284 Seif viz . — Observations on some Physical Properties of 
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membrane . The colloidal nature of the plasma-membrane, its structure 
and composition, how it is formed, and how it functions are all problems of 
great interest, but they are chiefly problems of colloidal and molecular ; 
physics, and therefore primarily of so theoretical a nature that few irrefutable 
statements can be made regarding them. However, the physical chemical ! 
evidence bearing on the existence of a plasma-membrane is of so fundamental, ; 
even though theoretical, a nature that it deserves very careful consideration. 
From familiar surface tension phenomena we know that the surface of 
a liquid is the seat of certain physical forces which give to the surface layer 
a structure differing from that of the interior. Furthermore, all chemical 
reactions are surface ones ; consequently, we can conclude that the region of 
contact between two immiscible systems is likely to be one of both physical 
and chemical activity. The chemical and physical forces active in the 
septum separating two fluid systems is often sufficient to convert the septum 
into a third system differing physically and chemically from either of the 
two systems which it separates. It would be very extraordinary indeed if 
so intricate a colloidal system as protoplasm should prove an exception to 
this and be quite inert at its surface. Quite the contrary condition is the 
likely one. The extremely complex nature of protoplasm suggests the ) 
occurrence of an unusual surface activity. We are led to expect this through 
consideration of those peculiar instances where substances, like colloidal ; 
(spongy) platinum, are active in the colloidal state, though they are inactive 
when not in this state. 
The theorem of Willard Gibbs tells us that any dissolved substance 
which lowers the surface tension of a solution is deposited at the surface, 
i. e. is positively adsorbed. Conversely, the concentration of any substance 
which raises the surface tension is less at the surface. Such selective 
adsorption results in the production of a region which, without any actual 
chemical reaction, differs in chemical make-up from either of the two 
original systems. But adsorption is always a preliminary to chemical 
action (although chemical union need not always follow adsorption. 
Whether adsorption is purely a physical union or a chemical one is simply 
a question of degree of firmness of the bond. A loose chemical union and 
a firm physical one are in this respect the same). Consequently, there occur 
in the region of separation chemical activities peculiar to the septum alone, 
which, in turn, produce substances foreign to either of the systems which the 
septum separates. Just how far this process goes in the formation of 
the plasma-membrane cannot be said. But, realizing the possibilities which 
so complex a colloidal system as protoplasm offers to physical and chemical 
surface activity, there is every justification for assuming, a priori, that the 
septum which separates protoplasm from its surrounding medium is itself 
a system differing in chemical composition as well as in physical structure 
from both of the systems which it separates. 
