temperature of 
melting ice, are accordingly,— 
‘ 1 
For the Centigrade Scale 746 — 0-00364166 

* 4 EE haere 
For FAHRENHEIT’S Scale 19498 = 0:00202314 
