378 Scientific Proceedings, Royal Diihtin Society. 



As the orthobarie volumes of vapour published at various dates in 

 different journals are now found, in many cases, to require correction at 

 the lower temperatures, they have been read from the reconstructed curves, 

 and are given in the tables at the end of this paper. 



4. In many of the papers, not only the specific but also the molecular 

 volumes are given, and in some cases the molecular volumes only. But 

 since the publication in 1889 of the first paper in which molecular volumes 

 wei'e recorded, the atomic weiglits of many of the elements have been 

 redetermined with greatly increased accuracy. In order to avoid confusion, 

 the atomic weights first adopted, those of F. W. Clarke {Constants of Nature, 

 1882), were adhered to in the whole series of papers, although it became 

 more and more evident as time went on that they required considerable 

 modification. 



In the present paper it lias not been thought necessary to give the 

 molecular volumes at all, as they can easily be calculated, if required, from 

 the specific volumes. 



5. After the vapour-pressures of each substance had been determined, 

 tlie logarithms of the pressures were plotted against the temperatures, and 

 curves were drawn through the points. Constants for Biot's formula 



log p = a + ba* + c/3* 

 were then in most cases calculated ; and tlie vapour-pressures were re- 

 calculated from the formula, and compared with those read from the curves 

 or actually observed. The constants and the recalculated pressures are 

 given in the original papers for all the substances except the ten esters. 

 The values for methyl formate and etliyl formate were actually calculated, 

 but were not published at the time ; those for the other eight esters were 

 subsequently calculated by Dr. J. E. Mills, and the whole of them were 

 publislied by him.' 



For the great majority of substances the observations extend from 0° to 

 the critical temperature ; but witli bromobenzene and iodobenzene it was not 

 found possible to extend the temperature range beyond 270°. Also in the 

 case of chlorobenzeue the constants of Biot's formula were calculated after 

 determinations of pressure liad been made up to 270°, but before those at 

 higlier temperatures had been carried out. 



It was noticed some little time ago both by Dr. Mills and by myself that 

 although there is good agreement in all three cases between the observed 

 and recalculated pressures, yet the pressures calculated for the critical 

 temperatures are manifestly impossible. Evidently, therefore, tlie trend 



' Journ. Phys. CI. em., x., p. 1, 1906. 



