[BARNES & COOKE] THE DENSITY OF ICE 145 
capillary tube A, and the movement was a direct measure of the 
expansion of the water on freezing. Only three determinations of 
this kind were made, probably owing to the fact that a new instrument 
had to be prepared for each experiment. 
The «mean value obtained by these three determinations was 
0:91580 + 0-000008. 
The determination made by Kopp in 1855 is scarcely worthy of 
mention. His method involved the use of a dilatometer, but one of 
poor design and involving probable errors of such a nature as to render 
his results quite worthless. His values came much lower than those 
of the preceding observers, a result which might have been anticipated 
from the fact that in every specimen of ice experimented on, obtained, 
of course, by artificial freezing, a small bubble was noticed. The 
value obtained was 0:9078 + -0007. 
In 1860 Dufour undertook a new determination of the density of 
ice. His method differed from both the preceding ones, being inca- 
pable of the same degree of accuracy obtained by the double weighing, 
or by the dilatometric method. 
The method consisted of submerging ice in a liquid, of which the 
density could be varied ; then, by adjusting this, a point could be 
reached where the density of the liquid was identical with that of the 
ice, this being ascertained by observing when the ice was in equilibrium 
in the liquid. The specific gravity of the liquid was then accurately 
determined, and the ice density thus arrived at. In his first series 
of experiments, a solution of alcohol and water was used. Here, 
however, the results were doubly indirect, as the alcohol attacked the 
ice, when the mixture was at 0° C., rendering it necessary to carry 
on the experiments at lower temperatures than this, and then allow 
for the cubical expansion of the ice, adopting the coefficient obtained 
by Plücker in his experiments, viz., 0-000158. From 22 experiments 
he obtained the value 0-9175 + 0-0007, the probable error here being. 
the same as that of Kopp’s results. 
The year following the publication of these results, Dufour again 
set himself to solve the same problem, using the same method, but 
employing a mixture of chloroform and petroleum, neither of which 
attacks ice. Here again he worked at temperatures below the freezing 
point, the results obtained being slightly higher than those of the first 
set of experiments — 0:9178 + 0-0005. 
We now come to the work of Bunsen, who began his determina- 
tions in 1870. The method employed involved the use of a dilato- 
meter of special design, the increase in volume being measured by the. 
quantity of mercury expelled from a capillary point in the apparatus, 
