1917] BROOKS—PERMEABILITY 511 
In the use of indicators we possess an extremely sensitive and 
reliable means of demonstrating the presence of small amounts 
of free acid or alkali in a solution. It would be possible therefore 
to detect the diffusion through the diaphragm of tissue of small 
amounts of hydrochloric acid or sodium hydroxide by adding a 
small amount of a suitable indicator to the distilled water in the 
upper cell of the apparatus. In the lower cells 0.1 M solutions of 
the acid and alkali were used. 
A period of 43 hours was insufficient to allow the passage of an 
amount of sodium hydroxide great enough to cause any change in 
the color of the distilled water containing about 0.01 per cent of 
phenolphthalein, as determined by comparison in a colorimeter 
with fresh distilled water. The change of hydroxyl-ion concentra- 
tion necessary to cause the first visible change in the color of the 
phenolphthalein would be that from 1 X107~° M to 1X1075 M. 
The turning point of Congo red lies at a hydrogen-ion concen- 
tration of 1X10~* M. An increase of less than 1X10~*M in the © 
hydrogen-ion concentration of distilled water containing Congo 
red will then cause the appearance of the blue coloration in the 
indicator. Experiments were conducted in which the lower cells 
were filled with o.1 M hydrochloric acid, and the upper cells with 
distilled water containing barely sufficient Congo red to cause a 
distinct red coloration; these showed that a period of 3-5 hours 
was sufficient to cause the color change in the indicator. Control 
experiments in which the lower cell was filled with pure distilled 
water showed no color change in the upper cells during 19 hours. 
In order to eliminate the possibility that the permeability to 
hydrogen ions was the result of the action of the 0.1 M hydro- 
chloric acid on the tissue, several of the cells in which there had 
been a diffusion of acid were simply rinsed out thoroughly, and the 
lower cell finally filled (after preliminary rinsing with the solution) 
with o.1 M sodium hydroxide. The upper cell was filled with 
distilled water containing a slight amount of phenolphthalein. 
There was no color change in the distilled water up to the end of 
the experiment, a period of 3 days. 
The inner epidermis of onion bulb scales, at least when its cells 
are dead, is therefore but slightly permeable to hydrochloric acid, 
and not perceptibly so to any other of the substances tried. These 
