13 PHYSICAL CHEMISTRY IN MEDICINE. 



two still other conditions, according to our judgment. 

 If the coagulation through surface tension is studied in 

 detail we find that we have to do with an extreme increase 

 in the concentration of a solution, or, what amounts to 

 the same thing, with the expression of the solvent from 

 such a solution. The energy necessary for this, which, 

 as is well known, expresses itself in osmotic pressure, is, 

 however, in the case of the colloidal substances, exceed- 

 ingly small in contrast to that observed in crystalloidal 

 substances. And so it is that the colloids are especially 

 well able to form solid surface films quickly and exten- 

 sively. 



A second important condition is the electrical charge 

 of the colloidal particles. The greater this is, the 

 more powerful are the repellent forces between the sim- 

 ilarly charged particles that antagonize the surface ten- 

 sion, which tends to crowd them together. The proteins, 

 as we know, carry only weak charges of electricity and 

 furnish in consequence favorable conditions for the col- 

 lection of the particles on free surfaces. To the formation 

 of such solid protein films RAMSDEN attributes the well- 

 known formation of a film on milk and the behavior of 

 the fat droplets of milk, which have long been imagined, 

 from their physical and chemical reactions, to be sur- 

 rounded by a denser film of protein. In pathology similar 

 phenomena might play an accessory part in air and fat 

 embolism. It can readily be shown experimentally that 

 the migration of air bubbles in tubes which are filled 

 with protein solutions meets with unexpectedly great 

 difficulties. In the living organism the danger of the 

 entrance of air bubbles or fat droplets into the circulation 

 is dependent, at least in part, upon similar processes, 



