64 WATER PASSES THROUGH EXCESSIVELY SMALL PORES. 



suppose a tube of such a length, with respect to its diameter, that a fluid in which it is 

 immersed shall rise to the top of it, and that some extraneous cause shall effect its re- 

 moval as fast as it reaches that position, it is evident that a continuous current will 

 traverse the tube. A case in point is the action of the wick of a lamp, along which 

 the oil continually ascends, because it is removed by chemical decomposition as fast as 

 it reaches the highest point of the capillary system of cotton fibres. Any other cause 

 which would effect its removal in as complete a manner, would equally produce a con- 

 tinuous current. The same explanation applies in the case of water passing through a 

 tissue of bladder to alcohol ; for as soon as the small columns which percolate through 

 that tissue meet with the alcohol, they are removed by uniting chemically with it, and 

 a continuous current, therefore, results. The current of alcohol that takes place in the 

 opposite way meets with a similar fate ; and the excess of one of these currents over 

 the other determines in what direction the hydrostatic level shall change. Even the 

 reputed decompositions brought about by endosmosis are not without very homely and 

 well-known analogues. The greasy wick, when dipped into a lamp containing oil and 

 water, removes the former without disturbing the latter. 



207. It has been shown that, to exhibit the phenomena of endosmosis, pores of a cer- 

 tain size are necessary ; that if their diameter exceed this, the mere leakage masks ev- 

 ery other effect. We might next proceed to investigate what are the actual dimensions 

 demanded. This inquiry is not alone one of mere curiosity, but meets with important 

 applications in every department of physiology ; and the problem, if successfully solved, 

 would cast a great deal of light on the interstitial communications that take place in 

 every part of organic structure. Vessels of an excessive degree of minuteness creep 

 through the finest tissues, which might almost be regarded as formed by the interlacings 

 of these narrow capillary tubes. The immediate apertures of communication between 

 the remote fibrils of the artery, the vein, and the duct of any gland, are of an indescri- 

 bable smallness ; yet, how great a share of the aggregate of the actions of life is car- 

 ried on in such little pores, which are too small for the injection of the anatomist to 

 reach, or even for microscopic vision to descry. 



208. These pores are, however, capable of approximative admeasurement; or, at least, 

 their dimensions may be determined within limits of error. The method by which this 

 can be accomplished essentially depends on the circumstance, that if any fluid will wet 

 two or more solids, it will rise in capillary pipes formed of them identically to the same 

 height, no matter what their chemical constitution may be, provided their diameter is 

 the same. Thus, water will rise in a tube of glass, of serous membrane, or in a straw, 

 to the same height, if the diameters be alike. 



209. It has been stated (199) that water will pass into a chink the width of which 

 is not more than the half of a millionth part of an inch, under the condition that it can 

 wet both faces of the chink. Sir I. NEWTON has shown (Optics, b. ii., p. i.) that if you 

 lay a convex lens of long focus on a glass plane, a series of coloured rings surrounding a 

 central black spot will emerge ; and it is known from simple geometrical principles, that 

 the greatest distance between the two glasses, in any part where the black spot appears, 

 does not exceed the half of a millionth part of an inch. Yet, if a drop of water be 



