78 Aids to Microscopic Inquiry. 



stances the surface- action becomes intensified. All porous 

 bodies may be regarded as assemblages of capillary tubes, and 

 hence they exert a greater effect. Charcoal, which is porous, 

 absorbs gases with great power, and holds them so tight that 

 they become condensed. Water and many other fluids rise in 

 capillary glass tubes. Mercury is not attracted by, and does 

 not attract, the glass in the same way. Pour water into a 

 narrow tube, and the tube will seem to suck it up at the sides, 

 producing a concave surface. Pour mercury into a similar tube, 

 and the tube will seem to thrust it away, so that its surface 

 will be convex, that is, highest in the middle when it is furthest 

 from the glass. 



When the surface of a capillary tube strongly attracts a 

 given fluid, it cannot run through it easily; but if the repulsion 

 of the fluid particles be increased, it will then flow freely. To 

 show this hang a miniature pail containing water on the prime 

 conductor of an electrical machine. Let a tube proceed from 

 the bottom of the pail, so small in its bore that the water only 

 drips slowly. Then turn the handle of the machine, and note, 

 as the water becomes electrified, how rapidly it runs. 



In passing through a narrow tube any fluid must experience 

 more friction in proportion to its bulk, than the same fluid in 

 larger quantity encounters in traversing a wider tube. Thus, 

 on the ground of friction alone, it is much more difficult to 

 force a fluid through a narrow tube than a wider one. If the 

 tube and the fluid attract each other, the fluid will ascend as 

 if by its own will. If they do not attract each other, much 

 force will be needed to force the fluid on ; and when circulation 

 or other transmission of fluids occurs in the extremely minute 

 vessels which the microscope reveals in thousands of organic 

 bodies, we should consider the nature of the impelling force 

 and the relation the fluid must bear to the sides of the little 

 tubes through which it runs. 



Some fluids will flow through tubes of a given size more 

 readily than others, and if we pass from tubes to what we may 

 regard as an assemblage of tubes — the pores of substances 

 — we shall find that they exercise a physical choice as to what 

 shall go through them and the rate at which the process 

 shall take place.* "When two fluids are separated by a 

 thin porous partition, either organic or inorganic, currents 

 in opposite directions are set up between them, to which the 

 names endosrnose and exosmose are given. These terms, which 

 signify impulse from within and impulse from without, were 

 introduced by Mr. Dutrochet/'t 



* It is not proved that endosrnose and capillary attraction arc exhibitions of 

 the same force ; but many authorities regard them as such, 

 t Ganot's Physics, by Atkinson, p. 87. 



