THE MOVEMENTS AND REACTIONS OF AMCEBA. 207 



factors in surface tension, the "surface tension proper" and the 

 " normal pressure." If the normal pressure, directed inward, were 

 decreased in a certain region, while the tangential factor, the "surface 

 tension proper," were not decreased, were perhaps even increased, 

 could not pseudopodia be formed as actually occurs, without any back- 

 ward current on the surface? Jensen seems to lean toward the possi- 

 bility of such action when he speaks of the variation of one of these 

 factors without the other (Jensen 1901, p. 374).* 



But with such an inquiry should we hot leave the field of realities to 

 wander among abstractions ? One who is not a physicist can, of course, 

 not speak positively on such a point. Yet, so far as I am able to dis- 

 cover from the results of experiments and from the theories of surface 

 tension, the state of the case is about as follows : The tangential tension 

 and the normal pressure are not two different things ; they are only 

 different aspects of one and the same thing. Viewed from the stand- 

 point of " energetics," what the physical experiments show liquids to 

 possess is surface energy, in virtue of which they tend to decrease their 

 surface and resist an increase of surface, however these changes are 

 brought about (see Ostwald, 1902, p. 197). When the " surface ten- 

 sion is decreased " in a certain region (x) of a fluid mass, this signifies that 

 the tendency to a decrease of surface and the resistance to an increase 

 of surface is lessened in this region. As a result, the remainder (y) of 

 the fluid decreases its surface at the expense of the region x; the latter 

 is thus compelled to increase its surface. This takes place by simulta- 

 neous passage of the contents of y into x and of the surface of x on to 

 y, the two operations being essentially one, and both having the result 

 of decreasing the surface of y and increasing that of x. There would 

 seem to be no ground, theoretical or experimental, for supposing that 

 in a fluid one of these operations could take place without the other. 

 An attempted explanation of this sort would be, if these considerations 

 are correct, not a physical explanation, but a purely hypothetical one, 

 working with conditions not known to exist. The whole value of the 

 surface tension theory lies in its direct reference back to the results 

 of physical experiments in its fidelity to the results of such experi- 

 ments. As soon as it leaves this ground it becomes of no more value 

 than the thousand and one other hypotheses that have been constructed 

 for the explanation of contractility. 



Further, even this purely hypothetical explanation could not account 

 for the forward currents on the upper surface of Amoaba, nor for the 

 transference of portions of the body surface to the surface of a pseudo- 

 podium. In any form we can give it, the theory that the movement is 

 due to local changes in surface tension is not in agreement with the 

 observed phenomena. 



* Though elsewhere he speaks of the necessity of their varying together. 



