THE CHEMISTRY AND PHYSICS OF THE CELL 49 



Precipitation and Coagulation of Colloids. Be- 

 cause of the rather slender margin by which the colloids are 

 separated from the suspensions, their persistence in solution is 

 generally in a rather precarious condition. Relatively slight 

 changes suffice to throw the colloids out of solution, and when 

 once precipitated, they are often incapable of again dissolving in 

 the same solvent. Solutions of albumin may undergo sponta- 

 neous coagulation on standing for some time, and agitation 

 rapidly produces the same effect in many proteid solutions. 

 Some inorganic colloids are as readily coagulated as the proteids. 

 A comparatively small rise in temperature, less than to 50 C. 

 with some proteids, renders the proteid perfectly insoluble. 

 Furthermore, we have coagulation of proteid solutions by enzyme 

 action. The inorganic " colloidal suspensions " may be precipi- 

 tated by the addition of very small quantities of electrolytes. 

 Colloidal solutions of the type of the proteids are not so read- 

 ily precipitated by most electrolytes, but if to the solution large 

 quantities of crystalloids are added, the proteid molecules are 

 practically crowded out of solution, as in the " salting-out " 

 process used in separating proteids by ammonium sulphate and 

 other salts. The effect of heat upon different colloids is pecu- 

 liar, in that some varieties, as silicic acid, aluminium hydrate, 

 and many proteids are rendered so insoluble that they cannot 

 again be dissolved in any fluid without first being modified in 

 some way ; whereas colloids of the type of gelatin and agar are 

 made more soluble by heat. The change of colloids into insolu- 

 ble forms, the " peetous " condition of Graham, requires the 

 presence of water, for the dry colloids may be heated to rela- 

 tively high temperatures without losing their solubility. On 

 the other hand, dehydration of colloids while in solution will 

 result in their precipitation and coagulation, as occurs in proteid 

 solutions when alcohol is added. 



Colloids are precipitated by many electrolytes, apparently 

 through the formation of true ion compounds, one or both of 

 the ions of the electrolytes uniting with the colloid ion ; although 

 some writers, as Spiro, believe that the combination is merely 

 an additive one between entire molecules. Mathews l has 

 advanced the theory that the solution tension of the salts is the 

 chief factor in determining the precipitation of colloids by elec- 

 trolytes. Colloidal particles have a high surface tension which 

 is always tending to reduce the volume of the particle, and in 

 colloidal solutions this is constantly opposed by the force of 

 solution tension. If the solution tension of the salt is of such 

 1 American Journal of Physiology, 1905 (14), 203. 



