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Laser power and energy measurements are commonly made in calorimeters operating in a constant temperature environment. Calorimeters of this type are analyzed in terms of the first law of thermodynamics and the boundary value problem describing heat flow in the calorimeter. This theory of the measurement suggests design features of the calorimeter, sources of error to be avoided in design and operation, and tests to demonstrate experimentally the adequacy of the design. The analysis shows how time—temperature data can be used to allow for the temperature gradient on the calorimeter and the heat exchange due to transients in the temperature.

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Equations are derived describing a model of an isoperibol calorimeter in which the calorimeter proper consists of two parts thermally connected, one surrounding the other and exchanging heat with the constant-temperature environment. These solutions provide insights into the behavior of real calorimeters. Inferences are drawn relative to the effect of the locations of the thermometer and heat source on the energy equivalent of the calorimeter and some possible errors are pointed out. Macleod's application of the two-body theory of King and Grover to high-temperature enthalpy measurements is discussed and weaknesses in theory and experiment are pointed out. Procedures having a better theoretical basis are outlined.

The absorption coefficient of NO2 (g) has been found to be both temperature and pressure dependent for the temperature range 299.71° to 376.52°K and pressures from 2 to 5 cm Hg, for the 5461-Å mercury ``line''. The equilibrium constants for the reaction N2O4 (g)=2NO2 (g), at the temperatures 299.71°, 303.70°, and 308.89°K, have been evaluated by combining precise transmittance measurements for the 5461-Å mercury line with the known vapor pressures of solid N2O4. ΔG°298.16 for the above change in state has been found to be 1.151±0.004 kcal/mole, which, when combined with the known value of ΔS°298.16, gives ΔH°298.16=13.640±0.100 kcal/mole.