346 PHYSICAL PROPERTIES 



This phenomenon has received extended consideration at the hands 

 of Hermann (44), Berthold (11), Ramsden (87) (88), and Shorter 

 (115), who have shown that protein solutions can be wholly 

 or partially coagulated by mere mechanical agitation, or by the 

 purely surface-action of fine powders, such as burnt clay or char- 

 coal. Ramsden also explains the formation of a film at the surface 

 of heated milk and other protein solutions in the same way.* 



The formation of these protein films by surface-action can be 

 very conveniently illustrated by the following experiments (100) 

 (103). If chloroform be shaken up with casein, gelatin or pro- 

 tamin solutions, it settles in fine particles or droplets, which, if 

 numerous, form a milky layer at the bottom of the vessel; by 

 transmitted light, however, they appear perfectly clear. These 

 droplets are extraordinarily stable, and do not coalesce, however 

 long they stand in contact; they may be repeatedly washed in 

 water until all traces of protein have been removed from the super- 

 natant fluid, and they still remain perfectly stable and distinct 

 from one another; they may be shaken up in chloroform itself or 

 treated with iV/10 potassium hj^droxide without impairing their 

 form or stability. If, however, they be heated to nearly the 



* The non-filterability of many proteins, that is, their inability to pass 

 through the pores of a clay filter has, by some authors, also been attributed to 

 surface-action. This factor alone, however, is insufficient to explain the total 

 non-filterability of these proteins through certain filters. True, when solutions 

 of substances which reduce the surface-tension at the surfaces of insoluble 

 powders are filtered through such powders, the first portion of the filtrate 

 which filters through contains less of the dissolved substance than the original 

 solution (Cf. J. J. Thomson (124)); for example, when potassium perman- 

 ganate solution is filtered through fine quartz sand the first portions of the 

 filtrate may be colorless. But subsequent portions of the filtrate contain 

 a greater proportion of the solute and, finally, the unaltered solution filters 

 through, owing to the fact that the surfaces of the pores of the filter are 

 now fully coated by a film of more than molecular thickness. If this were 

 the only factor concerned in holding back dissolved proteins from passing 

 through clay filters, the protein would sooner or later pass through the filter, 

 which, in many cases it does not (Cf. F. W. Zahn (133), J. Lehmann (57)). 

 True, it is possible that the first portions of the protein which are de- 

 posited upon the walls of the pores, partially clog up the pores and narrow 

 them, thus assisting the filter to hold the protein back; but even if this be the 

 case, the ultimate factor which determines the impermeability of such filters 

 for proteins must either be the grossness of the protein particles themselves, 

 or of their associated water-complexes, or else the existence within the protein 

 solution of a structure connecting together tke protein particles. 



