only be that the surface takes the smallest area possible to it, 

 and resists any attempt to make it larger. In other words, it 

 behaves as if stretched. 



It^will also be clear that the molecules at the surfaces where 

 any unlike substances touch one another are similarly exposed to 

 forces different from those in the interior of the substances. 



For the present, we are only concerned with that aspect of 

 dissimilar forces at the surface which results in surface tension, 

 a phenomenon which we can only detect when the molecules are 

 free to move, but unable to get away from the influence of their 

 neighbours ; that is, at the contact surface of liquids with gases 

 or other liquids. We can detect it indirectly at the contact of 

 liquids with solids, and there must also be related phenomena at 

 the contact of solids with each other and with gases/ It will be 

 clear that there cannot be anything of the kind with gases them- 

 selves, because their molecules are completely free to wander 

 away into the interior of both, so that gases in contact always 

 mix up together. In other words, if gases form a part of any 

 heterogeneous system, they can only form one phase, however 

 many different chemical species this phase may be composed of. 



The body-substance of an amceba, as is easily seen, does not 

 mix up with the water in which it lives. It forms a separate phase, 

 just as oil and water form distinct phases. But we know that 

 protoplasm consists largely of water, as can be seen when it dries 

 up and returns to life again when moistened, as sometimes happens. 

 Moreover, chemical analysis shows it to contain So per cent, pr 

 more of water. We have seen that it is a liquid, so that it must 

 be a solution of various things in water, and it contains also other 

 things floating in it. If an amceba is killed by a strong electric 

 shock (E., p. 165), its protoplasm is dissolved up and disintegrated 

 by the water around it. Why, then, does it not mix with water 

 in normal conditions?/ It must be surrounded by some kind of 

 a layer that protects it'. We have to find Out how such a layer or 

 film, sometimes called the " plasma- or cell-membrane," is produced, 

 taking into account the fact that it is not a permanent rigid case, 

 like the cell wall of a plant or the shell of an egg. This is obvious 

 enough from observation of a pseudopodium. As it is formed 

 and increases in size, there is no mixing of its substance with the 

 water. Hence the membrane must be continually being pro- 

 duced at the contact between water and protoplasm. We shall 

 see later, moreover, that a dye, such as aniline blue or congo red, 

 is unable to pass through the cell-membrane, and that it is equally 

 unable to pass into a bit of protoplasm cut off from the main 

 mass, although, when either is killed in any way, the dye freely 

 enters. 



