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Dr. W. M. Bayliss. 



[June 30 r 



There is one more hypothesis which might be made on the ground of the 

 colloidal behaviour of the anions in the case of Congo red. These large 

 organic ions may aggregate while retaining the combined charges of their 

 components. This seems to imply, however, so great a density of the charge 

 on the surface of the aggregates as to be improbable.* It could not, in any 

 case, apply to the !STa* ions, which cannot be assumed to form aggregates of 

 this kind. If, however, Ave suppose that the non-dissociated molecules, as 

 well as the anions, are so far aggregated as to give an osmotic pressure too 

 small to be detected by a mercury manometer, so that the Na* ions alone are- 

 osmotically active, the osmotic pressure at a dilution such that the dissocia- 

 tion is 50 per cent, would be accounted for, but not when the dilution is 

 greater than this, while the difficulty as to the large charge on the aggregated 

 anions remains. The total charge of the anions must, of course, be equal and 

 opposite to that on the cations. 



The following experiment gives evidence, so far as it goes, against the. 

 formation of complex ions. When solutions of Congo red are exposed to> 

 electrolysis in a parchment-paper cup so that the anode is inside and the 

 cathode outside, cations are driven out, as we have already seen. These ions 

 are not coloured, as seems would be the case if they contained acid com- 

 ponents. The outer fluid becomes strongly alkaline but remains colourless. 

 It may be, perhaps, that under the action of a current the naturally present 

 complex ions are caused to dissociate further, although the natural conclusion 

 would be that the Ha' ions were formed in the solution already .f It is of 

 interest to note that, in this experiment, when the current is reversed, the 

 escaped cations are driven back and that, after this has taken place, the 

 current practically ceases to pass, owing to the inability of the anion to pass 

 through the membrane. The apparatus is, in fact, a kind of rectifier. 



On the whole it is evident that there is something abnormal in the form of 

 the dissociation of salts, one of whose ions is colloidal. It may tentatively 

 be suggested that aggregation of molecules may play some part in the mode 

 of ionisation, so that this may be a function of the surface of the aggregates. 



The fact that the observed values of the osmotic pressure are less than 



* It is pointed out to rue by Mr. Hardy that the abnormally high velocity of the 

 negative ion shows, either that resistance to movement decreases with increase of volume, 

 or that the charge is increased. It might be argued that, as the normal velocity would 

 be about 28 x ]0 -5 cm. per second, the charge is 57/28 {vide ante), or twice as great as 

 that carried by an ordinary ion. 



t This experiment cannot, perhaps, be regarded as offering very strong evidence 

 against the presence of complex cations, since, if only a small fraction of the dye were 

 dissociated with formation of Na - ions, these would pass through the membrane, and their 

 place inside be filled by fresh dissociation, according to the law of mass action. 



