284 



Dr. W. M. Bavliss. 



if 



[Apr. 19, 



manifest. In order to stop this movement, I mixed a part of the diluted 

 solution, as used for the previous measurements, with an equal volume of a 

 2-per-cent. solution of gelatin, warmed just sufficiently to liquefy it. This 

 method was used by J. Duclaux * in his investigations of ferric hydroxide. 

 Although the gelatin used by me had been soaked in repeated changes of 

 toluene-water, it retained a certain amount of its adsorbed electrolytes ; 

 so that on adding the blue colloid to it, a distinct change of colour towards 

 purple resulted. Although the particles were seen to be immobilised, it 

 did not seem worth while to proceed further with the laborious deter- 

 minations, since the change of colour indicated a change in the colloid. 



The determinations of molecular dimensions given above are intended to 

 show the possibilities of the method. The exact numerical data are, no 

 doubt, capable of correction when a more satisfactory means of immobilising 

 the particles has been found. The values obtained appear high, even for 

 a molecule containing 70 atoms, such as the one in question. From 

 Zsigmondy's observations with colloidal gold it would seem that particles of 

 these dimensions should be resolved by the ultra-microscope. It is true 

 that the impression given to the observer is that the solutions of congo-red 

 are just on the limit. Moreover, the fact that molecules of congo-red are 

 unable to pass through parchment-paper shows that they far exceed in 

 dimensions those of crystalloid bodies.f 



Further experiments are in progress, as also others in the manner of those 

 of Perrinf with suspensions of gamboge. It would be premature to draw 

 conclusions from the results of the preceding pages as to whether the 

 particles as wholes are responsible for the osmotic pressure, or whether only 

 a part of each one, such as adsorbed ions, alone is active in this respect. 

 So much may be said, that my observations speak decidedly in favour of the 

 kinetic theory of the osmotic pressure of colloids. According to this theory 

 the " Brownian movement " of the particles corresponds to the molecular 

 movement assumed in the kinetic theory of gases. 



Important recent confirmation of this view is to be found in the experi- 

 ments of Perrin already alluded to, which show that the kinetic energy of 

 a colloidal particle is identical with that of a molecule. This observer shows 

 that, if we take the number of molecules contained in one gramme-molecule 

 of a rjerfect gas to be 



6 or 7 x 10 23 , 



* ' Comptes Eendus,' vol. 147, pp. 131—138, 1908. 



t As regards size of pores in parchment-paper, see Bechhold, ' Zeit. f. Phys. Chem.,' 

 vol. 64, pp. 328—342, 1908. 



1 'Comptes Eendus,' vol. 146, pp. 967—970, and vol. 147, pp. 530—532, 1908. 



