832 REPORT— 1898. 



I proposed, therefore, to determine the vapour pressures and the specific 

 volumes (both as liquid and as saturated vapour) of a considerable number of 

 substances from low temperatures to their critical points, and in the first place I 

 chose some compounds of elements belonging to the same group in the periodic table, 

 as it seemed possible that some points of interest might thus arise. The first 

 substances I examined were the four monohaloid derivatives of benzene, as well 

 as benzene itself,' and the result of the investigation was to show that, when the 

 haloid derivatives are compared together, the generalisation, as regards tempera- 

 ture and pressure, hold good accurately ; but there is this peculiarit}- about these 

 compounds, that their critical pressures are equal, or very nearly so, and therefore 

 ' corresponding ' pressures are in this case equal pressures, The critical pressure 

 of benzene itself is difterent, and when the hydrocarbon is compared with any of 

 its haloid derivatives, the diiferences between the temperature ratios are much 

 greater. As regards the volume ratios, the ditferences are small in all cases. 



The only other substances, bearing on the periodic arrangement of the 

 elements, which have been yet examined are the tetrachlorides of carbon and 

 tin.^ The critical pressures difler considerably, and the relationship resembles 

 that of the normal paraffins to each other, which will be referred to later. Many 

 of the chlorides of the elements are very hygroscopic, and attack mercury at 

 high temperatures, and it was thought better to postpone their further examina- 

 tion, and to obtain the data for a few series of homologoas organic compounds. 

 Up to the present, in addition to the three lowest alcohols, investigated by 

 Ramsay and myself, ten esters ^ and four paraffins ' have been studied, and with 

 the exception of the alcohols it has been observed in every case (and the same 



T 



remark applies to the tetrachlorides of carbon and tin), that the ratios — - at any 



reduced pressure increase with rise of molecular weight. No definite relation is 



. Y V . 



observable between the molecular weights and the ratios — and — in the case of 



the esters, but with the three normal paraffins (pentane, hexane, and heptane) and 

 the two tetrachlorides, the ratios — increase and — diminish slightly with rise 



T 

 of molecular weight. With the alcohols, on the other hand, the ratios and 



J- are irregular and — diminish. 



Another point of interest is the comparison of isomeric compounds ; hut up to 

 the present the only two pairs of isomers investigated are methyl butyrate and 

 isobutyrate, and normal and iso-pentane. In both cases there is a clear relation- 

 ship between the ratios and the constitution, the normal and iso-compounda 

 standing to each other in much the same i-elative positions as a higher and lower 

 normal paraffin. 



It is worthy of remark that the volumes of a gram of all four paraffins are 

 nearly the same (4'266 — 4-.30.3) at the critical point. 



Looking at the data for the twenty-six substances examined, it is evident that 

 they may be divided into groups — (1) benzene and its haloid derivatives, ether, 

 the tetraclilorides and the paraffins; (2) the ten esters; (.3) the alcohols; (4) 

 acetic acid. The members of group (1) may be regarded as normal : the devia- 

 tions of the ratios from constancy are small, though, as pointed out, they exliibit 



certain regularities ; in group (2) the values of - are rather higher, and of 



V ■ • • T i' 



rather lower than in group (1) ; in group (3) the ratios and are very 



' Trans. Chcm. Soc, vol. Iv. p. 48G. 

 ■' lhid.,\o\. lix. p. 911. 

 » Ibid., vol. Ixiii. p. 1191. 



* //;«(Z., vol. Isvii. p. 1071; vol. Ixxi. p. 44(1; vol. Isxiii. (575; rroc. Plnjs. Sue, 

 vol. xiii. p. 602. 



