CHAPTER VI. 

 CANE SUGAR. 



PRELIMINARY DETERMINATIONS OF OSMOTIC PRESSURE. 



The numerous determinations of the osmotic pressure of cane sugar 

 which are to be presented in this report will be classified as preliminary 

 or final, according as they were made before or after the method of 

 measuring the force had been developed to a point where, in the author's 

 judgment, full credence was to be given to the results. 



The final arrangements for the measurement of osmotic pressure 

 which have been described in the earlier chapters, and the methods 

 of manipulation which will be discussed to some extent hereafter, were, 

 as a rule, the products of a slow growth. In a general way, the whole 

 history of the investigation may be divided into three periods, as follows : 

 First, a period of four years, in which the attention of the writer and 

 his co-workers was given almost exclusively to the task of perfecting 

 the porous wall of the cell; second, a period of nearly equal duration, 

 in which they were measuring osmotic pressure with a view to discover- 

 ing and eliminating the sources of error in the method ; third, the period 

 within which — owing to the absence of any large sources of error — the 

 results are regarded as reliable in a high degree. 



During the second or evolutionary period, eight series of quantitative 

 measurements of the osmotic pressure of cane sugar were made, and 

 three series of measurements on solutions of glucose. The present 

 chapter gives an account of the work upon cane sugar. 



The value of the results increases quite continuously from the first 

 to the last series in the proportion in which it was found practicable 

 to diminish or suppress sources of error. Two sources of error — ther- 

 mometer effects, and dilution of the cell contents during or after a meas- 

 urement of pressure — were found to exceed all others in importance and 

 to vitiate the results more than all other defects of the method. They 

 were also the most difficult to deal with. t In fact, the whole four years 

 may be said to have been devoted to their elimination. 



The "thermometer effects" have been sufficiently discussed in a 

 former chapter. They were due, of course, to the imperfections of the 

 earlier arrangements for the maintenance of temperature. 



The dilution of the cell contents (which, at first sight, would naturally 

 be ascribed to leakage of the membranes) was found to be due to two 

 causes: First, to an acquisition of solvent during the closing and the 

 opening of the cells; and second, to an enlargement of cell capacity 

 under pressure. Accordingly, during the whole of the period within 



ill 



