28 GENERAL LAW OF THESE MOVEMENTS. 



ty of wetting or not wetting the surface of that tube. Of two liquids in a given tube, 

 that will rise highest which will wet the tube most perfectly. And, therefore, we can 

 see in these movements, that if two liquids be placed on opposite sides of a porous sys- 

 tem, or at the opposite ends of a capillary tube, which is wetted by one more perfectly 

 than by the other, that one which exerts the most energetic action will flow fastest. If 

 a piece of bladder be soaked in water and in alcohol, it will be readily seen that the 

 former acts more powerfully on it, giving it greater flexibility and translucency, and 

 having a stronger affinity for its tissues. For these reasons, if a mixture of water and 

 alcohol be tied up in a bladder, as is well known, the water will soak out and evapo- 

 rate away, but the alcohol will be retained. And for the very same reason, when these 

 two liquids are placed on opposite sides of such a porous body, the water moves fast- 

 est through it, as in the experiment of DUTROCHET (81). 



84. From these simple principles we deduce the following important law important, 

 because it gives us at once a clear explanation of the rise of sap in trees, and a beau- 

 tiful exposition of the true cause of the circulation of the blood : When two different 

 liquids are brought in contact in a porous solid, which is wetted by both, but by them, 

 unequally, that one which has the greatest affinity for the solid, or which tcets it most 

 perfectly, will pass most rapidly through it, and may even drive the other entirely be- 

 fore it. 



85. This passage is not accomplished with an insignificant force. Direct experi- 

 ment shows that (Ap., 145) water will thus pass into alcohol through a pervious mem- 

 brane with a force equal to the pressure of nearly two atmospheres. Nor is it alone 

 between liquids that the phenomenon takes place ; it is exhibited also by gases. 

 Here, again, the force with which the movement has been accomplished is surprisingly 

 great. Sulphurous acid will pass (Ap., 158) into atmospheric air against a pressure of 

 one hundred and ten pounds on the square inch, and sulphuretted hydrogen will move 

 through a membrane with a force that is superior to a pressure of twenty-four atmo- 

 spheres (Ap., 162). As a mechanical agent, capillary attraction, therefore, is fully able 

 to overcome any of the resistances which it has to encounter in elevating sap to the 

 tops of the loftiest trees, or driving blood from the remotest parts of the largest ani- 

 mals. 



86. In the Appendix I have given (CHAP. V.) an account of the physical principles 

 on which capillary attraction itself depends. It is needful, therefore, in this place, only 

 to sum up the evidence there brought forward, that it is nothing more than a manifesta- 

 tion of electricity, and that all capillary phenomena are cases of electrical attraction. 

 If we take a piece of flat glass, and place it on the surface of some mercury, the glass is 

 held to the metal with a considerable degree of force, so that it requires some exertion 

 to separate them. When this is done, and the electric condition of the mercury and 

 glass respectively examined, the one is found to be positively and the other negatively 

 electrified. They must therefore attract each other (Ap., 116) ; and it is owing to this 

 electric excitement, which always takes place upon the contact of bodies, that all the 

 phenomena of capillary attraction are due. Thus, if two pieces of plate glass are brought 

 in contact, they are found to cohere, because the one is positive and the other negative, 



