ON COLLOID CHEMISTRY AND ITS INDUSTRIAL APPLICATIONS. 125 
of the necessary molar concentration would be solid. Even congo- 
red would require to be 21 per cent., a value far exceeding its 
solubility in water. 
Hoéber (1910-1912) has shown, moreover, that there are free 
electrolytes present in cells. He made use of two methods. ‘The 
first was by determining the increase in capacity of a condenser 
when a conductor is placed between its plates. Red blood corpuscles 
showed a conductivity about equal to that of deci-normal potassium 
chloride. The second method depends on the damping of the vibra- 
tion of a rapidly alternating current in a coil by the presence of a 
conductor in the axis of the coil. By this method.the blood 
corpuscles had the conductivity of a solution of potassium chloride 
between 0:1 and 0-4 per cent., muscle between (1 and (0:2 per cent. 
Although the methods are probably not sufficiently sensitive to give 
exact measurements of the concentration, they show clearly that free 
electrolytes are actually present. 
Another experimental result pointing to the same conclusion may 
be referred to. When cells are immersed in dilute copper sulphate, 
alcohol, acetone or aniline, there is a marked increase in the 
conductivity of the outer solution, as shown by Stiles and Jorgensen 
(1915 and 1917). The additional ions must have come from the 
interior of the cells. 
Living cells oppose an enormous resistance to the passage of an 
electrical current and if it were possible to examine them free from 
external electrolytes, it is probable that they would be found to be 
non-conductors in respect of currents applied by electrodes external 
to them. But, if they contain free electrolytes, it seems to the 
writer that the only satisfactory explanation of inability to conduct 
electrical currents is that the cell is surrounded by a membrane 
impermeable to these electrolytes, so that there is no possibility of 
their conveying charges from one electrode to the other. ‘This 
phenomenon is shown in an interesting way in the method used by 
Morse (1914, p. 83).to prepare his copper ferrocvanide cells by 
electrolytic deposition. As the membrane becomes more perfect, the 
resistance goes up steadily and may amount, after soaking in water, 
to a million ohms. 
The cell membrane can be deprived of this property, of resistance 
to the passage of currents, by the action of heat, of anwsthetics and 
soon. After this treatment the cells conduct currents readily. 
We may conclude that, when examined under normal conditions 
and at rest, living cells are surrounded by a membrane impermeable 
to most salts, to strong acids and bases, and also, as shown by osmotic 
experiments, to glucose and to amino-acids. There are, however, 
certain crystalloids to which the cell appears to be permeable under 
all conditions. These are urea, ammonium hydroxide and some 
other ammonium salts, certain dyes of low molecular weight, 
alcohols, etc. But in some of these cases, it is not clear that no damage 
has been done to the membrane by the solution applied, a difficulty 
not always taken into consideration. 
But the physiologist will raise an objection. If the membrane is 
impermeable to glucose and sodium chloride how does the cell ever 
obtain these materials for its active processes? The answer is to be 
