The hourly observations were made from May 4 to 6 in the moat where Potamogeton cripsis was abundantly growing. We could make clear the state of O₂-consumption due to the respiration of the aquatic plant during the night. The amplitude of the change of O₂ on May 4 was 7, 09c.c./L. The minimum value of O₂ appeared at 6h. on 4, showing 4, 51c.c./L (63, 8%) and on 6 it came about two hours late in the bottom layer as it was cloudy in the morning. It was recognizable that a lineal relation between the increased volume (d) of O₂ from the minimum and the amount of the solar radiation (S) which was summed up from sun-rise till each time of the observation on May 4 as that described in the previous paper, while the relation on 5 was relatively irregular in the surface layer. This irregularity is due to the disturbance of the surface water caused by the south wind (about 9m./sec.) during 12 and 14h.
The phenolphthalein-alkalinity of the water was determined with the titration of N/10. HCl, using phenolphthalein as an indicator. The feature of its variation was resemble to that of pH.