﻿570 Prof. S. Young on the Vapour-Pressure 



A great number of determinations of the vapour-pressure 

 of the liquid were made at various temperatures, and in many 

 cases the volumes of liquid and vapour were read. The 

 results obtained prove conclusively that the vapour -pressures 

 of isopentane are independent of the relative volumes of liquid 

 and vapour; and it may be of interest to give all the observed 

 vapour-pressures at two temperatures, with the corresponding 

 volumes of vapour and liquid. 



The same apparatus was employed as in my previous work 

 on benzene and its derivatives, the esters, &c: it is similar in 

 principle though it differs somewhat in detail from that made 

 use of by Ramsay and nryself (Phil. Trans. 1887 A, p. 59) . 



The tube containing the isopentane was heated by the 

 vapour from pure liquids (Trans. Chem. Soc. 1885, p. 640 ; 

 1889, p. 483) boiling under reduced pressure. 



The pressures are corrected for (1) the difference in height 

 of the columns of mercury in the tube containing the isopen- 

 tane and in the air-gauge ; (2) the expansion of the heated 

 column of mercury ; (3) the pressure of the column of iso- 

 pentane ; (4) the deviation of air (in the air-gauge) from 

 Boyle's law, as determined by Amagat. 



Several series of determinations were made with different 

 quantities of liquid in the tube. As a rule four readings of 

 pressure were taken in each series at each temperature. 



It will be seen that although the relative volumes of vapour 

 and liquid vary within very wide limits, there is no such 

 corresponding variation in the vapour-pressures, the greatest 

 difference from the mean value at 140° being slightly less 

 than O'l per cent., and at 90° slightly greater — in no case 

 outside the limit of experimental error. 



It has happened occasionally in the course of this or pre- 

 vious investigations that a trace of air has entered the tube 

 or has been left in the liquid, and in a very few cases (ethyl 

 formate, propyl formate) a small quantity of permanent gas 

 has been formed by partial decomposition of the liquid by 

 prolonged heating at high temperatures. The presence of 

 permanent gas (or of very volatile impurity) is clearly indi- 

 cated by the increase of pressure required for the complete 

 condensation of the vapour, and the following points have 

 been noticed in such cases: — (1) The pressure does rise as the 

 volume of vapour diminishes ; (2) the readings taken with 

 diminishing volumes are higher than with increasing volumes. 

 This may be readily explained; for when the whole of the 

 substance is in the state of vapour the air is diffused uni- 

 formly through it, and when condensation is brought about 



