内容简介：
Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: cells, but of the physical qualities of the colloidal substances in the tissues. But must we conclude from this that the Na ions are the cause of the spontaneous rhythmical contractions of the Medusae ? I think not. The ions only bring about a certain labile equi- Fig. 4. Arrangement For Producing Automatically Pulsating Air-bubbles. (See text.) librium in the condition of the colloids of the contractile tissue which allows the true cause of the contractions to be effective. But what is this cause ? J. Rosenthal seems to have been the first to call attention to the fact that it is in no way essential for a rhythmical phenomenon to have a rhythmical cause, and that constant conditions can lead to rhythmical effects. If a small, constant stream of water flows into a pipette, it will pass out rhythmically in drops. The weight of the drop must be greater than thesurface-tension in the periphery of the opening of the outlet before the drop can break off. As long as the quantity of water running into the pipette, in the unit of time, remains below a certain limit, it will be some time before the drop will be heavy enough to fall. Quincke has given a simple and elegant method by which it is easy to produce rhythmical contractions in air bubbles (5). I will describe the experiment as shown in my lectures. A glass plate P (Fig. 4) is placed in a dish B, filled with water. The lower, narrow end of the thermometer tube T is under and at the middle of the air-bubble, while the upper end rests in a dish A filled with 95 per cent, alcohol. The alcohol rises in a fine stream toward the centre of the bubble. As soon as the alcohol comes in contact with the bubble, the alcohol spreads out on the limit between the air and the water, because the sum of the surface- tensions between air and ...