I] OF MATTER AND ENERGY 13 



Like other fluid bodies, its surface, whatsoever other substance, 

 gas, Hquid or sohd, 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, which 

 greatly afEect the form of the fluid surface. 



While the protoplasm of the Amoeba reacts to the slightest 

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

 as a fluid, it is evidently far less mobile than such a fluid, for 

 instance, as water, but is rather like treacle in its slow creeping 

 movements as it changes its shape in response to force. Such 

 fluids are said to have a high viscosity, and this viscosity obviously 

 acts in the way of retarding change of form, or in other words 

 of retarding the effects of any disturbing action of force. When 

 the viscous fluid is capable of being drawn out into fine threads, 

 a property in which we know that the material of some Amoebae 

 differs greatly from that of others, we say that the fluid is also 

 viscid, or exhibits viscidity. Again, not by virtue of our Amoeba 

 being liquid, but at the same time in vastly greater measure than if it 

 were a solid (though far less rapidly than if it were a gas), a process 

 of molecular diffusion is constantly going on within its substance, 

 by which its particles interchange their places within the mass, 

 while surrounding fluids, gases and sohds in solution diffuse into 

 and out of it. In so far as the outer wall of the cell is different 

 in character from the interior, whether it be a mere pellicle as 

 in Amoeba or a firm cell-wall as in Protococcus, the diffusion 

 which takes place through this wall is sometimes distinguished 

 under the term osmosis. 



Within the cell, chemical forces are at work, and so also in 

 all probability (to judge by analogy) are electrical forces ; and 

 the organism reacts also to forces from without, that have their 

 origin in chemical, electrical and thermal influences. The pro- 

 cesses of diffusion and of chemical activity within the cell result, 

 by the drawing in of water, salts, and food-material with or without 

 chemical transformation into protoplasm, in growth, and this 

 complex phenomenon we shall usually, without discussing its 

 nature and origin, describe and picture as a force. Indeed we 

 shall manifestly be inclined to use the term growth in two senses. 



