6o 



Emission and Transmission of Heat 



sphere is in halves which fit into 

 one another, the joint being made 

 water tight with wax. Thermome- 

 ters, not shown in the figures, give 

 the temperature of the water in the 

 outer vessel. Throughout an ex- 

 periment the water in the inner 

 sphere was agitated by an apparatus 

 similar to that employed in the ex- 

 Fig - 17 periments on cooling in air (782) 



and the water in the outer vessel was stirred by turning the 



crank K. 



The rate of cooling was measured in the same way as in the 

 experiments on cooling in air, and with the same corrections in 

 regard to the water introduced into the vessel before each obser- 

 vation in order to maintain it full. 



845. In the second method I have used a hollow cylinder of 

 the material of which I wished to determine the conductivity; its 

 interior was heated by steam and its exterior exposed to the air in 

 the constant temperature chamber used in the experiments on 

 cooling in air. When the r6gime is established, the quantity of 

 heat passing through the walls of the cylinder is evidently equal 

 to that escaping from its surface. Now there exists a very simple 

 relation between the inner and outer radii of the cylinder, the tem- 

 peratures of the steam, of the outer surface of the cylinder and of 

 the surrounding air, the rate of cooling of^ the outer surface, and 

 the conductivity of the material forming the cylinder. All these 

 quantities can be easily measured, except the temperature of the 

 surface of the cylinder. We shall show presently how this has 

 been determined. 



846. Figure 171 shows the general arrangement of the ap- 

 paratus. For powdered substances I used a cylinder of tin a b cd 

 eight inches in height, and of different diameters, covered with 

 paper; it was surmounted by a tube ef closed by a stopper at its 

 upper end through which passed the stem of a thermometer with its 

 212 mark just above the top of the stopper; this tube was pro- 

 vided with an outlet / communicating with a vessel producing 

 steam. Attached to the bottom of the cylinder was a tube h i k, 



