ASPECTS OF ADSORPTION PHENOMENA 215 



It is scarcely necessary to give instances of this difficulty, and I will 

 merely refer to three instances. Macallum 1 in searching for the cause 

 of the silver reaction of tissues met with considerable difficulty 

 in completely removing chlorides. Mellanby* states that there is a 

 molecular combination between neutral salts and globulin. Lange' 

 found that it was impossible to remove all ash-constituents from filter- 

 paper by extraction with hydrochloric and hydrofluoric acids and 

 washing. 



4. Dyeing 



On the whole, I think it must be taken as the true explanation 

 of this process that it is, in the main, an adsorption. The very slight 

 diffusibility of most dyes through parchment-paper seems to me a 

 considerable objection to Witt's theory of solid-solution. If the dye 

 is dissolved in the paper it should be readily given off again to water 

 on the opposite side in the same way as hydrogen passes through 

 palladium in the experiments of Ramsay. 4 The theory to which my 

 experiments lead is very much the same as that to which Picton and 

 Linder have independently arrived. 5 According to these observers, 

 there are two stages to be distinguished. 



Stage I. The 'coagulation stage' in which single ionic inter- 

 change takes place between the ' fibre substance ' (colloid) and the dye, 

 resulting in the separation of insoluble dye derivatives retaining a feeble 

 charge. 



Stage II. The 'colour absorption' stage, in which coagula pro- 

 duced in stage I attract and retain the oppositely-charged particles of 

 the dye substance. 



My experiments show that no actual precipitation of the dye must 

 take place except in the substance to be stained, and I should be 

 inclined to modify the above theory by omitting stage I, and reading 

 instead of 'coagula produced in stage 1' simply 'colloids of fibre- 

 substance.' These bodies usually having a negative charge, it is 



1. Proc. Roy. Soc. 76 B, p. 225, 1905. 



2. Journ. of Physiology, 33, p. 359, 1905. 



3. Ber. Deutsche. Chem. Ges., 1878, p. 823. 



4. Phil. Mag., 38, pp. 206, 2 1 8, 1894.. 



5. Journ. Chem. Soc., 88, p. 1,935, I 95- 



