ARTIFICIAL IMITATIOSH OF AMFli'Hn .\/0\ FMFST 267 



face layer which is more or less readily permeable to solvents and to 

 substances in solution, and suspended in a fluid of quite different com- 

 position. 



Surface Tension. — Such a drop of fluid suspended in another diirerent fluid 

 obevs well known laws of i)iivsics. Tlie pailicles of each fluid are all under the 

 influence of a verv considerable force, called the cohesion pressure, wincli tends 

 to draw tliem to^nHlier closely. Within the drop each particle is subjected to 

 this force alike from all sides, so that the forces neutralize one anotber, and 

 each particle is as free as if there were no cohesion pressure. But the particles 

 on the surface are subjected to unequal pressure, for that of the fluid outside 

 the drop is difl'erent from that inside, and so the pressure on the surface particles 

 is equal to the diflerence of the cohesion pressure of the two fluids: tins con- 

 stitutes the surface tension. It is this tension that pulls in upon the surface 

 continuallv, causing it to tend always to reduce the free surface to a niinnnum, 

 which condition exists perfectly in the sphere. The amount of cohesion aflinity 

 is yery diflerent in difl'erent fluids, and therefore some haye a high surface 

 tensioii and some a low. When a substance dissohes in another the surface ten- 

 sion is a resultant of the surface tension of the two substances, and hence the 

 surface tension of a liciuid may be raised or lowered by dissolying yarious sub- 

 stances in it. 



ARTIFICIAL IMITATIONS OF AMEBOID MOVEMENT 

 Imagine a drop of fluid suspended in water— let it be a drop of 

 protoplasm, or oil, or mercury; the drop owes its tendency to as- 

 sume a spherical shape to the surface tension, which is pulling the free 

 surface toward the center and acting with the same force on all sides. 

 The result is that the drop is surrounded by what amounts to an 

 elastic, well-stretched membrane, similar to the condition of a thin 

 rubber bag distended with fluid. If at any point in the surface the 

 tension is lessened, while elsewhere it remains the same, of necessity 

 the wall will bulge at this point, the contents will flow into the new 

 space so offered, and the rest of the wall will contract ; hence the drop 

 moves toward the point of lowered surface tension. Conversely, if 

 the tension is increased in one place, the wall at this point will eon- 

 tract with greater force than elsewhere, driving the contents toward 

 the less resistant part of the surface, and the drop will move away 

 from the point of increased tension. The resemblance of these changes 

 of form and the type of motion produced, to ameboid movement, is 

 apparent, and much experimenting has been done to determine how 

 far the processes of motion as shown by ameba? and leucocytes can 

 be reproduced by fluid drops under various conditions of experiment, 

 and to ascertain if such ameboid movement of living cells can be 

 entirely explained by the laws of surface tension. 



Gad'"'- in 1878, pointed out the resemblance to ameboid motion of 

 the changes in shape observed in drops of rancid oils in weak alkaline 

 solution. These changes in shape are due to the formation of soaps 

 which lower the surface tension of the drop in places, so that the 

 fluid flows toward these places and produces pseudopodium-like pro- 

 jections. 



52DuBois Reymond's Arch. f. Physiol., 1S7S, p. ISl. 



