If a drop of weak acid be put into the water in which the paramœcia are swimming,—for instance, in the water between a cover-slip and a slide,—it will be found, after a time, that many individuals have collected in the drop. It was at first supposed that the paramœcia are attracted by the diffusion of the acid in the water, and turn toward the source of the chemical stimulus; but Jennings has shown that this is not the way in which the aggregation is brought about. If the individuals are watched, it will be found that they swim forward in a spiral path without regard to the position of the drop of acid. If one happens, by chance, to run into the drop, there is no reaction as it enters, but when it reaches the other side of the drop, and comes into contact with the water on this side, it suddenly reacts. It stops, backs into the middle of the drop, rotates somewhat toward the aboral side (_i.e._ away from the vestibule), and then starts forward again, only to repeat the action on coming into contact with the edge of the drop again. The paramœcium has been caught in a veritable trap. All paramœcia that chance to swim into the drop will also be caught, until finally a large number will accumulate in the region. The result shows, that, in passing from ordinary water into a weak acid, no reaction takes place; but having once entered the acid, the animal reacts on coming into contact with the water again. On the other hand, there are some substances to which the paramœcium may be said to be negatively chemotropic. If a drop of a weak alkaline solution be put into water in which paramœcium is swimming, an individual that happens to run against it reacts at once. It stops instantly, backs off, revolving in the opposite direction, turns somewhat to one side, and swims forward again. The chances are that it will again hit the drop, in which case it repeats the same reaction, turning again to one side. If it continues to react in this way, it will, in the course of time, turn so far that when it swims forward it will miss the edge of the drop, and then continue on its way. If an individual were put into an alkaline drop, it would leave it, because it would not react when it passed from inside the drop into the surrounding water.