LIVING SUBSTANCE 97 



The liquid nature of living substance is its most important 

 physical characteristic. It requires that in its physical relations 

 living substance must obey the laws of liquids. Accordingly, and 

 in opposition to the idea that vital phenomena are associated only 

 with a solid organisation, it will be seen that such phenomena may 

 be understood only upon the supposition that their substratum is in 

 a condition in which the particles are more or less capable of 

 shifting. The structures that have a rigid consistency, like 

 tendons, connective-tissue fibres, cell-membranes, and the ground- 

 substance of bone and of cartilage, show no active vital 

 phenomena, and the old dictum, " Corpora non agunt nisi soluta," 

 although its universality may be attacked here and there, applies 

 perfectly to living substance. 



2. The Specific Gravity of Living Substance 



Among the physical properties of living substance its specific 

 gravity possesses an important value for the understanding of 

 certain vital phenomena. If cells of different kinds or pieces of 

 tissue as pure as possible be allowed to fall into distilled water, it 

 is observed that usually they sink to the bottom. It follows, 

 therefore, that the cell-contents, as a whole, is in general heavier 

 than water, i.e., possesses a specific gravity greater than 1. Very 

 recently Jensen ('93, 1) has made a careful determination of the 

 specific gravity of the one-celled ciliate infusorian, Paramcecium 

 aurelia, in the following manner. It is well known that the 

 specific gravity of a liquid can be raised by the addition of soluble 

 salts, and can be graduated very finely by increase of the concentra- 

 tion. Jensen placed Paramcecia in a weak solution of potassium 

 carbonate, the strength of which he raised until they no longer 

 sank to the bottom, but remained suspended in the solution a sign 

 that the solution possessed the same specific gravity as the bodies 

 of the Paramcecia. Then the specific gravity of the solution was 

 determined by means of an areometer. It was thus found that 

 the cell-body of Paramcecium possesses a specific gravity of 

 approximately 1*25. In general, the specific gravity of living 

 substance cannot be much greater than this. So far as our 

 knowledge at present extends, it is always a little greater than 1. 



But there are certain cases in which the aggregate weight of 

 the cell deviates from this general principle, in which the specific 

 gravity of the cell as a whole is less than 1. These cases can be 

 understood at once, if it is recalled that protoplasm is not a homo- 

 geneous substance. E.g., in the case of cells in which fat-droplets 

 are stored in the ground-substance of the protoplasm it is possible 

 that, although the ground-substance is heavier than water, the cell 

 as a whole possesses a less specific gravity, since the accumulation 

 of fat, which is considerably lighter than water, reaches such an 



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