558 



Dr. R. Norris on the passage of 



[June 15, 



ceivability is no test whatever of possibility. To comprehend these phe- 

 nomena it is necessary to bear in mind the ultimate constitution of the 

 animal membranes, which form alike the capsules of the corpuscles and the 

 parietes of the capillaries*. All the membranes which enter into the 

 animal body may, from a physical point of view, be divided into two orders, 

 — the very fine structureless homogeneous films which must be regarded 

 as simple cohesion-membranes, in contradistinction to the second order of 

 coarser membranes, to which certain mechanical arrangements are super- 

 added, which have the effect of increasing the strength, such, for example, 

 as structure, the result of interlacing fibres ; in films of collodion, gelatine, 

 albumen, india-rubber, and soap we have examples of the first class of mem- 

 brane. It is with this class that we are now concerned, and these are sus- 

 ceptible of two states, the fixed or rigid condition, and the contractile or 

 elastic state, dependent upon the presence of the principle of "flow," 

 which principle may be operative in every shade and degree, from perfect 

 liquidity to absolute rigidity. 



It will be sufficient to state here that the more colloid and plastic those 

 membranes are, or, in other words, the more they approximate in their con- 

 stitution to liquids, so do they proportionately cease to obey exclusively the 

 laws of rigid bodies, and begin to exhibit intermediate properties or quali- 

 ties, some of which belong to solids and others to liquids. 



We may take the soap-film as the best illustration we can find on a 

 large scale of the class of homogeneous cohesion-films, possessing in the 

 greatest perfection this principle of " flow," and as exhibiting to the fullest 

 extent phenomena which I have generalized under the term progressive 

 cohesive attraction f. 



By the study of the soap-film we may acquire a knowledge of many 



* The parietes of the capillaries are held by modern histologists to consist of pro- 

 toplasm, a substance which is universally considered to be of a viscid semiliquid nature, 

 and in which it is easy to demonstrate the presence of the property of flowing within 

 certain limits. 



f The term progressive cohesion is here used in contrast with that operation of cohe- 

 sion which simply maintains contact between the particles of two like or unlike sub- 

 stances or bodies, and which, when it occurs between the particles of unlike bodies, is 

 called adhesion. The attraction of cohesion evidently operates for some distance beyond 

 the atom or particle, so that actual contact is not essential to its display. When two 

 small globes of mercury, or of any other liquid, are made to touch at one point, they 

 merge, as is well known, with great rapidity into each other, and the materials which 

 compose them become arranged around a common centre, that is to say, one larger 

 sphere results. The mode of union of these two spheres is clearly a progressive one, the 

 particles nearest to those in actual contact being the next to come into contact, and so 

 on, until the globes become intimately united. In the presence of gravitation there 

 can be no mass-attraction between the two globules. 



Again, when a solid is partially immersed in a liquid having a cohesive affinity for 

 it, e. g. a sheet of glass, the liquid, as is well known, rises considerably above the water- 

 level. This shows that with unlike bodies the action extends beyond contact, and is 

 progressive in its operation from one line or row of particles to the next above. This 

 term therefore includes all effects of cohesion which arise from and display its opera* 



