Values of fluorescence yields for 18 compounds in solution at 23°are reported and they are compared with previous literature values. Modifications of the technique of Weber and Teale for measurement of fluorescence yields are described. Fluorescein in 0.1 N NaOH and anthracene in ethanol are the best fluorescence standards on the basis of agreement of fluorescence yields with other work. The use of quinine as a fluorescence standard is complicated by variation of its fluorescence yield with H2SO4 concentration and the excitation wavelength. Several other compounds, including chrysene, show no variation of fluorescence yield with a change of excitation wavelength. Radiative lifetimes estimated from integrated absorption spectra are found to be consistently shorter than the radiative lifetimes estimated from fluorescence yields and lifetimes. The fluorescence yields of pyrene-dw and anthracene-dm in ethanol are significantly lower than those of corresponding solutions of pyrene-and anthracene-, respectively. No significant effect of compound deuteration upon the fluorescence yields of seven other compounds is found.

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1. Birds lose water in evaporation from the respiratory tract and, in many species, through the skin. Anatomical arrangements in the nasal passages to conservation of water and hear from the expired air in the absence of heat loads. However, most species still expend more water in evaporation than they produce in metabolism when either quiescent or vigorously active. Certain small birds, several of them associated with arid environments, represent exceptions to this and their more favorable situation appears in part to reflect as an ability to curtail cutaneous water loss. 2. Birds typically resort to panting in dealing with substantial heat loads developing in hot environments or accumulated over bouts of activity. In a number of species this form of evaporative cooling is supplemented by gular fluttering. 3. The ubiquitousness of active heat defense appears to reflect more the importance for birds of dealing with heat loads existing following flight or sustained running than any universal affinity for hot climates. Panting can be sustained for hours, despite progressive dehydration and, in some instances, hypocapnia and respiratory alkalosis. The prominent involvement of thermoreceptors in the spinal cord in its initiation is of considerable interest.

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