THE THERMAL CONDUCTIVITY OF METALS. 167 
the heating box, B, is an asbestos covering, M, to prevent heat from reaching the 
calorimeter from the sides of the box. In order to keep the temperature of the air 
surrounding the calorimeter constant, the latter is surrounded by a water-jacket, R, 
through which water at the temperature of the air is kept circulating. This water- 
jacket is simply a cylindrical vessel, made of copper, with double walls, between 
which the water circulates. The inside diameter is about 1 centim. greater than that 
of the ball. The top of the water-jacket is covered with three or four layers of 

Clay 


Full size. 
paper, so that the air surrounding the ball is completely separated from the outside 
air, and is kept at a constant temperature. Surrounding the wire, all along its 
length, is a tube of cardboard, G, of inside diameter 1 centim. Between the wire 
and the inside of the tube cotton-wool is loosely packed, so as to prevent, as 
effectually as possible, circulation of air about the wire. 
From fig. 2 it will be seen that the heat from the Bunsen is prevented from 
