of Vapours to Mariotte and Gay-Lussac's Law. 305 



the well-known tension-curve, which constantly withdraws itself 

 from the axis of abscissae as the temperature increases. At the 

 abscissa « + 9'5 the curve Pj coincides with the curve p v but 

 afterwards at higher temperatures approaches nearer to the axis. 

 And here the vapours of chloroform and of bisulphide of carbon 

 show that the curve P, may have a maximum. Now, since we 

 cannot assume that, beyond the maximum, Pj constantly decreases 

 with an increasing temperature, therefore the curve P x after the 

 maximum must have a minimum, in order that it may withdraw 

 itself more and more from the axis of abscissae, as is approxi- 

 mately shown in fig. 3. 



Consequently we may draw a parallel to the axis of abscissas 

 from a point on the curve p x which shall cut the curve Pj three 

 times ; i. e. the same tension of the vapour may correspond to 

 the superheated condition for lower temperatures and to the ga- 

 seous for higher temperatures, then the vapour may enter again 

 into the superheated, and finally into the gaseous state. This 

 conclusion is connected with that drawn previously from the 

 course of V v since the product PjVj must increase proportion- 

 ately to the absolute temperature (the abscissa in the diagram) . 



§10. 



From what has been said in the last two paragraphs, a surpri- 

 sing conclusion is arrived at concerning the behaviour of the co- 

 efficients of dilatation of vapours of constant volume and under 

 a constant pressure. 



Since the superheated condition shows a smaller product^ 

 than there would be in the corresponding gaseous condition, it 

 follows that whenever a constant volume v is taken, the pressure 

 p must be smaller for superheated vapour than it is, under other- 

 wise similar circumstances, for an ideal gas. Hence it follows 

 that when the vapour under a constant volume and with a gra- 

 dually increasing temperature passes gradually from the super- 

 heated condition into the gaseous condition, it must exhibit a 

 greater coefficient of dilatation for a constant volume than that of 

 an ideal gas would be; and, conversely, in a gradual progress 

 from the gaseous to the superheated condition with an increasing 

 temperature the coefficient of dilatation of the vapour under a 

 constant volume must be smaller than that of an ideal gas. All 

 this holds good when the volume v is interchanged with the pres- 

 sure p for the coefficient of dilatation under a constant pressure. 



From the particulars of the curve Y ]J as represented in fig. 2, it 

 follows that the coefficient of dilatation can exhibit the behaviour 

 of the vapour in its dependence on the temperature when under 

 a constant volume, as is given in fig. 4, where the abscissae x re- 

 present the temperature, the ordinates y the coefficient of dila- 



