NATURE AND PROPERTIES OF COLLOIDS 545 



tions in water; rubber forms colloidal solutions in ether, but not in 

 water. The term colloidal solution does not, therefore, refer to a true 

 solution in the sense of a crystalloid, but to a colloidal state of suspen- 

 sion (the so-called colloidal solution). 



3. Colloids are non-diffusible, or lack the power of passing through 

 animal and parchment membranes. Not all colloids possess the same 

 rate of diffusion, this property being relative, rather than absolute; 

 however, solutions of salts (crystalloids) pass through so readily that 

 they are easily separated from proteins (colloids) by dialyzation, a 

 process that is in constant practical use. 



4. Colloids have an extremely small osomotic pressure. They may, 

 to a very slight degree, exert some influence upon osmotic pressure, the 

 freezing and boiling-points of fluids, but in all cellular processes in which 

 manifestations of osmotic pressure or diffusion are present the crystal- 

 loids may be considered as almost entirely responsible for these. 



5. The colloids exhibit surface tension to a high degree in other words, 

 colloid fluids possess the force that strives to reduce its free surface to a 

 minimum. As partial expressions of this force, the formation of emul- 

 sions when oil and water are mixed and the ameboid movements of the 

 ameba and leukocytes may be mentioned as examples. 



6. Colloids do not separate freely into ions when dissolved, and accord- 

 ingly do not conduct electricity to an appreciable extent. When an electric 

 current is passed through a colloidal fluid, most of the colloids move 

 toward the anode; this phenomenon, known as cataphoresis, is also 

 generally exhibited by suspensions, and in this particular the colloids 

 resemble suspensions. 



7. Colloids are usually easily precipitable and coagulable, and this is 

 readily understood when the slender margin that exists between many of 

 the colloids and the suspensions is borne in mind. Relatively slight 

 changes, such as exposure, gentle heat, the presence of large quantities 

 of crystalloids, the action of enzymes, etc., may throw an organic colloid 

 out of solution, and when once precipitated, it is often incapable of again 

 dissolving in the same solvent. Colloids are also precipitated by many 

 electrolytes, apparently through the formation of true ion compounds. 



8. The physical structure and size of colloids. This subject has been 

 studied extensively by Hardy. 1 Cells contain but one, type of colloids, 

 the proteins that form non-reversible coagula. So long as a colloid is in 



1 A good general outline of the subject of colloids may be found in Pauli's " Phys- 

 ical Chemistry in the Service of Medicine," 1907, translated by Fischer (Chapman 

 and Hall). 



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