48 COLLOIDS IN BIOLOGY AND MEDICINE 



pressure. When we consider that the finest gold particles of a col- 

 loidal gold solution have a diameter of only 2^ (a dimension one- 

 fifth that of ultramicroscopic visibility); according to SVEDBERG in 

 order to make a normal colloidal gold solution we should have to cram 

 50 kilos of gold into a liter, which would have to contain 6.1 X 10 23 

 such gold particles. It is naturally impossible to do this; the most 

 that can be dissolved, experimentally, is one gm. per liter. Under 

 favorable conditions such solutions have an osmotic pressure of 

 4.5 X 10~ 4 atmosphere, i.e., they would rise 4.65 mm. in an osmom- 

 eter, or be in equilibrium with electrolytes which modify the electro- 

 lytic or hydrolytic dissociation of colloid electrolytes. 



It is unnecessary to catalogue all the dozens of fruitless investi- 

 gations of the molecular weight of colloids by physical methods. 

 They either gave surprisingly low molecular weights, when it could 

 be shown on testing that the colloid was contaminated with crystal- 

 loids, or the values were so small (the molecular weight so large) 

 that they fell within the limits of error of observation, which means 

 that it became doubtful whether the colloid studied had any osmotic 

 pressure at all. The molecular weight is the expression of a chemical 

 point of view which cannot be determined by physical methods for 

 colloids. What we obtain by these methods are more or less nu- 

 merous groups of molecules, usually in adsorption balance with irre- 

 movable traces of crystalloids which cannot be separated, or which 

 are in equilibrium with electrolytes that influence the electrolytic 

 and hydrolytic dissociation of the colloid electrolytes. 



In the present state of the science we may only strive to determine 

 the size of the particles in solutions of colloids having various equilibria. 



1 [According to Zsigmondy particles 5^A in size may be seen with the aid of 

 strong sunlight. Tr.] 



