PASSAGE INTO SOLUTION 291 



tion in the effect of glycerol upon the rate of solution of casein 

 in dilute alkali. In glycerol we have a substance which, like 

 alcohol, reduces the tension of a solid-water interface but which, 

 unlike alcohol, does not coagulate (polymerize) proteins in solu- 

 tion. Accordingly we find that glycerol decreases the rate of 

 solution of casein progressively as its concentration increases. 

 No sign of reprecipitation of dissolved casein is observed in any 

 of the mixtures. The penetration formula x = Kt"" applies to 

 the rate of solution in all of the mixtures employed and the value 

 of the coefficient of penetration progressively decreases in a 

 smooth curve with decreasing acceleration as the concentration 

 of glycerol increases. 



These results illustrate the part which may be played by capil- 

 lary phenomena in heterogeneous systems which contain proteins, 

 the importance of which has, of recent years, been especially 

 insisted upon by, among others, Wo. Ostwald and H. Freund- 

 lich (67). 



It is of interest to estimate, by exterpolation from the pene- 

 tration formula, the amount of time which would be required to 

 "saturate" an alkaline solution with casein by stirring up excess 

 of undissolved casein in it. We have seen that the relation be- 

 tween the percentage ( = x) of casein dissolved in a given solution 

 of alkali and the time ( = t) of stirring is expressed by the equation 

 X = Kt"". For a mixture of 5 grams of solid casein with 100 cc. 

 of 0.00870 N KOH the values of K and m at 18-20 degrees are, 

 respectively, 1.48 and 0.146. The quantity of casein required 

 to "saturate" 100 cc. of this solution would be 7.63 grams. 

 Calculating the value of t corresponding to this value of x we 

 find that it would take no less than thirty-one years to fully 

 "saturate" the solution with casein, at the rate of stirring em- 

 ployed.* This very clearly indicates the great importance which 

 the time factor may assume in heterogeneous systems which 

 contain proteins. 



* For more dilute solutions of alkali mixed with the same number (5) of 

 grams of casein per 100 cc. the time required for "saturation" would appear 

 to be shorter, thus for 0.00435 N KOH, calculating as above, it is only about 

 17 days. This is because not only the concentration of alkali but also the 

 mass of casein, as we have seen, plays a part in determining the rate of solu- 

 tion. The ratio of casein to alkali was of course greater in the more dilute 

 alkaline solutions in inverse proportion to the concentration of the alkaU. 



