VI. C A T. O Tt I M E T R I C MEASURE M E \ T S 187 



Biinscii's ice calorimeter, and ])ailiculaily the use of this apparatus 

 for microcalorimetr}^, the reader is referred to the recent book by 

 SwietoslaAvski (17). 



Combustion calorimetry by flie vise of the calorimetric bomb 

 may be classified among adiabatic measurements, even if no adiabatic 

 jacket is used, because the heat flow between calorimeter proper and 

 environment (or jacket) plays only a minor role. It is then designated 

 as "heat leak" and, in the final calculation, figures as a correction 

 term. Heat leak and coml^ustion caloiimetiv will be discussed in the 

 following sections. 



3. Measurement of Heat Flow through Walls 



Instead of preventing heat flow as in adiabatic (calorimeters, one 

 may measure it on the l)asis of Fourier's law, which, for steady state, 

 may be formulated as follows: 



AQ/At = \SiTi - T,)/d (11) 



where AQ/At = rate of heat flow (for example, calories per minute), 

 X = heat conductivity of insulating layer, d = thickness of insulating 

 layer, .S' = area of heat stream — in this case, area of insulating surface 

 layer, 7\ = temperature of inner surface of insulating layer, and Tg = 

 temperature of outer surface of insulating layer. 



By empirical calibration one may determine a heat conduction 

 constant, L = 'KS/d, for any given calorimeter, and then formulate: 



AQ/At = UT, - T,) (12) 



This principle of measuring heat flow was applied especially in early 

 animal calorimetry by Richet and by Rubner (18,19). In their calo- 

 rimeters the animal room is surrounded by a layer of air (At, Fig. 1). 

 This air space is surrounded bj' an insulating layer /, which in turn is 

 enclosed in an outer layer of air, Ag. Each of the two air layers is 

 hermetically sealed and is connected to the other through a ma- 

 nometer, M. After an empirical calibration the difference in pressure, 

 p, between the two air layers indicates the difference in temperature, 

 Ti — Ts, for the calculation of heat flow. 



Instead of using an air thermometer for measuring T< — T„ one 

 might nowadays advantageously measure this difference with a set of 

 thermoelectric junctions distributed on the inner and outer walls of 

 the insulating layer. 



