NATURE AND PROPERTIES OF COLLOIDS 547 



the neutralization of their electric charge, and, as previously stated, this 

 can be accomplished either by electrolytes or by colloids: 



(a) Precipitation by electrolytes is best illustrated by the action 

 of a strong acid on an albuminous solution. The negatively charged 

 particles attract to themselves the positively charged hydrogen ions; 

 their charge is now neutralized, and the force of attraction due to their 

 surface tension is no longer counterbalanced by an electric repulsion. 

 The particles are drawn together, form larger and larger masses, which 

 finally come under the influence of gravity and precipitation takes 

 place. 



(6) Precipitation of colloids by colloids is illustrated by the precipi- 

 tation of albumin by acetic and ferrocyanic acids. The colloid must be 

 of opposite sign. As a result of the acid the particles acquire a positive 

 charge, if they are not so charged already. This charge is then neutral- 

 ized by the colloidal ferrocyanic acid of negative sign; the surface ten- 

 sion is no longer neutralized by an electric repulsion, and particles come 

 together to form larger masses that are finally deposited as a precipitate. 



Instead of precipitating the other, an excess of one colloid may act 

 in a reverse manner. For example, as Neisser and Friedmann have 

 shown, a suspension of particles of mastic in water (made by dropping 

 an alcoholic solution in water) takes on a negative charge, and can be 

 precipitated by positive colloids or ions, such as ferric chlorid. If the 

 dose of ferric chlorid is increased gradually, the precipitate becomes 

 more and more abundant, until an excess of ferric chlorid is present, 

 when the reaction ceases and the precipitate may be redissolved. This 

 has been explained on the assumption that when two colloids of opposite 

 sign are mixed, they tend to fuse and form masses; the addition of an 

 excess of either colloid tends to electrify the masses, causing mutual 

 repulsion and possibly resolution of the masses. Hence the precipitate is 

 soluble in an excess of both substances, just as a precipitate is soluble 

 in an excess either of precipitin or of its antigen. 



10. Absorption is the taking up of dissolved or volatile substances 

 by finely divided or colloidal bodies. It is a combination between two 

 substances dependent on physical attraction rather than on chemical 

 affinity, and taking place in variable ratios, rather than in simple and 

 constant ones, as occur in a true chemical union. It is believed by many 

 that the two substances entering into the phenomenon of absorption 

 exist as such side by side in the compound, which is to be regarded 

 as an intimate admixture of the two, rather than as a new compound. 



The lack of definite ratios by which colloids are absorbed has been 



