vin MECHANICAL AGGREGATION 275 



surface, the proteid- molecules are subjected along with the water- 

 molecules to the effect of surface tension, and may be supposed to form 

 collectively an elastic proteid membrane. The latter appears clear, 

 because the component molecules are all similarly arranged, forming a 

 homogeneous layer. By shaking, the surface particles are, suddenly, 

 partly released from tension and partly subjected to still greater stress, 

 and being unable to resume at once the original shape, owing to their 

 viscosity, masses of distorted and compressed molecules will be pro- 

 duced. On continuing the process of shaking, the already -formed 

 aggregates act as nuclei to which other surface molecules attach them 

 selves, and thus strands and fibres are formed of sufficient size to 

 become visible to the naked eye." 



In further support of this view it was pointed out by the author l 

 that "the tendency of weak alcohols to prevent the formation of 

 mechanical coagula is very interesting, and is probably due to a double 

 action, namely, partial dehydration of the proteid molecules, and also 

 to an alteration in the surface tension of the fluid." 



More recently Ramsden, 2 by ingenious experimentation, has suc- 

 ceeded in showing that true membranes are actually formed, and that 

 an essential condition for the formation of these aggregates is the pres- 

 ence of a free (i.e. gas or air) surface, and that aggregates similar to 

 those formed by egg-white may be obtained with all proteid solutions, 

 including even the simplest peptones, and with a very large number 

 of other colloidal substances (soap, quinine, ferric hydrate, saponin, 

 aniline dyes, etc.). 



At any appropriate surface particles of the previously dissolved 

 colloid pass spontaneously out of solution and form a delicate surface 

 pellicle which exhibits many of the characteristic properties of solid 

 matter. This rearrangement of the system : 



Water ^ dissolved colloid ^ gas, 



is due to the fact that the total energy of surface tension is 

 thereby diminished. If such a surface with its coating of solid 

 particles be forced to diminish rapidly in area, these particles are 

 heaped up and become visible as 'mechanical surface aggregates,' 

 which in some cases (serum-albumin, quinine, peptone, aniline dyes, 

 etc.) are completely resoluble in the mother liquid ; in other cases 

 (egg-albumin, ferric hydrate, acid- and alkali-albumin) portions of the 

 aggregates are found to have been rendered permanently insoluble, 

 i.e. to have been ' coagulated.' In a paper not yet published, Ramsden 



1 Physiological Histology, 1902, p. 104. 

 2 W. Ramsden, Proc. Roy. Soc. 72. 156 (1903). 



