384 SCIENCE PROGRESS. 



acid previously considered, it is evident that some additional 

 disturbing factor must operate in the former case, and the 

 data clearly point to the existence in strong solutions of 

 complex molecules of dissolved substance which become 

 dissociated into simple molecules as the dilution increases. 



Nature of the dissolved substance.- — Now it is one of the 

 most interesting facts discovered in this field that almost all 

 associated solutions are solutions of hydroxy-compounds, 

 and further, that the great majority of hydroxy-compounds 

 when dissolved in hydrocarbon solvents give rise to asso- 

 ciated solutions. The following tables, based on data 

 summarised by Auwers (1893) illustrate the behaviour ot 

 some of the more important members of four different 

 classes of these compounds, the solvent being benzene. 

 Under g, is given the limits of concentration between which 

 the observations were made ; under M ', the observed 

 limiting values of the molecular weight ; and under M, the 

 theoretical molecular weight. 



These tables show that in very dilute solutions the 

 alcohols give normal molecular weights which increase at 

 an extremely rapid and approximately constant rate as the 

 concentration increases. The curves obtained on plotting 

 molecular weight as ordinates against concentration as ab- 

 scissae are therefore almost straight lines steeply inclined to 

 the horizontal axis. The shape of the curves for the acids 

 indicates that, in general, in extremely dilute solutions the 

 molecular weights would also be normal ; the values increase 

 very rapidly, however, as the concentration increases, until 

 they are almost twice as large as theory demands ; further 

 increase in concentration is then attended by a slow increase 

 in this double molecular weight of the same order as that 

 usually occurring in the case of unassociated solutions. 

 The curves for the oximes resemble those for the acids 

 with the difference that the increase to the double molec- 

 ular weight proceeds at a very much slower rate. The 

 two lowest phenols give curves resembling those of the 

 alcohols ; the behaviour of the others is normal, although, 

 with the exception of thymol, the effect of concentration can- 

 not be accurately ascertained on account of their limited 



