592 
Proceedings of the Royal Society 
3. Account of a Preliminary Experiment on the Thermal 
Conductivity of Ice. By A. Crichton Mitchell. Com- 
municated by Professor Tait. (Plate XXI.) 
The following paper is an account of a preliminary experiment 
performed in the Edinburgh University Physical Laboratory, with 
the view of determining the thermal conductivity of ice. 
The method employed was that of Angstrom, which consists in 
heating and cooling alternately, to a definite amount and for definite 
periods of time, one end of a block or bar of the substance whose 
conductivity is required. The thermal changes produced in this 
manner are transmitted throughout the substance in a species of 
wave. Taking two points in the substance in a line perpendicular 
to the surface which is being heated and cooled, these changes are 
produced until the temperature indications at the two successive 
points have become periodic. The conductivity can then be calcu- 
lated from the ratio of the amplitudes of the temperature indications 
per unit length along the line joining the two points, and from the 
interval in time between the maximum temperatures per unit length. 
Obviously, the ranges of temperature we must wmrk with in the 
case of ice lie below 0° cent. ; and accordingly, the periodic cooling 
and heating were effected by successive applications of a freezing 
mixture and exposure to the surrounding air, which was throughout 
the experiment never above 1° cent. To carry this out, a tinned 
iron box, with open top, had two thermo-electric junctions of iron- 
German silver — inserted through india-rubber corks, fitted into 
circular holes, in opposite sides of the box; one hole being so much 
above the other as to separate the junctions by a distance of 1*27 
centimetres from each other, in a line perpendicular to the bottom 
of the box. The cube was then filled with water, previously boiled 
to expel dissolved air, aud placed on the roof of the College for 
forty-eight hours during frost. The ice formed in this manner, with 
the thermo-electric junctions in its interior, was free from cracks and 
air particles, and was evidently quite homogeneous. 
It was at first attempted to cool and heat the surface of the ice 
by simply laying on it a freezing mixture of snow and salt for a 
definite period of time, and subsequently flooding the surface of the 
