102 
Walter Stiles 
So far only pure substances, or solutions of one substance, have 
been dealt with. In the organism on the contrary we always have 
to deal with mixtures. 
In general if there are two substances which can be adsorbed they 
displace one another to some extent. As the concentration of one 
of the substances increases, more of it is adsorbed and displaces the 
other to a greater extent, but relative to the concentration the 
amount of displacement decreases with increasing concentration. An 
exception to this is found in those viscous substances which tend 
to form rigid membranes at the surface. The presence of a viscous 
solute has no influence on the adsorption of a solute of low 
viscosity. 
Another peculiarity of such substances is that the extent of ad¬ 
sorption depends not on their concentration, but on the absolute 
amount present. It is not clear how far the peculiar properties of 
such substances are due to high viscosity, low diffusivity or colloidal 
nature. Sugar as is well known considerably increases the viscosity 
of water, but the adsorption of sugar follows the ordinary laws and 
is reversible. 
So far we have considered the consequences of the tendency of 
surface tension to reduce itself to a minimum. A few words are no\v 
necessary regarding the tendency of the area of the surface to 
diminish as far as possible. This is illustrated by the fact that liquids 
free from external forces always take up a spherical form, while if 
a liquid is divided into droplets scattered through another liquid of 
the same density, and with which the first liquid is immiscible, the 
droplets tend to unite into larger ones and finally into one single 
sphere. 
In the case of living matter of a gel nature, however, we have a 
substance which possesses a certain amount of rigidity. Any change 
in form of droplets of such a substance will therefore be opposed 
by the resistance offered by the rigidity of the substance to change 
of shape, and a condition of equilibrium will be reached when these 
two tendencies are equal. 
The tendency of surface tension in gels is therefore to bring about 
the coalescence of separated droplets. This union is called agglutina¬ 
tion when microscopic particles coalesce into particles visible with 
the naked eye, and coagulation when the particles that unite are 
ultra-microscopic or very finely microscopic. Agglutination and 
coagulation may therefore be produced either by an increase in 
surface tension or by a decrease in rigidity of the dispersion medium. 
