CLARK : MOLECULAR WEIGHT DETERMINATIONS. 249 



the tube. The barometer was read between the two readings of 

 the Beckmann thermometer for each condition. The tube was 

 carefully cleaned and dried just before each set of readings. 

 The volume of water was measured with a pipette graduated in 

 tenths. No correction was made for the amount of water cling- 

 ing to the sides of the tube or of the condenser while the tube 

 was in use. The water used was double distilled from acid and 

 alkaline permanganate solutions, condensed in a block-tin con- 

 denser, and kept in a rubber-stoppered Jena flask. No test was 

 made of the purity of the water. 



Cane sugar, common salt, and urea are the only solutes in- 

 vestigated so far. The first was a commercial article ; the last 

 two were chemically pure, the latter having been made by C. 

 A. F. Kaulbaum. The molecular weight was calculated ac- 

 cording to the formula, M = ^ ; in which w is the weight of the 

 solute in grams ; R is the elevation of the boiling-point in degrees ; 

 Wis the weight of the solvent (assumed to be numerically equal 

 to the volume) ; and C is the molecular elevation constant for 

 water, obtained from the equation, C = -j^, but here assumed 

 to be 520 — a value about one per cent, too large at this altitude. 

 The salt and sugar were compressed into pellets to avoid loss in 

 handling, and large crystals of urea were selected for the same 

 reason. As there is about 6.5 volts difference of potential be- 

 tween the ends of the heating coil when the liquid is boiling 

 steadily, the salt was decomposed sufficiently to give a strong 

 odor of chlorine, after the tube and contents were allowed to 

 cool. Thus no dependence is placed on this form of apparatus 

 for work with conducting solutions, and the results obtained 

 with sodium chloride are not here recorded. 



As will be seen from table A, the calculated molecular weight 

 of cane sugar varies from 314 to 271 with increasing concentra- 

 tion varying from 17.5 g. of sugar per 100 c.c. of water to 82.6 

 g. per 100 c.c. ; this is much less variation than was obtained 

 by Kahlenberg.^ 



Table B shows the molecular weight of urea, varying from 

 66.1 with a concentration of 1.51 g. per 100 c.c. of water, to 

 63.8 with a concentration of 3.03 g., to 64.8 with a concentra- 

 tion of 7.53, and falling to a minimum of 60.4 with a concen- 



3. Jour. Phys. Chem., 5, 339 ( 1901 ). 



