A machine has been described which combines volumetric and refractometric means of observing changes in carbon dioxide and oxygen concentrations and amounts in a closed system suitable for containing a respiring system. A means of thermoregulation has been described briefly. The theory upon which estimation is based has been outlined in considerable detail and generality; indicating how dependence upon absorption of gases by reagents can be eliminated completely, although they may be used in certain instances as a means of independent check. An experiment of this nature has been reported as an illustration of one use of the machine. Data of other experiments in which a moist environment was employed for the respiring system will be given in another communication.

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