278 



Dr. W. M. Bayliss. 



[Apr. 19, 



normal calcium sulphate was prevented, although in the absence of the 

 protein complete precipitation occurred. In order to be quite certain of 

 adequate protection, I added 10 c.c. of the albumin solution to 50 c.c. of the 

 dye and placed the mixture in a Schleicher and Schiill diffusion thimble of 

 parchment-paper tied on to a glass tube fitted with a cork and a long 

 narrow tube to act as manometer. This was placed in water contained in a 

 large test-tube and then immersed in a thermostat at 30 o- 7 C. The osmotic 

 pressure rose to 200 mm. of water in the course of 30 hours, being 94 per 

 cent, of the theoretical value. Calcium sulphate in one-hundredth normal 

 solution was then put into the outer tube instead of the water. In about 

 24 hours the pressure had fallen to 50 mm. and no further fall took place. 

 In this case a certain protection was shown, since, without the albumin, the 

 pressure would have gone down nearly to zero. 



The apparent disagreement between the results of osmotic pressure 

 measurements and the naked eye appearances are, I think, to be explained 

 in the following way. When the contents of the osmometer in the last 

 experiment were poured into a glass vessel and observed carefully, it was 

 obvious that, although no precipitation had taken place, the solution was 

 distinctly more turbid than a similar one to which no calcium sulphate had 

 been added. On examination under the ultra-microscope, the former was 

 resolvable into a multitude of distinct, but not brilliant, particles ; whereas 

 solutions of congo-red itself, as already shown, are not resolvable. It is 

 clear, therefore, that a certain degree of aggregation had in reality taken 

 place, although the particles formed are much smaller than those formed 

 when calcium sulphate acts upon congo-red in the absence of a protecting 

 colloid. In this latter case, they are large enough to fall as a precipitate. 

 The actual values of the osmotic pressure observed, 200 mm. and 50 mm., 

 show that, under the conditions of this experiment, aggregates of four 

 molecules are formed. It appears that, so far as congo-red is concerned, the 

 mode of action of a stable colloid is to form, under the influence of an 

 electrolyte, a colloidal complex with the dye, which complex, although 

 consisting of several molecules and therefore, when formed, causing a large 

 fall in the osmotic pressure of the solution, is yet in particles sufficiently 

 minute not to fall as a precipitate. The reason why the aggregates formed 

 are small is, no doubt, connected with the lowering of surface-tension caused 

 by the protein. It is to be noted that, unless the solution of dye + albumin 

 + electrolyte had been compared with a similar solution without the 

 electrolyte, it would have been supposed that complete protection from 

 the action of the electrolyte had been brought about, since no true precipi- 

 tation occurs. 



