132 PRINCIPLES OF GENERAL PHYSIOLOGY 



chloroform-lipoid phase becomes deeply coloured when brought into contact with a 

 watery solution of methylene blue. The explanation given by the adherents of 

 the lipoid-membrane theory is that methylene blue is more soluble in kephalin 

 than in water and that the staining of the lipoid is due to a true solution of the 

 dye in it. Now Loewe brings strong evidence against this interpretation of the 

 fact. Suppose that the dye is dissolved in true solution ; there is a certain ratio 

 between its concentration in the water phase and that in the chloroform-lipoid 

 phase, known as the " partition coefficient," and, if the molecular weight is the 

 same in both solvents, this ratio will not vary with the concentration. Loewe 

 finds, on the contrary, that the ratio varies very considerably with the concen- 

 tration, but that it follows the parabolic law of adsorption, viz., the ratio varies as 

 some power of the concentration. The exponent 1/n has values between O35 and 

 0'16, according to the particular lipoid used, kephalin, cholesterol, residual brain 

 lipoids, etc. On the other hand, for each individual lipoid, the value is fairly 

 constant. The conclusion drawn is that we are dealing with a case of adsorption, 

 but it must not be forgotten, as Loewe appears to have done, that the partition 

 between solvents also has an exponential ratio if the molecular weight of the 

 solute is not the same in the two. Take, for example, acetic acid dissolved in 

 benzene and in water ; in the former the molecular weight is double that in the 

 latter, owing to the association of two molecules together. In such cases, the 

 exponent expresses the ratio of the molecular weight in the two solvents, so that 

 it must be a whole number. Now, in the case of methylene blue in lipoid and 

 water, a ratio of whole numbers can only be obtained by assuming a very large 

 association in both solvents ; a quite impossible degree in fact as regards water, 

 where, judging by its electrical conductivity, there is no association. It appears, 

 then, that Loewe's interpretation is correct. Moreover, as this investigator poinN 

 out, if the phenomenon is a partition owing to different solubility, the dye would 

 readily be removed when the lipoid phase is put into contact with pure water. 

 But this is not so, and the case is precisely similar to that of paper stained with the 

 dye. It will be remembered that paper, owing to its negative charge, has a 

 strong adsorptive power for electro-positive dyes and is in equilibrium only when a 

 very deeply-stained paper is in contact with a very dilute solution of dye. So 

 that, as Freundlich points out, very little dye is removed by pure water. Another 

 fact observed by Loewe, which shows the staining of lipoid by methylene blue to 

 be a surface condensation only, is that, if a mass of kephalin be placed in contact 

 with a watery solution of methylene blue, the dye does not diffuse into the lipoid. 

 Further, if a solution of dye, to which gelatine has been added in order to prevent 

 mixing of the various layers, be covered with a layer of lipoid and over this water 

 be placed, no dye passes into the water. Similar facts were noticed with regard to 

 other substances supposed to be soluble in lipoids, such as narcotics, nicotine and 

 tetanus toxin. As concerns other lipoids, cerebroside (a galactolipine) and the 

 lipoid residue from brain after removal of kephalin and cerebroside, all behaved 

 like kephalin. Cholesterol was found to obey the partition law, but dissolved very 

 little dye. Thymol in chloroform was found to be partly in a colloidal form, partly 

 in true solution, but obeyed the adsorption law and not the partition law. In 

 this last case, apparently, only the colloidal particles took up the dye. There is, 

 finally, another difficulty involved in the acceptance of the solubility partition 

 theory. If we take a particular case of Loewe's, say the first on Table II. (p. 161 

 of his paper), we see that the final concentration of the dye is greater in the lipoid- 

 chloroform phase than in the water phase. Remembering that the dye is 

 practically insoluble, j^ chloroform itself, the result means that the solvent power 

 of chloroform for he dye has been raised by the addition of 0'5 per cent, of 

 kephalin to at least that of water. If we compare this effect with the increase of 

 the solvent power of alcohol for cane-sugar, produced by the addition of as much 

 as 3'28 per cent, of water, which was found by Scheibler (1872) to be raised only 

 to 0'36 per cent., we are compelled to admit the inherent improbability of explana- 

 tion on these lines. 



So far as Loewe's experiments go, it appears that a lipoid membrane, so far 

 from being an assistance to the passage of " lipoid - soluble " substances into 



