246 Mr. William Sutherland on the 



and of the quantities involved in its calculation by the ther- 

 modynamic relation, has found the most perfect harmony 

 between the results of the two methods. Now at 30° 0. 

 Perot gives as the saturation- volume of the vapour 400*4, and 

 the saturation-pressure "635 metre, while Ramsay and 

 Young's values are 374 and *648 metre; but if ethyl oxide 

 were a perfect gas, under Perot's pressure of '635 metre it 

 would have a volume of 400*8, almost identical with his value: 

 yet we cannot imagine that ethyl oxide under this pressure 

 and at this temperature is so nearly a perfect gas as this would 

 imply, unless there is some remarkable discontinuity in its 

 behaviour at high volumes. Accordingly, in spite of the 

 thoroughness of the researches of both Perot and Ramsay and 

 Young, we are on the horns of a triple dilemma, from which 

 only some experimental repetition can deliver us, and de- 

 monstrate where the cause of these discrepancies lies. Wiillner 

 and Grotrian (Wied. Ann. xi.) have put on record the results 

 of experiments which indicate the cause; they find the pressure 

 of condensation measurably different from the ordinarily 

 measured saturation-pressure, — a fact explaining the difficulty 

 of measuring v s accurately, and showing also that the values 

 of dP/dT are not so reliable as usually supposed. 



Our last equation is verified by, and shows us the cause of, 

 an interesting relation that has been independently discovered 

 and expressed in different forms, between the molecular latent 

 heat and the boiling-point, by Pictet {Ann. de Ch. et de Ph. 

 ser. 5, t. ix. 1876), Trouton (Phil. Mag. xviii. 1884), and 

 Ramsay and Young (Phil. Mag. xx. 1885), namely, that the 

 molecular latent heats of fluids are nearly proportional to 

 their absolute boiling-points. Now we have seen that 

 T =120Z/409R* (Section 4), and I have noticed that a large 

 number of substances have their ordinary absolute boiling- 

 points nearly equal to 2T /3, and k is nearly proportional to 

 v ly say is equal to 2*8 v v as it is for ethyl oxide. Hence we 

 have 



3 T _ 120/ 



409R2-8V 



.-. MZ=14-3MRr 1 T 6 = 1190t> 1 T M 



when the megadyne is the unit of force; hence from our 

 equation for Ml in terms of X we have 



1190T 6 =66-5M\-101T 6 ; 



.*. 1291 T 6 ~ 66*5 M\ or MA, = 19*4T d , 



