GENERAL LAW OF THESE MOVEMENTS. 



29 



as may be proved by examining them with an electrometer. Even in those cases in 

 which attractive forces are developed without an apparent electric disturbance, these 

 principles apply- Two leaden bullets, which have been brought (Ap., 12G) into con- 

 tact with one another, cohere strongly, but we are not able to show the development 

 of the separate electricities on each, because of their high conducting power. So, too, 

 when a piece of glass is laid on the surface of some water, and, on being lifted off, is 

 carried to the electroscope, no development can be detected, and the reason is obvious, 

 for there has been only an apparent, and not a true separation of the liquid and the 

 glass from each other ; the particles of the liquid have been simply torn apart, and have 

 not been separated from the glass. 



87. Referring, therefore, to that chapter for the proof that capillary attraction origi- 

 nates in electric disturbance, it is sufficient for our present purpose to remark, that it is 

 therefore due to the very same cause as chemical affinity itself. The quality which 

 liquids possess of wetting or not wetting the surface of solid bodies is, therefore, nothing 

 more than an indication of the affinity which is between them. Quicksilver will not 

 wet glass, because they have little affinity, but it will wet a surface of gold or of tin with 

 facility, because its affinity for those bodies is energetic. These observations, which 

 appear so simple, have very important applications ; the intensity of affinity between a 

 given liquid and a solid with which it is brought in contact determines their capillary 

 relations, and thereby determines, also, the phenomena of movement. No farther proof 

 of this importance is, perhaps, required than the result to which we shall be presently 

 led, that even in MAN the circulation of the Wood is caused by the oxydating action of that 

 liquid on the solid structures with which it is brought in contact. 



88. Let us, therefore, finally remember that the explanation of the circulation of nu- 

 tritious juices, both in the vegetable and the animal kingdom, rests upon this simple 

 physical principle, that if two liquids communicate with one another in a capillary tube, 

 or in a porous or parenchymatous structure, and have for that tube or structure different 

 chemical affinities, movement will ensue ; that liquid which has the most energetic affin- 

 ity will move with the greatest velocity, and may even drive the other Jluid entirely be- 

 fore it ; that this is due to common capillary attraction, ichich, in its turn, is due to 

 electric excitement. 



89. These things being understood, let us proceed now to apply our principle (84) 

 to the cases in hand, and commence with giving the theory of the flow of sap in plants. 



90. The liquid of which the ascending sap is constituted is derived from the ground 

 by the action of the spongioles, and consists of water holding in solution the different 

 saline bodies which are necessary to the plant, along with carbonic acid, &c. This 

 compound fluid passes upward by the woody fibre and ducts of the alburnum, making 

 its way to the leaf, on the upper surface of which, in common cases, a change in its 

 chemical constitution occurs through the influence of the sunlight. It obtains a cer- 

 tain quantity of carbon, and from being a thin watery solution, becomes much concen- 

 trated, and gains the under face of the leaf. This elaborated sap, or latex, as it is fre- 

 quently called, returns now to the bark, and descends through its cellular tissue and 

 inter-cellular spaces, finding its way by the route of the medullary rays to all parts of 



