270 



Dr. W. M. Bayliss. 



[Apr. 19, 



so that it seemed desirable to investigate the behaviour of colloids of known 

 chemical constitution and molecular weight, the latter to be as small as 

 possible, in order that the osmotic pressure should be sufficiently great. By 

 this means it might be possible to estimate the number of molecules taking 

 part in the formation of " solution-aggregates " or colloidal elements, and also 

 to obtain more definite information as to the effect of electrolytes on the 

 osmotic pressure. 



Certain of the aniline dyes form colloidal solutions, if we may take 

 Graham's criterion of non-diffusibility through parchment-paper as decisive. 

 One of these dyes is congo-red, whose constitution and molecular weight are 

 well known. My attention was first called to the fact that solutions of this 

 body have a considerable osmotic pressure by phenomena met with in purify- 

 ing it by dialysis. It was striking and, in fact, a matter of some 

 inconvenience, to find that the contents of the parchment-paper tubes rapidly 

 increased in volume by taking up water and, unless some of the fluid was 

 removed, continuously overflowed. Congo-red, therefore, formed the starting- 

 point of the observations to be recorded in the present paper. 



Owing to their high colouring power, the aniline dyes present many 

 advantages for the study of colloidal properties. In the investigation of 

 osmotic pressure, for example, the slightest leak in the membrane of the 

 osmometer is detected at once. 



The particular form of osmometer used was that of Moore and Eoaf,* 

 modified in order to change at will the fluid on the side of the membrane 

 opposite to the solution under investigation. Eepeated changes of distilled 

 water could be made until no further change in the osmotic pressure 

 occurred, while the effect of the presence of various electrolytes or other 

 bodies could be examined. A diagram of the apparatus is given in fig. 1. 



Congo-red, as obtained from Kahlbaum, was found to contain an 

 appreciable amount of sodium chloride. In order to remove this, hydro- 

 chloric acid was added until the red colour had vanished and the free acid, 

 precipitated, was washed on a filter with distilled water. It was soon found 

 that the free acid went into a beautiful deep blue colloidal solution, which 

 passed through the filter. (This observation has been published by Pelet- 

 Jolivet and Wild,f since my experiments were made.) I was obliged, 

 therefore, to resort to prolonged dialysis against distilled water. This 

 dialysed solution was placed in the osmometer. It gave a very small 

 osmotic pressure, about 6 mm. Hg. Subsequent experiments, to be described 

 below, were made to determine the osmotic pressure of this blue colloid 

 * Loc. cit. 



t ' Kolloid-Zeitsch.,' vol. 3, p. 175, 1908. 



