CHAPTER VIII. 

 CANE SUGAR. 



FINAL DETERMINATIONS OF OSMOTIC PRESSURE. 



The determinations of osmotic pressure which were made after the 

 method had been perfected as described in Chapters VI and VII are 

 designated as "final," because they are believed to be in a high degree reli- 

 able. It is characteristic of them all that there was neither any material 

 variation in bath temperature during any experiment, nor any dilution of 

 the cell contents which could be detected by the polariscope. It is not 

 meant thereby that we were able to maintain absolutely constant tem- 

 peratures in the large baths which were employed. There were frequent 

 fluctuations which amounted sometimes to 0.02°, but usually to not more 

 than 0.01°. If the variation in bath temperature did not exceed 0.02° 

 during an experiment, it was considered to have remained sufficiently 

 constant. Occasionally accidents happened to the regulating devices, 

 and the baths were temporarily thrown "off temperature" in consequence. 

 If the difficulty was soon discovered and quickly remedied, the resulting 

 fluctuation in bath temperature was small, and the experiment was saved 

 by discarding all readings of pressure until the cell contents had had 

 ample time to recover from any thermometer effects due to the accident. 

 If the trouble occurred during the night and was not, therefore, dis- 

 covered until the temperature of the bath had risen or fallen a consider- 

 able fraction of a degree, the determination was usually discarded. The 

 most frequent cause of difficulty with the regulating devices was a tem- 

 porary interruption of the main current at its source, i. e., at the power 

 house. 



The sugar which was employed for the "preliminary" determinations 

 described in Chapters VI and VII was "rock candy," which was not puri- 

 fied by recrystallization. This material is known, however, to contain, as 

 a rule, some mother liquor and to be otherwise impure, notwithstanding 

 its fine appearance. Moreover, it had been observed that the material 

 obtained from rock candy by reprecipitation gave somewhat higher pres- 

 sures than had been obtained with the unpurified sugar. It was decided, 

 therefore, to subject the sugar which was to be used for the "final" meas- 

 urements to a thorough-going purification. The method employed was 

 essentially that of Cohen and Commelin.* 150 pounds — approximately 

 70 kilograms — of the best rock candy were procured and subjected to the 

 treatment described below. Kilogram quantities of it were dissolved, 

 each in 500 c.c. of previously boiled distilled water which, when making 



* Zeitschrif t fur phvsikalische Chemie, lxvi, 1. 



159 



