CANE SUGAR. 163 



chapter — except one which is introduced to illustrate concentration in the 

 cell — the two portions of the solutions were found to have identical rota- 

 tions. In other words, all experiments in which the solutions were found 

 to have suffered a change in concentration have been discarded. When- 

 ever a gain or loss in concentration has occurred in the course of the work, 

 it has usually been due to a faulty adjustment of the initial pressure, i. e., 

 the interval between it and the final pressure has been left too large. The 

 osmotic pressures of solutions whose concentration has changed in the 

 cells are readily correctible, if one could only -prove that the cells have not 

 leaked. But the one certain proof that no solute has escaped through 

 the membrane is the fact that the solution taken from the cell at the close 

 of an experiment has the same concentration as the one which was put 

 into it in the beginning. All other demonstrations of the integrity of the 

 membrane have one or more weak points. 



It will be seen that the possibility of a sensible dilution or concentration 

 of the solution in the cell depends on the relation of the nitrogen volumes 

 at initial and at equilibrium pressures. If the difference between these is 

 very small as compared with the volume of the solution, there can be 

 no material change in concentration. It follows that, so far as actual 

 pressures are concerned, the preliminary adjustments of pressure must be 

 much closer in the case of dilute than in that of concentrated solutions; 

 moreover, that the difference between initial and final pressures must be 

 made smaller when manometers of large capacity are used, than when 

 those with only moderate gas volumes are employed. Since the cells all 

 have a capacity of about 20 c.c, it is only necessary, when adjusting the 

 initial pressure, to consider whether the subsequent contraction or expan- 

 sion of the nitrogen will constitute an appreciable fraction of that volume. 



The work included in the present chapter required three years for its 

 completion. The number of measurements reported is 270. The average 

 rate of progress was, therefore, 90 determinations per year, or 10 for each 

 working month. It is to be remembered in this connection, however, that 

 the labor required for the mere measurement of osmotic pressure is insig- 

 nificant when compared with that which must be bestowed upon the cells, 

 the membranes, and the manometers during the intervals between meas- 

 urements. If the measurements reported in the present chapter were 

 arranged in a strictly chronological order, it would be observed that a cell, 

 once used, reappears only after a long interval. 



It was intended, in the beginning, to carry the measurement of the 

 osmotic pressure of cane sugar from 0° to 100°, or as near to the latter 

 temperature as possible. The temperature-intervals selected were 5° 

 between 0° and 30°, and 10° between 30° and 100°. The work progressed 

 steadily until the temperature of 80° was reached, when it was necessary 

 to discontinue the measurements for the three summer months. The 

 cells were allowed to cool down to the temperature of the air and were 

 then placed in thymol water to soak through the summer. No serious^A/^pF*^ 



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