62 
Proceedings of the Royal Society 
2. On the Thermal Conductivity of Ice, and a new Method 
of Determining the Conductivity of Different Substances. 
By Professor George Forbes. 
The value of the coefficient of conductivity for ice is an impor- 
tant desideratum in several branches of physics; and it derives 
additional importance from an application, explained in the second 
part of this communication, to the determination of the thermal 
conductivity of different substances in absolute measure. The 
brilliant researches of Neumann,* and some ingenious experiments 
by M. Lucien De La Bive,t afford us at present the only two deter- 
minations that we possess of this important quantity. The value 
found by Neumann is 0T14, while De La Bive makes it 0T38. 
The discrepancy justifies the publication of experiments on a some- 
what large scale, which gives us a close approximation to the truth. 
Sir William Thomson suggested the method of imitating the 
freezing of a lake by means of a freezing mixture, and to deduce 
the conductivity from measures of the thickness of the ice formed 
in a definite time. In order to carry out this idea, I ordered an 
apparatus to be constructed by means of which a disc of ice could 
be formed twelve inches in diameter by a freezing mixture placed 
above a vessel of water kept constantly at 0° 0. A means was 
devised for measuring the thickness of ice formed at successive 
intervals of time. The freezing mixture was drained constantly 
during the course of each experiment by means of a syphon. 
Temperature was read frequently at the base of the vessel in which 
it was contained. It was found possible to read the thickness of 
ice formed to within ^-th of an inch. The experiments lasted from 
four or five hours to twenty-one hours, a watch being kept continually 
on the drainage and temperature of the mixture for the first six or 
eight hours in experiments of long duration. The ice formed was 
quite uniform, very clear, and when cloven by planes perpendicular 
to the plane of freezing, split easily, showing the crystalline struc- 
ture with great clearness. 
Six whole days of frosty weather were employed in perfecting and 
completing the series of observations, during which time seventy-two 
readings were taken, capable of giving a value for the conductivity ; 
* Phil. Mag. 1863. t Soc. de Ph. d’Hist. Nat. de Geneve, 1864. 
