274 



Dr. W. M. Bayliss. 



[Apr. 19, 



compound, in that its composition varies with the relative concentrations of 

 the two colloids present in the solution. As I have shown elsewhere,* the 

 behaviour of congo-red in respect of adsorption by cotton and other 

 materials is that of an electro-negative colloid. 



The statement is sometimes made that colloids have no definite point of 

 saturation. Congo-red, on the contrary, has, in a certain sense, a somewhat 

 indefinite limit of solubility. It appears, however, that many colloids, 

 especially the inorganic ones, tend to aggregate and deposit when their 

 particles are brought into too close apposition ; it may be that traces of 

 electrolytes are responsible for this behaviour, which is thus not the same 

 thing as the crystallisation of a super-saturated true solution. 



Kecent research tends to show that there is no real line of demarcation to 

 be drawn between colloids and crystalloids. Congo-red is evidently one of 

 those interesting cases which have some of the properties of both classes. In 

 any case, it does not seem reasonable to expect fundamental differences as 

 regards properties dependent on dimensions of the active elements between 

 a large molecule such as congo-red, containing some 70 atoms, and a particle 

 of colloidal gold containing a similar number of atoms. The properties 

 referred to are those dependent on diffusion, such as osmotic pressure and 

 those dependent on surface development. 



The fact that, in the case before us, true solution and colloidal solution are 

 one and the same thing suggests several interesting questions. At what 

 molecular size do bodies begin to show properties due to surface develop- 

 ment, although still in the condition of single molecules ? Again, why 

 should we not be able to reduce the number of molecules in the aggregates 

 of colloidal gold until they consist of single molecules ? In this case we 

 should have a true solution of gold in the metallic state, not in the ionised 

 condition. It is possible that differences of electric potential and surface- 

 tension oppose obstacles to such a phenomenon. 



The great sensitiveness of congo-red to traces of electrolytes has already 

 been incidentally referred to and this fact makes it a matter of considerable 

 difficulty to obtain the maximum readings of the osmotic pressure as given 

 above. For example, a solution of congo-red, containing - 84 per cent, of 

 the dye, dialysed against repeated changes of ordinary distilled water until 

 no further rise of pressure took place, gave an osmotic pressure of only 

 118 mm. Hg, whereas on using water which had been distilled after the 

 addition of potassium permanganate and sulphuric acid and a second time after 

 the addition of barium hydroxide, being kept from contact with air by soda- 

 lime tubes, the pressure rose to 207 mm. Hg. The ordinary distilled water 

 * ' Biochem. Journ.,' vol. 1, pp. 175—232, 1906. 



