Summary. 1. In a pond in which Spirostomum occurred in vast quantities the pH of the water was 7·4, that of an aquarium in which Spirostomum were present in large numbers was 7·5, and that of the water containing flourishing cultures of Spirostomum was 7·4 to 7·6. 2. The Spirostomum showed the greatest activity in solutions of pH 7·4. Below this value the activity is reduced, but down to pH 6·0 at any rate the effect is not lethal. In solutions above pH 7·6 to 7·8 the Spirostomum are killed within a few hours if the pH greatly exceeds these values, and in a longer time if they are barely exceeded. In dying as result of the alkalinity of the water being too great the animals assumed a characteristic pear‐shaped malformation. 3. Pütter (1903) supposed that the death of Spirostomum which occurred when the animals were placed in shallow vessels exposed to the atmosphere was due to oxygen poisoning. This is shown not to be the case but to be due to the pH rising above the lethal value. 4. Spirostomum, when placed in tubes in which the pH of the water at one end is 8·0 and 6·8 at the other, are found to migrate rapidly into the water of pH 6·8 and then finally settle in the middle of the tube where the water is pH 7·4. This preference for water of a particular pH, which is stronger in its effects than those of light and gravity, disappears in the darkness. 5. The normal behaviour of Spirostomum in the aquarium, in the pond and in the collecting jars may be explained to a large extent by the preference for water of a particular pH and by the presence or absence of a vertical pH gradient. 6. No evidence was found to connect an epidemic of conjugation with the pH of the water. 7. The temperature at which the experiments were performed or the observations made varied from 9 to 15· Centigrade, but the temperature was uniform throughout any tube or jar at the time it was observed. There was no temperature gradient.
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J. T. Saunders (1924) studied this question.