I] OF MATTER AND ENERGY 17 



principal properties of matter with which our subject obhges us to 

 deal. Let us imagine, for instance, the case of a so-called "simple" 

 organism, such as Amoeba; and if our short list of its physical 

 •properties and conditions be helpful to our further discussion, we 

 need not consider how far it be complete or adequate from the 

 wider physical point of view*. 



This portion of matter, then, is kept together by the inter- 

 molecular force of cohesion; in the movements of its particles 

 relatively to one another, and in its own movements relative to 

 adjacent matter, it meets with the opposing force of friction — 

 without the help of which its creeping movements could not be 

 performed. It is acted on by gravity, and this force tends (though 

 slightly, owing to the Amoeba's small mass, and to the small 

 difference between its density and that of the surrounding fluid) 

 to flatten it down upon the solid substance on which it may be 

 creeping. Our Amoeba tends, in the next place, to be deformed 

 by any pressure from outside, even though slight, which may be 

 applied to it, and this circumstance shews it to consist of matter 

 in a fluid, or at least semi-fluid, state: which state is further 

 indicated when we observe streaming or current motions in its 

 interior. Like other fluid bodies, its surfacef, whatsoever other 

 substance — gas, hquid or solid — it be in contact with, and in varying 

 degree according to the nature of that adjacent substance, is the 

 seat of molecular force exhibiting itself as a surface-tension, from 

 the action of which many important consequences follow, greatly 

 affecting the form of the fluid surface. 



While the protoplasmj of the Amoeba reacts to the shghtest 

 pressure, and tends to "flow," and while we therefore speak of it 



* With the special and impprtant properties of colloidal matter we are, for 

 the time being, not concerned. 



t Whether an animal cell has a membrane, or only a pellicle or zona limitans, 

 was once deemed of great importance, and played a big part in the early contro- 

 versies between the cell-theory of Schwann and the protoplasma-theory of Max 

 Schultze and others, Dujardin came near the truth when he said, somewhat 

 naively, "en niant la presence d'un tegument propre, je ne pretends pas du tout 

 nier i'existence d'une surface." 



% The word protoplasm is used here in its most general sense, as vaguely as when 

 Huxley spoke of it as the "physical basis of life." Its many changes and shades 

 of meaning in early years are discussed by Van Bambeke in the Bull. Sac. Beige 

 de Microscopie, xxn, pp. 1-16, 1896. 



