COLLOIDS 59 



to those already described (page 4) can be employed. Most of the 

 recent work has been done either with collodion sacs, or with unglazed 

 clay cups impregnated with some gel, such as silica or gelatin. When 

 such an osmometer, filled with some colloidal solution (like a solution of 

 pure albumin) and provided with a vertical glass tube, is placed in an 

 outer vessel containing water, the fluid will be seen to rise in the ver- 

 tical tube, the height to which it rises being proportional to the osmotic 

 pressure. 



But the observed pressure does not necessarily give us the osmotic pressure of 

 the pure colloid, for to this, even when highly purified, there is almost certain to 

 be attached a considerable amount of inorganic salt, which may be responsible for 

 the osmosis. It has indeed been maintained by some observers that electrolytes form 

 an integral part of certain colloids, being bound to them perhaps by adsorption (sec 

 page 66), and that they are essential to the maintenance of the colloidal state. In 

 any case, since electrolytes are always present, the osmotic pressure of the pure 

 colloid can be measured only when means are taken to discount their influence. Sev- 

 eral devices have been used, of which the following may be mentioned: 



1. Addition to the fluid outside the osmometer of a percentage of salt equal to that 

 found by chemical analysis to be present in the colloid. (This method is untrust- 

 worthy.) 



2. The use of a limited quantity of fluid on the outside of the osmometer so that 

 equality of saline content soon becomes established, by diffusion, in the fluids on the 

 two sides of the membrane. 



3. The use of a membrane which is permeable to electrolytes but not to colloids. 

 Even when the greatest care is taken in its measurement, the osmotic pressure of 



a given colloid has been found to vary considerably not only according to the method 

 used in its preparation, but also according to the amount of mechanical agitation 

 (shaking, stirring, etc.) to which the colloid solution has been subjected. Regarding 

 the influence of the method of preparation, it was found in one series of experiments 

 that albumin that had been repeatedly washed (but still contained considerable ash) 

 gave no osmotic pressure, whereas another preparation that had been purified by crystal- 

 lization twice (and contained much less ash) had a pressure of 3.38 mm. Hg. Ac- 

 cording to these results the ash content of the colloid is not fundamentally responsible 

 for its osmotic pressure. As to the influence of mechanical agitation, the osmotic pres- 

 sure of a gelatin solution is increased by shaking, while that of a solution of egg albu- 

 min is decreased. 



The property upon which the osmotic pressure depends is undoubtedly the state of 

 dispersion of the colloid particles, and until we know all of the factors which may in- 

 fluence this, measurements of osmotic pressures of colloids can scarcely be of very 

 much value. Nevertheless, that this property has some physiologic bearing is clear 

 from the effect which colloids have in restoring the blood pressure after hemorrhage 

 (page 141). 



Further evidence that the osmotic pressure of colloids has not the significance that 

 it has in the case of molecular solutions is furnished by the fact that the osmotic pres- 

 sure is only approximately proportional to the concentration of the solution ; it may 

 either increase or decrease relatively to the strength of the solution. Temperature also 

 has quite a different influence on the osmotic pressure of colloids from that which it 

 has on the osmotic pressure of molecular solutions, and it frequently has an influence 

 which persists after the solution is brought back to its original level. 



