COLLOIDS. 33 



the crystalloids will diffuse out of it, leaving the colloids, which 

 consist mainly of proteins. The physical reason for this failure 

 to diffuse is the large size of the molecules, in comparison with 

 the small size of those of the crystalloids. By causing a beam of 

 light to pass through a colloidal solution and holding a micro- 

 scope at right angles to this beam, the colloidal particles become 

 evident, just as particles of dust become evident in the air of a 

 room in a beam of daylight. In confirmation of this view of the 

 cause of the indiffusibility of colloids is the fact that filters can 

 be made of unglazed porcelain impregnated with gelatin, in 

 which the pores are therefore very minute, through which col- 

 loids cannot pass, though water and inorganic salts do so. When 

 blood serum is filtered through such a filter, the filtrate contains 

 no trace of protein. The colloidal molecules can also very readily 

 be caused to fuse together, thus forming aggregates of molecules 

 which become so large that they either confer an opacity on the 

 solution or actually form a precipitate. 



Tli is fusing together of colloidal particles can be brought about 

 either by adding certain neutral salts or by mixing with certain 

 other colloids. The explanation of these results is as follows: 

 colloidal molecules carry either a positive or a negative electrical 

 charge, and when this is neutralized, the colloidal molecules fuse 

 together, i. e., become aggregated. This neutralization of elec- 

 trical charge can be brought about either by adding an electro- 

 lyte, one of whose ions will supply the proper electrical charge, 

 or by a colloid having an opposite charge. Thus the S0 4 anion 

 of Na 2 S0 4 , in virtue of charges of negative electricity which it 

 carries, will very readily precipitate such a colloid as colloidal 

 iron (ferrum dialysatum, U. S. P.), which is charged with posi- 

 tive electricity ; or again, this colloid itself will readily precipitate 

 arsenious sulphide, another colloid carrying a negative charge. 

 The physiological importance of these reactions lies in the fact 

 that they probably explain many of the peculiarities of behavior 

 of mixtures of different animal fluids, such as toxins and anti- 

 toxins (see p. 149). 



A property of colloids which is closely related to the above is 

 that of adsorption. This means the tendency for dissolved sub- 



