CANE SUGAR. 161 



in all the specimens by other means. Much time was spent in attempts to 

 establish the limits within which these might be present. Finally, how- 

 ever, the whole question of the purity of the materials was referred to the 

 Bureau of Standards at Washington. The report which was received 

 from the Bureau is given below. 



Sample A. — Reducing substances in terms of invert sugar, 0.08 



per cent ± 0.005 per cent. 

 Sample B. — Reducing substances in terms of invert sugar, 0.01 



per cent ± 0.005 per cent. 

 Sample C. — Polarization, 99.93°. Reducing substances in terms 



of invert sugar, 0.01 per cent =*= 0.005 per cent. 

 Sample D. — Polarization, 99.95°. Reducing substances in terms 



of invert sugar, 0.005 per cent ± 0.005 per cent. 



The material employed for the "final" determinations of osmotic pres- 

 sure was that designated by the letter C, in which the Bureau of Standards 

 had found 0.01 per cent of reducing sugar. The sample D which had been 

 three times recrystallized was doubtless somewhat purer, but it was feared 

 that the quantity of D in hand would not suffice for all the determinations 

 which were to be made, and uniformity of material was of quite as much 

 importance as absolute purity. 



The baths which were devised for the regulation of temperature have 

 been sufficiently described in Chapter III, and it will only be necessary to 

 explain in the present chapter certain points as to their use in the measure- 

 ment of pressure. 



It has been stated elsewhere that the cells, whether in or out of use, 

 are maintained at all times at the temperature at which they are to be 

 employed for the determination of pressure. This statement is correct 

 for all low and moderate temperatures. But when they are to be used at 

 high temperatures, e. g., above 40°, it is necessary to maintain them at a 

 temperature a little higher than that at which the measurements are to be 

 made, in order to compensate the cooling effects of exposure while the 

 cells are being filled and closed. 



The same is also true of the solutions. They are made up at the tem- 

 perature of the room, and then cooled or warmed, as the case may require, 

 in closed flasks, in the baths. The baths which are used for such purposes 

 are maintained at the temperature at which measurements are to be made, 

 if the temperature in question is a low or moderate one; otherwise, at a 

 slightly higher temperature. The amount of the provision which is thus 

 made for the cooling effect of exposure while filling the cells is entirely a 

 matter of judgment and experience. The mercury in the manometers is 

 always at the temperature of the room when the cells are filled, and its 

 subsequent expansion in a bath of higher temperature must be taken into 

 account; for this partially compensates any contraction of the solution 



