278 PHYSICAL PROPERTIES 



equivalents of base in the water employed to dissolve the casein. 

 The experimental fact is, however, that neither of these equations 

 expresses the relationship between the time for which a mixture 

 of solid casein and alkaline water is stirred and the mass of 

 casein dissolved (86). 



In the experiments about to be described the casein which 

 was employed was especially prepared so as to readily sink in 

 water (Cf. Chap. II, 2), that is, it was not absolutely anhydrous 

 but contained a trace of water insufficient to disturb the accu- 

 racy of the results, but very material to the success of the ex- 

 periments. Since the presence and amount of associated traces 

 of water play such an important part in determining the readi- 

 ness with which casein is wetted by a solvent, it proved to be 

 necessary, in obtaining comparable experimental results, always 

 to employ the same preparation of casein, since no two samples 

 of incompletely anhydrous casein can be relied upon to contain 

 exactly the same proportion of water. 



A measured amount (usually 100 cc.) of the fluid employed 

 as solvent was placed in a beaker of squat form and 400 cc. 

 capacity and was agitated by a flattened glass rod which was 

 bent at right angles, the plane of the horizontal arm being some- 

 what inclined to the vertical, so as to communicate an upward 

 thrust to the rotating liquid. The horizontal arm of the stirrer 

 was about 2| cm. long and as near as possible to the bottom of 

 the beaker; this was rotated at an approximately constant rate 

 of about 1600 revolutions per minute by a small motor. While 

 stirring, a weighed amount of the casein was dropped into the 

 fluid. At stated intervals samples of the mixture were almost 

 instantaneously abstracted by means of a 10 cc. pipette which 

 was provided with a rubber bulb. The samples were then very 

 rapidly filtered through lightly packed glass wool. The time 

 occupied in filtration was never more than 30 seconds for the 

 5 and 10 minute samples, or more than 1 minute for the later 

 samples. The refractive index of the filtrate from each sample 

 was then determined. Denoting the refractive index of any 

 given sample by n and that of the pure solvent by wi, the quotient 



J'' ^^ is the number of grams of casein dissolved in 100 cc. 

 0.00152 



of the solvent at the moment when the sample was extracted 

 (Cf. Chap. XIV). 



