Permeability 219 
up by the epithelial cells of the kidney and accumulate there; never¬ 
theless, some acid dyes are not taken up. Hober and his co-workers 
came to the conclusion from their experimental results that when 
a dye is not absorbed by the epithelial cells of the kidney it is highly 
colloidal; whereas if a dye is only slightly colloidal or semi-colloidal 
it is easily absorbed. 
In the next year Ruhland (1908 a, b) came to the conclusion from 
experiments with Spirogyra, that whatever the degree of dispersity 
of the dye, the cells in question readily absorb a basic dye but not 
an acid dye. In the same year Hober (1909) examined the capacity 
of 34 dyes to stain the epithelial cells of the kidney and came to the 
same conclusion as he had done previously, namely, that those dyes 
which do not stain these kidney cells are unable to do so on account 
of their low degree of dispersity. He rightly pointed out that his 
results only hold for kidney cells and were not necessarily applicable 
to cells in general. 
In further work which will be dealt with in some little detail later 
in this chapter, Ruhland (1912 b, 1913 a, b, c) examined the intake 
of a great number of dyes, both acid and basic, by a different method 
and as a result changed his point of view and now holds that it is 
solely the size of the particles of the dye which determines whether 
the dye is taken up or not. 
The importance of the hydrogen-ion concentration of the cell 
contents is emphasized by Bethe (1916, 1922) and Rohde (1917), who 
find that the capacity of living cells to absorb dyes depends in high 
degree on the reaction of the external medium and of the cell contents. 
Acid reaction in the interior of the cell favours the accumulation of 
acid dyes and acts against staining by basic dyes, while inversely, 
basic dyes are extraordinarily strongly absorbed by cells with an 
alkaline reaction while acid dyes scarcely stain such cells at all. 
Bethe (1922) adduces in support of this view experiments in which 
basic and acid dyes and one amphoteric dye were allowed to diffuse 
through parchment paper under different conditions of hydrogen-ion 
concentration. With acid dyes diffusion is furthered by an acid re¬ 
action and reduced by an alkaline one; with basic dyes the reverse 
is the case. Similarly, if the dye is allowed to diffuse from an aqueous 
solution through parchment paper into a protein, the dye accumu¬ 
lates in the solution when an acid dye diffuses into an acid protein 
sol or when a basic dye diffuses into an alkaline protein sol, but not 
when the reaction of the sol is acid and the dye a basic one, nor when 
the reaction of the sol is alkaline and an acid dye is employed. 
