1877.] 



Properties of Homologues and Isomers. 



239 



complete, but that with regard to the others little is known but their 

 approximate boiling-points and densities. 



Among the alcohols of the C w H(^+i) . OH series, for example, there 

 are two (methyl and ethyl alcohols) with whose physical properties 

 we are well acquainted. The expansion of these bodies has been deter- 

 mined by Kopp, Pierre, Mendelejeff, and others ; the vapour-tensions, 

 latent and specific heats by Eegnault ; the latent heat by Andrews also ; 

 the refractive indices by Gladstone and Dale. Nearly all these researches, 

 and many more which might be enumerated, have been accompanied by 

 measurements of density and boiling-point. 



"When we pass to the other members of the series we find, however, 

 that in the case of normal propyl alcohol, with the exception of numerous 

 determinations of density and boiling-point, experiments have been 

 limited, as far as I am aware, to the measurement of the expansion by 

 Pierre and Puchot. (The table of vapour-tensions given in the same 

 paper is erroneous, as I hope to prove in a future communication.) The 

 same is to be said of all the other alcohols of this series, the rates of 

 expansion of some of which have been examined by Kopp, Lieben and 

 Eossi, "Wanklyn and Erlenmeyer, and others. 



In order partially to supply this want I have undertaken a series of 

 experiments, of which the following pages are the firstfruits. 



I. Density, Expansion, and Vapour-tension of Propyl and Isopropyl 



Iodides. 



The selection of these as my first experiments was prompted by the 

 following considerations : — If, as is possible, all the particular physical 

 characteristics of a given substance result from one main characteristic, 

 which is, as it were, the fundamental definition of the substance (at 

 least from a physical point of view), then the various phenomena ob- 

 served must be dependent one on the other, and the measurement of 

 the vapour-tension, for instance, or of the latent heat, of the given 

 substance must involve implicitly all other measurements, such as 

 those of the specific heat, rate of expansion, &c. ; that this is not with- 

 out foundation, is proved by the many partially successful attempts to 

 connect the various results obtained for certain bodies by mathematical 

 reasoning. 



A connexion between the vapour-tension and rate of expansion of a 

 liquid organic body is rendered probable by the well-known researches 

 of Hermann Kopp and others. From a large number of observations 

 conclusions were formed which may be thus summed up :- — The atoms of 

 the various elements entering into the composition of substances of the 

 same chemical type always occupy the same space when the tension of 

 the vapour of the liquid is equal to the pressure exerted by a column of 

 mercury 760 millims. in height. Hence the molecules of isomeric sub- 



