~ 
ON COLLOID CHEMISTRY AND ITS INDUSTRIAL APPLICATIONS. 125 
thetic, it freely mixes with the surrounding watery solution. In the 
normal state, it must be surrounded by a film or membrane of some 
kind, which prevents escape of the cell contents. We may draw a 
similar conclusion from the fact that the products arising from 
digestion of the food particles in protozoa, although consisting of 
such ireely diffusible substances as glucose and amino-acids, are not 
washed out. Much discussion has arisen as to the nature and even 
the existence of such a membrane. In the present Report, space will 
not permit of discussion of all the work which has been devoted to 
the problem. Attention must be directed to those aspects of it which 
appear to the writer to be of essential importance and to the conclu- 
sions to be drawn therefrom. These conclusions will sometimes 
differ from those of the investigators themselves and the reader may 
not always be inclined to agree with the interpretations given in this 
place. 
It has been pointed out (Bayliss, “General Physiology” p. 128) 
that, according to the Principle of Willard Gibbs, which is a con- 
sequence of the tendency of free energy to decrease, as stated by the 
second law of energetics, if there are in the cell contents any 
substances which lower surface tension, these must be concentrated 
at the interface between the protoplasm and the surrounding liquid, 
Moreover, Ramsden (1904) has shown that, in many cases, as solutions 
of bile salts, quinine or saponin, this concentration at the surface 
may be so great as to exceed the solubility of the substance in 
question, which is then deposited in a solid form, producing a film 
of more or less rigidity. In addition to this, Chambers (1917) finds 
that the extreme outer layer of the protoplasm has the properties of a 
gel. The change from sol to gel has been discussed ‘in the preceding 
section. In the present case, it is doubtless due to the molecular 
forces at the surface. It should be noted that this outer layer of the 
protoplasm is not identical with the membrane to which the cell 
owes its semi-permeable properties. When a dye is unable to enter 
a cell, it does not stain this “hyaloplasm”’. stratum, so that the 
membrane which stops it has a more external situation. We see that 
there is every reason to expect the presence of a film on the surface 
of cells and that it would be reversible, existing only on the boundary 
surface and formed anew whenever any fresh surface is produced. 
_ These considerations also give us a clue as to the chemical composition 
to be looked for. In addition to the components of the hydrogel, 
whatever they may be, but almost certainly protein of some kind, we 
shall expect to find more especially those constituents of the cell 
which lower the surface tension of water to a marked degree; in 
other words, all compounds of a fatty nature, and of these lecithin 
is known to be universally present. 
If we add certain dyes, such as aniline-blue or congo-red, to the 
water in contact with the protoplasm of a cell, we notice that it does 
not enter; the cell remains unstained. That this is not due to an 
obstruction anywhere but on the surface is shown by the fact 
observed by Kite (1913) that, if introduced by a capillary tube into 
the interior of a cell, congo-red freely and rapidly diffuses throughout 
the contents. Now it was observed by Nageli (1855), and confirmed 
by Pfeffer (1890) and others, that if a mass of protoplasm be broken 
