158 
of the solvent. We have chosen mannite as dissolved substance for 
our experiments, because the molecular weight (182) is large com- 
pared with that of water, and becanse it can easily be determined 
by weight after drying at 100°. Mannite (Ph. Ned. IV) was 
recrystallized from alcohol and dried. (Spt. 166'/,°). To obtain 
sufficiently reliable values for the diffusion constant, we have made 
a great number of determinations at different temperatures, and 
determined the mean values of the series of experiments; the results 
are recorded in table II, III, and IV, the mean values are given in 
a D—t diagram (fig. 5) (see p. 157). (D=[em’. 24 hours AJ): Al 
any point the number of observations is given which has contri- 
buted to that mean. 
TABLE 
Number of the 
cylinders 1 2 3 4 5 6 7 8 
Capacity in mg. ; 
mercury (18°) 854.3 | 611.7 | 683.0 | 470.8 | 555.8 | 520.4 | 515.9 | 529.1 
| Height in cm.) | — | — = | 4,748 2.050 1.895 1.935 = 
4. Conclusions concerning the radius of the mannite molecule. 
Equation (3) is valid for infinitely diluted solutions, in which the 
dissolved particles move through the medium independently of each 
other. & represents the internal friction of the solvent, in our case 
water, and D the diffusion constant for infinite dilution. 
Ornotm derived 0.513 for D, from his experiments with the 
normalities 0.5, 0.25, and 0.125 at 20°, which value is about 6°/, 
higher than the value 0.485, which was found by him for a solution 
of 0.25 normal. This is comparable with our observations, in which 
the strength of the solutions varies between 30 and 70 grams per 
liter (+ 0.2 and 0.4 norm.). The values found by OkHOLM for this 
concentration have been inserted in the graphical representation *). 
If the same change of the diffusion constant with the concentration 
1) The height was determined by means of a piece of a knitting needle which 
was slid into the diffusion cylinders till it reached the flat bottom. The total length 
and the part projecting outside the cylinder were measured by the aid of the 
ocular micrometer and of a millimeter division on glass. 
2) Ornotm. Meddelanden Nobelinstitut. 2 No. 23 (1912). The value found by 
one of us before, viz.: Divo = 0.38 is, evidently slightly too high, which will 
probably be owing to variations of temperature, which were inevitable on account 
of the long duration of the experiments (2i—27 days).(Ber. der Deutsch Chem. 
Gesellsch. 15. 797 (1882)). 
