34 COLLOIDS IN BIOLOGY AND MEDICINE 



the surface of aqueous solutions and form a layer, which at first is 

 easily movable. In a short time there is a change in this layer. In 

 the case of staining solutions the surface appears dull after an hour 

 and gradually there is formed a solid layer which histologists and 

 bacteriologists know to their sorrow. It is, therefore, necessary to 

 filter aqueous solutions of stains each time before using, even though 

 they have been protected from dust. The dye concentrated at 

 the surface undergoes chemical changes which the investigations 

 showed to be independent of the gas upon the surface (it might have 

 been attributed to oxidations with oxygen or to CO2, etc.). What 

 has been said of dye solutions occurs also in the case of peptone 

 solution, as METCALF demonstrated. 



The thickness of the layer which will just form a solid skin has 

 been measured, and found to be, for peptone 3 ju/x (METCALF), for albu- 

 min 3 to 7 /x/i (DEVAUX). Thus it is probably many times greater 

 than the hypothetical diameter of a molecule, perhaps even equaling 

 the radius of molecular attraction. The process of skin formation 

 may be very much hastened by amplifying the surface, i.e., by shak- 

 ing the fluid or passing gas through it. Thus W. RAMSDEN * was 

 able to remove by shaking practically all the albumin from an 

 albumin solution. The albumin passed into the foam and there 

 formed solid skins. This phenomenon is of greatest practical impor- 

 tance, since the solidity of meringue, whipped cream and beer foam 

 evidently depends upon it. In the case of beer, the rising bubbles of 

 C02 carry foam-forming colloids at their surfaces and conversely the 

 beer foam exerts a tension (pressure) which hinders the escape of 

 C0 2 and thus keeps the beer fresh for a longer time. Everyone who 

 has worked with colloidal solutions knows how high the gas pressure 

 must be in order that a stream of gas may be forced through a solu- 

 tion which has once formed a layer of foam. The formation of a 

 skin on boiling milk is evidently to be classed among these phe- 

 nomena. The significance of this process for the coagulation of fibrin 

 has been indicated on page 299. 



To these phenomena belongs "inactivation of ferments by shaking" 

 described by E. ABDERHALDEN and GUGGENHEIM * and independ- 

 ently by SIGNE and SIGVAL SCHMIDT-NIELSEN.* 



The formation of surface skins is so sharply characteristic, that 

 mixtures of substances, which diminish the surface tension of water 

 to different degrees, may be separated by shaking. Until now, we 

 have had only the qualitative investigations of W. RAMSDEN,* who 

 determined the predominance of saponin in the foam on shaking a 

 mixture of saponin and albumin. (Saponin lowers the surface 

 tension of water more than albumin.) 



