Letter | Published:

Temperature of the Breath


THE interesting observation made by Dr. Dudgeon (NATURE, vol. xxii. p. 241, and vol. xxiii. p, 10) to the effect that breathing on the bulb of a thermometer through several folds of flannel or silk raises the temperature of the instrument several degrees above that of the mouth and body, is easily verified. There is no doubt about the accuracy of the observation; but the explanation of it offered by Dr. Dudgeon is not satisfactory. He supposes that the heightened temperature is due to the expired air being hotter—not cooler, as is usually believed—than the mouth and body. A simple experiment sufficed to show that this view was untenable. A clinical thermometer was inserted in the cavity of the mouth, and the stem grasped by the teeth in such a way that the bulb lay free in the oral cavity. Inspiration was carried on by the nostrils, and expiration was effected by gently forcing the breath between the loosely-closed lips and the stem of the instrument. The bulb was thus placed in the centre of the stream of expired air and kept free from contact with the tongue and cheeks. Experimenting in this way, I found, at the end of five, and also of ten, minutes that the thermometer marked 97.2°—the temperature under the tongue at the time being 98.4°. Had the breath been hotter than the mouth the instrument could not have failed to register a higher temperature than 98.4°, but being really cooler, the instrument, of course, recorded a lower temperature.

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