6 JNTRODUCTlOX TO GENERAL PHYSIOLOGY 



the molecules among which they move does not increase. On the 

 other hand, the molecules of a solid are not free to move about ; 

 they can only vibrate backwards and forwards about the same 

 mean position. 



The attractive force between molecules is doubtless due to 

 the structure of the atom as consisting of electrically charged 

 smaller constituents, "electrons," with negative charges, moving 

 in various kinds of orbits around a positively charged central 

 body. 



In a liquid, then, the molecules are in constant movement, 

 hitting one another and rebounding. If a solid particle, large 

 enough to be hit by many molecules at the same time, be immersed 

 in water, the resultant force acting upon it will either be zero or 

 very small, because there will be about the same number of hits in 

 one direction as in the opposite one. The mass, again, of a large 

 particle would require to be hit in the same direction by a large 

 number of water molecules at the same time in order to move it. 

 But, if the particle is small, while still large enough to be visible 

 when adequately illuminated, it will be exposed to unequal 

 bombardment in opposite directions, and receive enough impulses 

 to send it moving until it is met by impacts sending it in another 

 direction (p., p. 86). This is Brownian movement, and we see that 

 it is a true representation of the molecular movements in the 

 liquid itself, so that, by looking at it, we get an idea of the way 

 molecules are in movement, as stated by the kinetic theory of 

 gases and liquids. 



Amoeboid Movement and Surface Tension 



We have noticed already how an amceba moves about by 

 means of a local protrusion of a part of its protoplasm and the 

 drawing up of the other part of the organism. The movements of 

 the protoplasm in a plant cell, such as one of those making up the 

 hairs on the stamens of Tradescantia, should also be examined, as 

 showing another form of protoplasmic movement (E., p. 166). 

 Nw, while it would be rash to state that the process can be 

 completely explained in a simple way, there is no doubt that what 

 is known as ' surface tension " plays a large part in it. Since 

 this property has important relations to numerous physiological 

 phenomena, we must give some attention to it. 



First of all, let us convince ourselves by some experiments with 

 a soap bubble or film of soap solution that the film behaves as if 

 it were stretched (E., p. 167). The fact that a drop of oil suspended 

 in a liquid of its own specific gravity, so that it does not rise or fall, 

 takes a spherical shape also serves to show that the surface of the 



