Alfred L. Loomis

The pressure of aqueous vapour in the alveolar air

THE pressure of aqueous vapour of the alveolar air is generally accepted to be a physiological constant. In calculations of alveolar gas pressures the value 47 mm. Hg is used, a value which represents the pressure of aqueous vapour in air saturated with moisture at 37.20 C. To our knowledge no direct measurement of the pressure of water vapour in the alveolar air has ever been made, and even from theoretical considerations it would seem probable that the figure which has been accepted is too high. It would also seem possible that this value might readily show physiological variations. This is evident when we consider the assumptions made in assuming a vapour pressure of 47 mm.-It is assumed that (a) the osmotic pressure of the pulmonary epithelium is equal to that of the blood; (b) the alveolar air is always saturated with moisture; (c) the temperature of the alveolar air is 37.20 C. and does not vary. The validity of these assumptions will be discussed later.

Modern Developments in Precision Clocks

ULTRA-PRECISE practical timekeepers of today can be placed within two classes, depending upon whether the restoring force is gravity, as in pendulum clocks, or elasticity, as in quartz crystal oscillators. The second class is a development of the last decade and was made possible by the vacuum tube and associated electrical circuits. Intercomparisons between timekeepers of the two classes provide a most valuable means for t he study of changes in gravity and related phenomena.