The apparatus which one selects for the study of gaseous exchange by living tissue is largely determined by the nature of the problem with which one is confronted. Choice usually lies between those methods where a continuous stream of air is passed over the respiring material and alteration in its carbon dioxide content measured, or those in which the oxygen or carbon dioxide content of the air surrounding the respiring material in a closed system is periodically determined. The former system is frequently used when one is only concerned with relatively large quantities of carbon dioxide. The gas, which is usually absorbed by passing the air through solutions of sodium or barium hydroxide contained in Pettenkoffer tubes or Reiset towers, may be subsequently determined by titration of the alkali with standard acid. This arrangement is much used owing to the readiness with which it may be set up employing normal laboratory equipment. The labour involved can, however, be considerably curtailed by adopting the electrical conductivity method for determining carbon dioxide. This has been described by Newton (1935). It may be made more accurate and more sensitive than the titration methods and, moreover, enable determinations to be made quickly and at short intervals of time. Leach (1944) has made the method automatically recording.