142 SCIENTIFIC APPARATUS. 



ratures, the thermo-electric current produced in a circuit of these 

 metals is directly proportional to the absolute temperature dif- 

 ference of their junctions. By arranging three metals in a circuit, 

 two of them in multiple arc, it is easily possible to obtain the same 

 result even when the specific heat of electricity has different values 

 in all three. 



(f) Electric Resistance Thermometers. [Siemens, 



IV. SOURCES OF HEAT. 



(a) Furnaces. 



(b) Bunsen Lamps, Blowpipes, &c. 

 (f) Electric arc. 



(<f) Sun heat : burning lenses and mirrors. 

 (e) Chemical Combination. Heat of Combination. [Andrews, 

 Favre and Silbermann, Thomsen, c.] 

 (/) Freezing Mixtures. 

 Arrangements for producing cold by Expansion, Evaporation, c. 



V. TRANSFERENCE OF HEAT. 



Heat may pass from one part to another of the same body, as 

 by Conduction ; from one body to another (even through vacuum) 

 as by Radiation ; or from place to place with one and the same 

 body, or part of a body, as by Convection. 



(a) CONDUCTION. 



We owe our knowledge of the laws of the Conduction of Heat 

 in Solids mainly to Fourier, who first satisfactorily gave the defi- 

 nition of Thermal Conductivity, or Conducting Power, and with 

 it the original and beautiful mathematical methods requisite for 

 its application. 



The Conductivity is measured by the number of units of heat 

 which pass, per unit of time, per unit of surface, through an infinite 

 slab of any material, of unit thickness, whose sides are kept at 

 temperatures differing by any assigned amount say i? C. The 



