210 Drs. Ramsay and Young on 



EF. For these large volumes of gas, it is almost certain that 

 the density would become normal at a sufficiently high tem- 

 perature ; but at low temperatures and pressures it cannot be 

 proved, owing to condensation, whether the molecular formula 

 would be C 4 H 8 4 ; that is, whether the line CD would coincide 

 with the line EF. 



On Plate III. are also represented similar relations for ether, 

 at volumes of 4000 and 1000 cub. centims. per gram ; the 

 points representing the observed relations of temperature to 

 pressure fall on the isochoric lines ; that is to say, the diver- 

 gence from Boyle's and Gay-Lussac's laws is too small to be 

 detected by experiment. At smaller volumes, however (those 

 at 300 and 250 cub. centims. per gram are shown), the line 

 passing through the observed points falls below the theoretical 

 line, but is not quite parallel to it. It follows therefore, that 

 if the isochoric lines are perfectly straight, they would cut the 

 normal isochoric lines at an extremely high temperature. 

 The physical meaning of this behaviour is that, if the tempera- 

 ture of a gas, at constant volume, be raised sufficiently high, 

 the density must equal and then fall below the normal. It is 

 evident that this must be the case. For the pressure of a gas 

 depends on the number of molecules present in unit volume, 

 on the average velocity of each molecule, and on the number 

 of impacts on unit area of the surface of the containing vessel, 

 in unit time. With constant volume, since the mean distance 

 between the molecules remains constant, the cohesion of the 

 molecules is assumed to be constant. But the rise of pressure 

 produced by rise of temperature of a theoretical gas is based 

 on the assumption that each impact takes place at the centre of 

 each molecule; that is, that the actual volume of the molecules 

 themselves is nil. But as this is not the case, as impacts must 

 take place at some distance from the centres of the molecules, 

 they must necessarily be more frequent. The effect of cohesion 

 is to reduce the pressure of the gas, by reducing the average 

 velocity of the molecules, and this, for any given volume, by 

 a constant amount. Hence, below a certain temperature, the 

 pressure will be less than that of a normal gas, and if the 

 temperature be reduced sufficiently, will become negative. 

 With rise of temperature, the average velocity of each 

 molecule will increase at the same rate as in the case of a 

 theoretical gas ; but the number of impacts, and, consequently, 

 the pressure, will increase at a greater rate than if the gas 

 were perfect ; hence a temperature will ultimately be reached 

 when the pressure is as much decreased by cohesion, as it is 

 raised by the more frequent encounters of the molecules ; 

 and at that temperature the density of the gas will be normal. 

 At still higher temperatures the pressure, and therefore the 



