Permeability 
9 
Osmotic Pressure of Colloids 
Whether colloids exert an osmotic pressure or whether they do 
not is a question which has been the subject of some discussion. 
Now as colloidal properties are due either to the large size of the 
molecule, or to the aggregation of a number of molecules, we should 
expect colloids to exhibit the same behaviour in regard to osmotic 
pressure as other substances. The influence of association of molecules 
in the case of hydration has already been touched upon. 
It was Starling (1896, 1899) who first showed experimentally that 
a colloid exerted an osmotic pressure. Blood serum was filtered 
through gelatine under pressure, and in this way the crystalloids 
were separated from the colloids, which alone were held back. The 
filtrate was then separated from unfiltered serum by a gelatine mem¬ 
brane, and the hydrostatic pressure developed was measured; any 
osmotic pressure produced is then attributable to the colloids. In 
* this way it was shown that the colloids of blood serum exerted an 
osmotic pressure of 3 to 4 cm. of mercury. Further determinations 
of the osmotic pressure of colloids have since been made by other 
observers, notably by Hiifner and Gansser (1907) who found the 
osmotic pressure of haemoglobin corresponds to the molecular weight 
calculated from chemical data (cf. also Reid, 1905), and it may now 
be assumed with confidence that colloids do actually exert a definite 
osmotic pressure. Owing to the large size of the molecule or molecular 
association this osmotic pressure will naturally be small as compared 
with that produced by equal weight of a crystalloid. 
Theories of Osmotic Pressure 
Explanations that have been put forward of the origin of 
osmotic pressure fall into two groups. In the first group osmotic 
pressure is supposed to arise in a similar way to gas pressure by the 
bombardment of the molecules (or ions) of the solute upon the sepa¬ 
rating membrane. 
This theory may be called the kinetic theory of osmotic pressure. 
It was first put forward as a possible explanation by van’t Hoff and 
is suggested at once by the fact that the osmotic pressure of dilute 
solutions is equal to the pressure the solute would exert as a gas 
occupying a volume equal to that of the solution. The contrasted 
hydrostatic theory, due in the first place to Jager (1891 a, b ) finds an 
origin for osmotic pressure in the different surface tensions of solution 
and pure solvent. This view, as elaborated by Moore (1894) regards 
