Fig. 1. (A) Sample plot of a recording trace in which the relative humidity was increased stepwise from 84% to 94%. The left ordinate shows the measured heat signal (µW) and the right ordinate shows the relative humidity (RH), plotted as the broken line. Exothermic heat flows are negative. The black and red plots show data for two different animals; the green plot is a blank recorded with an empty calorimetric cell. The (negative) peaks at each RH increment represent the exothermic heat of water adsorption to the walls of the calorimetric vessel. Clear deviations from the blank in the equilibrated trace show exothermic heat flow attributable to WVA. The abrupt decrease in heat flow in the red trace at 23 h shows that the animal terminated WVA at this time and the black trace shows a similar termination of WVA at ca. 18 h. Further explanation is provided in the text. (B) The heat signals recorded in the second (water) channel for these same animals during the later period of data collection. Depression of the RH in Cell 1 during WVA results in a corresponding increase in evaporation from Cell 2 and augmented endothermic (positive) heat flow compared to the blank. The red trace shows the onset of WVA in this animal at 16 h. The black trace shows an animal that is initially absorbing, but ceases WVA at 18 h. Fuller discussion is given in the text.