viii THE GENERAL PHYSICAL PROPERTIES OF ALBUMINS 269 



that the surfaces of colloidal particles are semi -permeable, which 

 means that they allow of the ready passage of ions of the opposite 

 electrical sign to that carried by themselves, i.e. of either kations or of 

 the anions, while the other ions which cannot enter the colloidal 

 particles remain in the solvent. This explanation amounts to the 

 same as that given by the author, namely, that the [colloid + the 

 entered ion] is an ion. Very interesting in this connection is an 

 observation made by A. Fischer, 1 who noticed that " the basic dyes 

 are absorbed at once by the acid nucleoproteids, while with acid 

 stains there is a delay, in about the proportion that methyl green will 

 have stained already intensely, when acid fuchsin only just shows the 

 faintest indication of staining. In the course of ten minutes, 

 however, this difference disappears in material which was fixed in 

 indifferent reagents." Reversely "the acid dyes diffusing through 

 the sections stain at first only the cytoplasm, and several seconds 

 later the nuclei, which ultimately are also stained as intensely as the 

 cytoplasm." Fischer failed to understand the importance of his own 

 observations, for he uses his facts to prove the absence of any real 

 difference in the absorptive powers of nucleins with regard to acid 

 and basic dyes ; while to the author, 2 Fischer's observations have 

 this significance : each particle, either kat-ion or an-ion, has an aversion 

 for ions of its own kind or those of the same electrical sign ; thus the 

 positive kat-ion H will not only repel other H ions, but also, for 

 example, those of potassium, K. On the other hand, positive 

 kat-ion s will readily unite with negative an-ions. 



The view that colloids are electrolytes is further supported by the 

 fact that colloids of opposite electrical sign precipitate one another, as 

 has long been known to histologists. Romanowsky 3 in 1891 com- 

 bined equi-molecular proportions of the basic methylene-blue and the 

 acid eosin, and thus obtained the water-insoluble eosinate of methylene- 

 blue. 4 Quite recently the same phenomenon has been studied by 

 Biltz, 5 who calls these unions ' adsorption-compounds ' (see p. 270). 



After giving various further examples of the fact that hydrosols 

 of opposite electrical sign mutually precipitate one another if they 

 are mixed in equivalent amounts, he shows that mixtures of 

 hydrosols possessing the same electrical sign such as the purple of 



1 A. Fischer, Fixirung, Farbung und Ban des Protoplasmas, 1899, p. 94 



2 Mann, Physiological Histology, 1902, p. 339. 



3 Romanowsky, Zur Fragc d. Parasitol. u. d. Therap. d. Malaria, St. Petersburg, 

 1891. 



4 Other instances are given in the editor's Physiological Histology, pp. 441-444. 



5 W. Biltz, "Mutual Interactions of Colloidal Substances," Ber. d. dcutsch. chem. 

 Gesell. 37. 1095 (1904). 



