282 



Dr. W. M. Bayliss. 



[Apr. 19, 



of 24 hours until uo further rise in the osmotic pressure took place. The 

 value attained was 91 mm. of water, or 6'8 mm. Hg at 30 o- 9 C. There seems 

 no possibility whatever of the presence of a salt of the dye in the above con- 

 ditions. In any case, there was a fairly large pressure when free hydrochloric 

 acid was detected in the outer fluid. A further reason for denying the 

 presence of alkaline bodies in the water used is that, if this had been so, the 

 pressure would have continued to rise as long as fresh water was run in, 

 until finally the great pressure of the salts of congo-red would have been 

 reached and the contents of the osmometer converted entirely into the salt. 



The conclusion must be that a definite osmotic pressure can be exerted by 

 a solution consisting of an undoubted suspension of particles, resolvable under 

 the ultra-microscope. When water distilled from over barium hydroxide was 

 run in, there was not much further rise in the osmotic pressure, the maximum 

 being only a few millimetres higher. It appears, therefore, that this colloid 

 is not particularly sensitive to traces of free acid. 



If now we proceed to calculate, from the concentration of such a colloidal 

 solution as that of the above experiment, what the osmotic pressure should be 

 if the dye were present in single molecules, we find that it is 20 times that 

 actually found. Assuming for the present that the kinetic theory of the 

 osmotic pressure of colloids is correct, this means that the average number of 

 molecules forming a colloidal particle of the free acid of congo-red is 20. 



This being so, and the separate particles being easily seen in the ultra- 

 microscope, it seemed to be a point of interest to attempt to estimate the- 

 dimensions of the particles in the manner described by Siedentopf and 

 Zsigmondy.* For this purpose a part of the contents of the osmometer 

 at the end of the experiment was diluted 1300 times, so that the particles 

 might be sufficiently far apart to be counted. This counting was somewhat 

 difficult, owing to the rapid movement of the particles. The mean of 

 a number of determinations was between 8 and 9 in a volume of 

 56 x 10 -6 cubic millimetre. The undiluted solution, therefore, contained 

 2 x 10 11 in 1 c.c. There is, however, a possibility not to be lost sight oL 

 As I shall show in a later paper, this acid of congo-red appears to be very 

 slightly soluble in water at 100°, in true solution, ionised. Although the 

 amount dissolved at room temperature is infinitesimal, it may be sufficient 

 to vitiate conclusions drawn from solutions necessarily very dilute. Accord- 

 ingly, I have made determinations, similar to the above, with solutions of 

 one and a-half and of twice the dilution of that one. The three values are 

 placed together in the table for comparison. It will be seen that the values 

 are, within the limits of error, proportional to the dilution. 



* ' Drude's Ann.,' vol. 10, pp. 17, 21, and 22, 1903. 



