144 PROTOPLASM 



trated gel, the diffusing particles will have more difficulty in 

 getting through; but if the gel is open in structure, then there 

 will be little hindrance to the diffusing substance. 

 . Ruhland has characterized the aqueous solutions of a large 

 number of dyes by determining their rate of diffusion in a con- 

 centrated gel (20 per cent gelatin). One can thus ascertain with 

 moderate accuracy whether the diffusing substance is molecu- 

 larly or colloidally dissolved, i.e., whether the particles are mole- 

 cules or micelles. 



Osmosis. — Osmotic pressure (Chap. XI) is a measure of the 

 pressure that a solution would exert were it confined in a sack 

 under certain ideal conditions. It was named by Graham as 

 one of the forces not possessed by colloidal solutions. When a 

 sugar solution is enclosed in a sack of parchment paper immersed 

 in water, a hydrostatic pressure is set up within the sack owing 

 to the entrance of water. The entering water is in excess of the 

 outgoing water, because of the inability of the sugar to diffuse 

 out. The greater the number of sugar molecules the greater 

 is the difference between the incoming and outgoing water, and 

 therefore the greater the hydrostatic pressure developed within. 

 Protein molecules and colloidal particles are large. They occupy 

 space, and there are fewer of them per gram of substance; 

 consequently, in equimolecular concentrations of crystalloids 

 and colloids, there are fewer of the latter per unit volume in the 

 osmotic sack, which means that the difference between incoming 

 and outgoing water, and therefore the hydrostatic pressure, will 

 be less. Colloidal substances do, however, possess a definite 

 though slight osmotic pressure. Pfeffer measured it for gum 

 and glue solutions. A colloidal substance with a molecular 

 weight of 20,000 will give an osmotic pressure of 7 mm. of mer- 

 cury — a low value compared to a crystalloid, such as cane sugar, 

 with a molecular weight of 342, which will yield a pressure of 

 25.69 atmospheres, or 1,850 mm. of mercury, at a concentration 

 of 0.1 M. The amount of osmotic pressure exerted by colloidal 

 solutions is so slight that it is customary to regard its absence 

 as characteristic of the colloidal state. 



Size of Particles in Hydrophilic Sols. — The slow diffusion 

 rate and the almost negligible osmotic pressure exerted by lyo- 

 philic colloidal systems are due to the large size of the dispersed 

 particles. High molecular weight suggests that the "particles" 



