CHANGES WROUGHT IN PATHOLOGY. 129 



at the surfaces of all liquids is familiar to every one. 

 While the particles within the liquid are surrounded on 

 all sides by particles of the same kind and are in con- 

 sequence in a state of equilibrium through the uniform 

 distribution of the attractive forces about them, this is 

 not the case with the surface particles. For here the 

 forces which act upon the particles and are directed 

 toward the centre of the liquid encounter no correspond- 

 ing antagonistic force. In consequence of the effort of 

 the surface particles to follow the attraction toward the 

 centre, the surface endeavors to become as small as 

 conditions will permit. Many substances are able 

 through solution in a solvent to bring about a decrease 

 in its surface tension. Under these circumstances the 

 liquid may follow its tendency to decrease its surface as 

 much as possible by allowing the particles of the sub- 

 stance which decrease the surface tension to take the 

 place of the particles of the liquid which are being pulled 

 toward the centre. In this way the surface becomes 

 gradually richer in the dissolved substance. In the end 

 the concentration of the solid particles at the surface 

 becomes so great that a film is formed which may be 

 removed and which is renewed after each removal. This 

 behavior, which has been studied in great detail by 

 RAMSDEN, is shown chiefly by the colloids, more especially 

 protein solutions. If such solutions are shaken together 

 with air or immiscible liquids, such as oil, chloroform, or 

 mercury, all the dissolved colloid can finally be precipi- 

 tated through the surfaces which are constantly re- 

 newed. 



That this process should be demonstrable most clearly 

 on colloids and especially on protein solutions is due to 



