234 



Dr. W. M. Bayliss. 



[June 30, 



that of saccharose in a dilution of 30 litres. The drops of dye solution at 

 first took up a trace of water from the sugar solution and were then in 

 equilibrium with it. Estimating from the change in dimensions of the drops, 

 the dye solution had the same osmotic pressure as that of saccharose in a 

 dilution of 29 - 4 litres. In the second experiment the drops of dye solution 

 increased slightly in size when alternating with drops of saccharose at 

 50 litres dilution, but decreased when sugar at 45 litres dilution was 

 substituted. The decrease in the latter case was greater than the increase in 

 the former, so that we may take the actual osmotic pressure, or rather vapour 

 pressure, of the dye solution to have been the same as that of sugar at a 

 dilution of 48 litres. The results are sufficient to show that the direct 

 measurements give correctly the total osmotic concentration. A very 

 minute impurity, if of small molecular weight, would be capable of accounting 

 for the very slightly higher vapour pressure of the dye solution than would 

 be expected from the osmometer values ; this, being diffusible, would not be 

 shown by the direct method of estimation. 



If the Na' ions were inactive osmotically, it is impossible that pressures 

 higher than what I have called " molecular " could ever be obtained, even in 

 solutions of great dilution. Now Biltz and v. Vegesack* themselves have 

 obtained such. I have myself in dilutions of about 1000 litres seen on 

 one or two occasions pressures of 101 — 102 per cent, of the " molecular "; but, 

 since the actual values did not exceed 21 mm. Hg, I do not feel justified in 

 drawing conclusions from them. On the other hand, with Chicago blue and 

 with sodium caseinogenate pressures are always obtained higher than could 

 be accounted for on the theory of Biltz. 



Chicago blue is the tetrasodium salt of a substituted dinaphthylamine- 

 tetrasulphonic acid.f If normally dissociated, it should give four sodium 

 ions and one quadrivalent anion. If the former were inactive, the osmotic 

 pressure of this dye should be the same as that of Congo red. In point of 

 fact, a dilution of 353 litres gave a value of 93 mm. Hg or practically double 

 the " molecular " osmotic pressure. It was the same, however, as that of a 

 Congo-red solution of the same electrical conductivity. Another experiment, 

 with a dilution of 1003 litres, gave an osmotic pressure of 35 mm. Hg, again 

 double the " molecular " pressure. 



The sodium salt of caseinogen, containing sufficient base to be faintly 

 alkaline to phenolphthalein, gave an osmotic pressure of 313 mm. Hg for a 

 solution of 3"23 per cent. If the anion alone were active, the maximum 



* 'Zeit. f. physik. Chem.,' 1910, vol. 73, p. 490. 



+ I am indebted to the kindness of the Berlin Aniline Company for a supply of this dye. 



