228 SCIENCE PROGRESS. 



of the sulphur compound. Testimony of the same kind is 

 obtained from the behaviour of solutions of hydroxy com- 

 pounds in hydrocarbon solvents (v. infra), abnormally high 

 molecular weights being obtained if the solutions are of 

 medium concentration ; indeed it has been proposed to 

 utilise the properties of such solutions as a means of 

 detecting the presence of hydroxyl in the molecule. 



Guye (16) has noted other three peculiarities which, 

 although they do not show so definitely that the disturbing 

 factor is molecular complexity, further illustrate the excep- 

 tional behaviour of hydroxy compounds. He finds that 

 those liquids, for which the latent heat of vaporisation 

 reaches a maximum, are hydroxy compounds. Again, if at 

 the same pressure we measure the densities of a substance 

 both as liquid and as saturated vapour, for most substances 

 the mean of the densities is a linear function of the tem- 

 perature ; liquids giving a curvilinear function are hydroxy 

 compounds. As a rule vapour-pressure curves do not cut 

 one another ; the most decided exceptions are given by 

 water, the alcohols, and acetic acid. Yet another point 

 of the same kind is recalled by a recent investigation by 

 Longuinine (17), who has determined the heats of vapor- 

 isation of the normal fatty alcohols at their boiling-points. 

 According to Trouton's rule the molecular heat of vapor- 

 isation divided by the absolute boiling-point should be the 

 same for different substances. Longuinine finds that this 

 is true for the alcohols, the mean value of the quotient 

 being 26*3 He concludes, however, that the value varies 

 from series to series. A survey of the entire data, at 

 all events, will serve to show, I think, that if hydroxy 

 compounds be excluded, Trouton's rule appears to be very 

 closely obeyed, considering the experimental difficulties. 



Although certain properties — such as density, refractive 

 index, and, according to Perkin (18), magnetic rotation — 

 which are distinguished from those treated above, as they 

 are concerned with the volume relations of atoms and mole- 

 cules, give, as might be anticipated, little indication of the 

 exceptional behaviour of hydroxy compounds, there is 

 ample evidence of its existence, and much of this evidence 



