VI. CALORIMETRIC MEASUREMENTS 197 



large; thus the twin calorimeter of Giieker for the measurement of 

 small heats of dilution in dilute electrolyte solutions had a volume of 

 1 liter for each of the two chambers. The table of Whii)p {36} best 

 illustrates typical characteristics of some microcalorimeters (see Table 

 HI). 



Common to all these instruments is the use of electric thermo- 

 couples with a very high temperatiu-e sensitivity of about a micro- 

 degree per milUmeter deflection of the galvanometer. The greatest 

 heat sensitivity is reached in Hill's instrument used for measuring heat 

 production of nerves in milliseconds. A deflection of 1 mm. of the 

 galvanometer spot indicates 0.5 microcalorie, a microcalorie being 

 one-millionth of a calorie. This heat sensitivity is about ten times 

 as high as the corresponding sensitivity in the measurement of oxygen 

 consumption reached in the Cartesian diver microrespiration appara- 

 tus, namely, one-millionth of a cubic milliliter of oxygen per milli- 

 meter of manometer change. 



This high sensitivity of microcalorimeters makes processes im- 

 portant that have no noticeable effect on the result in macrocaloi'im- 

 etry. Thus Hill attributed large prolonged galvanometer deflec- 

 tions in his apparatus to "rainstorms" set off in the tiny chambers by 

 electric charges on nerve-stimulating wires. He also calculated 

 that a pinch of a rubber tube causing a volume change of one part 

 in a million would produce an adiabatic temperature change of 10 

 microdegrees, which in tm-n would make the galvanometer spot travel 

 50 cm. (^7, p. 119). 



Microcalorimeters require special insulation. Dewar vessels are 

 often used for this purpose. Very accurate regulation of environ- 

 mental temperature is especially important. Tian (28) has invented 

 a thermal damping system in which several concentric layers of al- 

 ternately good and bad heat conductors form a multiple jacket. The 

 temperature inside this jacket is nearly unalTected by temperature 

 fluctuations outside, so that the inside temperature represents very 

 accurately the mean environmental temperature. Trends of envi- 

 ronmental temperature penetrate the damping system very slowly. 

 With a Tian damper, Hill Avas able to decrease the "zero creep" of a 

 thermostat to about 2 microdegrees in ten minutes. 



For detailed information and critical discussion of niicrocalorim- 

 etry the recent book of Swietoslawski should be consulted (/?). 

 An earlier article by Meyerhof appeared in Abderhalden's handbook 

 (29). 



