276 



Dr. W. M. Bayliss. 



[Apr. 19, 



When excess was present, as shown by the permanent alkaline reaction 

 of the solution, the pressure fell again. The explanation is, no doubt, that 

 the aggregating action of the cation made itself felt. In view of the results 

 of Moore and Eoaf* on the augmentation of the osmotic pressure of protein 

 solutions under the action of alkali, the fact noted by me is of some interest. 

 It seems probable that, in the case of proteins, the rise of osmotic pressure 

 is the result of the formation of new colloids by chemical action, these 

 " salts " having smaller " solution-aggregates " than the original colloid. f 



If we were unaware of the chemical nature of congo-red and observations 

 were being made of the osmotic pressure of the solution of the free acid, we 

 might imagine that alkali caused a large increase in the osmotic pressure of 

 this body. The fact is that the blue colloidal solution of the free acid,, 

 as will be shown later, consists of large particles, easily resolvable by the 

 ultra-microscope, and producing only a small osmotic pressure. When 

 alkali is added, the salt, ordinary congo-red dye, is produced, and this, as 

 shown above, exists in solution in single molecules with high osmotic pressure. 



The fact that, as an electrolyte is added in stages, pausing sufficiently long' 

 between each addition to allow equilibrium to be established, there is 

 a definite osmotic pressure for each step, so that a continuous smooth curve is- 

 obtained, shows that the action of a low concentration of electrolyte must be 

 exerted on a part only of the molecules present. For example, it is not 

 every molecule that unites with another one, since, if so, there would be no 

 intermediate stages between full and half osmotic pressure. From the fact 

 that these stages do exist it follows that a number of molecules are left 

 single. The process is analogous to the association which takes place in 

 ethyl alcohol when dissolved in benzene, where the apparent molecular 

 weight of the alcohol rises from 50 to 208 in regular gradation as the 

 concentration rises from - 494 to 14 - 63 grammes to 100 of benzene.j This, 

 can only be explained by the assumption of a steadily increasing number of 

 molecules becoming associated with others, while the rest remain free. 



Ultra-microscopic observations of the actions of electrolytes on congo-red, 

 although somewhat difficult to interpret, confirm the results given by measure- 

 ments of osmotic pressure A dilute solution, showing only a very few 

 scattered bright points, on the addition of a solution of carbonic acid or a 

 natural salt contains a greatly increased number of these bright particles, 

 which vary considerably in size. In this respect they contrast with the- 

 particles seen in the colloidal solution of the free acid, which are strikingly 



* hoc. cit., p. 66. 



t See also Lillie, < Amer. Journ. of Physiol.,' vol. 20, pp. 127—169, 1907. 

 \ Walker, ' Introduction to Physical Chemistry,' 4th ed., p. 205, 1907. 



