1. The effect of H2 tension, CO2 tension, pH, time, light intensity, density of suspension, salt content of the medium, and certain spectral regions on the rate of photoassimilation of H2 and CO2 by Streptococcus varians has been studied.

2. The method of making light absorption measurements with thin suspensions of bacteria is described.

3. A light source, optical system, and filter for isolating 852 mµ with 894 mµ in sufficient intensity for photochemical work and an improved design of thermostat are given.

4. The photoassimilation of 2H2 with 1CO2 apparently involves little over all energy change but nevertheless requires 4 quanta.

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