370 Proceedings of Royal Society of Edinburgh. [sess. 
the scale of the micrometer in the eyepiece of the microscope 
corresponds on the average to 0 '000384°. 
Sources of Error. 
To estimate the degree of ionisation with the accuracy we desired, 
it was necessary for us, with depressions of about O' 2°, to secure an 
error not exceeding 0*0005°. With regard to the accuracy attain- 
able in cryoscopic measurements, we cannot do better than quote 
the words of the originator of the method, as given in his last paper 
on the subject (Raoult, loc. cit.). “The final result of this investiga- 
tion is, that with the apparatus above described, and with due 
observance of all precautions, it is fairly easy to determine the freez- 
ing-point depressions of dilute solutions with an accuracy of O'OOl °. 
“ It is a matter of great difficulty, however, to carry accuracy 
any further, chiefly on account of chance variations in the zero 
point ; but if one is so fortunate as to possess a thermometer in 
which these variations are small and regular, one may indulge the 
hope of reaching an accuracy of 0'0002°. In my experiments on 
the molecular depression of dissolved oxygen I actually attained 
this degree of accuracy.” 
In the following paragraphs we discuss the various sources of 
error to which our experiments were liable, and the degree of 
uncertainty to be expected in each case. 
The Thermometer . — We experienced some trouble in obtain- 
ing a thermometer suited to our experiments. The Beckmann 
thermometers belonging to the Chemistry Department of this 
College were found to be unreliable when readings were pushed 
beyond the thousandth part of a degree, owing to the tendency 
of the mercury to stick at certain places of the scale, no amount 
of tapping or other mechanical agitation of the thermometer being 
sufficient to ensure consistent readings. A thermometer, however, 
belonging to the Physiological Department proved on investigation 
to be entirely satisfactory, and we have here to express our thanks 
to Professor Way mouth Reid for kindly placing this instrument 
at our disposal during the experiments. The thermometer, made 
by Goetze of Leipzig, was a fixed zero instrument, but as there 
was a small bulb at the top of the stem, it was possible with care 
