1902 - 3 .] Freezing-Point Depression in Electrolytic Solutions. 371 
to detach a small amount of mercury, so that the zero point was 
only a few tenths of a degree from the bottom of the scale, which 
was just visible above the copper tube C. Very little of the 
mercury column was thus exposed to the room temperature. 
The scale divisions of the thermometer corresponded to 0'01°, 
and each of them was divided by the micrometer scale of the 
microscope into 26 parts on the average. Since it was easily 
possible to estimate the quarter of a division by the eye, the 
merely optical accuracy of the thermometric reading corresponded 
to O’OOOl 3 . In the interval of 0*3° in which all our observations 
lay, we carefully determined the value of each thermometer 
division in terms of micrometer divisions. The values varied 
from 25 to 27 micrometer divisions for each scale division. The 
difference between two observations of the value of the same 
thermometer division did not in any case exceed 0*3 micrometer 
divisions. The relation of the above part of the tube to the 
whole scale was then determined by the ordinary calibration with 
mercury threads. The tube was found to be of remarkably 
uniform bore, with the exception of the extreme upper part, 
which we never used in our experiments. Finally, the value of a 
degree of the thermometer was determined for us by Professor 
Kuenen and Mr Robson by comparison with a platinum resistance 
thermometer. The value of the degree of our thermometer was 
found to be identical with that of the air thermometer within the 
error of experiment, which was estimated at 0*4 per cent. 
As has already been mentioned, very little of the stem of the 
thermometer which contained mercury was exposed to the room 
temperature. Special experiments showed that at 0°, i.e ., at the 
point where the greatest column of mercury was exposed, a 
variation of 2° in the room temperature was barely perceptible on 
the freezing points as read off on the thermometer, i.e., probably 
did not amount to 0 , 0002°. The temperature of the basement 
room in which we worked varied very slowly, never more than 1° 
during a complete depression experiment, so that the effect of 
variation of room temperature on the exposed stem may be 
neglected. 
Since at the freezing point of water, O’ 2° more of stem was 
exposed than at the freezing point of the solutions we mostly 
