76 Q IT A R T E R L Y J n U R N A L . 



that tlic (laid into which the fine tube is plunged, should be 

 able to wet the tube ; thus water wets a glass tube, and hence 

 rises above its hydrostatic level, but quicksilver will not wet 

 glass, therefore it does not rise. It is not necessary that the in- 

 strument should be in the form of a tube in order that the fluids 

 may be elevated above their hydrostatic* levels j a sponge, the 

 cellular tissue, or any porous body, is, to all intents and purposes, a 

 capillary instrument, capable of lifting water ; even the passages 

 between cells are exceedingly short tubes. 



2. From these facts the following law is deduced, viz: When 

 two fluids are brought together in contact in a porous solid, or se- 

 ries of capillary tubes, which is wetted by both, but unequally, that 

 one which wets the porous solid most, or for which the attraction 

 is strongest, will pass most rapidly through it, and may drive the 

 other entirely before it. To make the law^ plainer, it is observed, 

 that the structure of plants and animals is like a congeries of 

 capillary tubes, particularly in vegetables, the leaf and the 

 spongiole of the root. 



Now for the application of the principles to the circulation of 

 sap. The ascending sap is derived from the ground by the action 

 of the spongioles ; it passes upward by the woody fibre and ducts 

 of the alburnum to the upper surface of the leaf. A change takes 

 place here by sunlight ; it obtains carbon, and forms a thin 

 watery solution, becomes a mucilage; this mucilage being now 

 elaborated sap gains the under surface of the leaf, and returns back 

 through the cellular tissue, finding its way by the medullary rays 

 to all parts of the plant. The descending sap in the spongiole is 

 mucilage, and from this fact is derived the reason why water will 

 enter the spongiole from without. The experimental fact and 

 proof is deduced from the following : Put sweetened water in a 

 bladder and immerse it in simple water, the latter flows in, and the 

 former out. The bladder is a congeries of capillary tubes ; the 

 flowing in and flow-ing out is nothing more than capillary attrac- 

 tion. It is the mucilage or the descending sap which reaches the 

 spongiole which enables the water to enter, and having entered, 

 rises in the stem; So in the leaf, with the mucilage on one side 

 and the water on the other, the latter drives the former before and 

 makes it descend, the tissue of the Icnf havuig a greater attraction 

 for water than for mucilage. 



