1902.] on Becent Developments in Colouring-Matters. 97 



not colourless, as we used to think. But even if we leave out of the 

 question such faintly coloured substances as air and water, if we 

 restrict our consideration to compounds endowed with a very intense 

 power of selective absorption and at the same time soluble in the 

 water which we employ for preparing our dye-baths, we do not 

 arrive yet at the true definition of the dye-stuflf. Cupric salts, 

 soluble chromates and many other intensely-coloured bodies are no 

 dye-stuffs, as may be easily shown by experiment. Yet these com- 

 pounds penetrate into the interior of textile fibres which are immersed 

 into their solution. They must do so, according to the laws of Osmose 

 so ably expounded by Thomas Graham, because they are crystalloids 

 and the fibres are invariably colloids. 



We know now that the laws of Osmose are identical with the 

 laws governing solution, and that crystalloids are able to wander 

 into the interior of colloids because they are soluble in their sub- 

 stance. Osmotic processes may be observed between two liquids 

 which cannot be mixed with each other, just as well as between a 

 liquid and a colloid immersed into it. Consequently we are justified 

 in assuming that the same powers are at work in both cases. 



In my first experiment (Exp. I.) * I intend to show you that a 

 crystalloid, dissolved in some liquid such as water, and brought into 

 contact with another liquid, not miscible with the first, such as ether, 

 will either remain indifferent to the ether altogether, or it will leave 

 the water and wander into the ether, or it will be distributed accord- 

 ing to a certain ratio between the two solvents. In this latter case 

 we have reason to believe that a constant interchange of molecules 

 takes place between the two solutions. Clearly, this will only happen 

 if there exists no great difference in the solubilities of the crystalloid 

 in water and in ether. In that case the water will continually abstract 

 nearly as many molecules of the crystalloid from the ether as the 

 latter will take up from the water, and thus an equilibrium will be 

 reached. If, on the other side, there is a great dissimilarity in the 

 solubility of the crystalloid in the two solvents, then this process of 

 mutual interchange will become so one-sided that it practically 

 amounts to the absorption of the whole of the crystalloid by one of 

 the solvents. 



My second experiment (Exp. II.) f is a more striking illustration 

 of these fundamental facts. If we mix together two coloured solu- 

 tions, one an aqueous one of a substance much more soluble in ether 

 than in water, and the other an ethereal one of a substance more 

 soluble in water than in ether, then the two solutions, on shaking, 

 change colour, and their shades are reversed. 



* Details of experiment ; An aqueous solution of magenta does not yield its 

 colouring-matter to ether ; indophenol, on the contrary, is entirely taken up by 

 ether. The dye-stuff, which is partly taken up by ether, is also a member of the 

 indophenol group, the constitution of which is not yet fully established. 



t The ethereal solution used contained magenta acetate, whilst the aqueous 

 one was prepared with trichloro-indophenol, 



YoL. XVII. (No. 96.) H 



