PHYSICAL PKINCIPLE OF THE CIPtCULATION. 131 



In Chapter VI. is a general statement of the phenomena and laws of 

 capillary attraction ; the principle now to be employed is closely connect- 

 ed therewith. It may be stated as follows : 



If two liquids communicate with one another in a capillary tube, for 

 the substance -of which they have affinities of different intensities, move- 

 ment will ensue : the liquid having the highest affinity will occupy the 

 tube, and may even drive the other before it. The same eifect will en- 

 sue in a porous structure. 



Fig. 51. Thus, let &, #, Fig. 51, be a capillary tube 



of any kind, which is occupied conjointly by 



Motion in a capillary tube. two liquids, a and v, meeting each other in 

 its middle, c; a having a high and v but little affinity for the substance 

 of which the tube consists, a will occupy the tube, pressing out v before 

 it. Of course, it is to be understood that the liquids a and v respect- 

 ively communicate with reservoirs that can furnish them a necessary 

 supply. 



The various phenomena described under the designation of endosmo- 

 sis* are experimental illustrations of the same kind. Thus, A Hed to thc 

 when water is put on one side of a piece of bladder, and al- explanation of 

 cohol on the other, the water, having the highest affinity for ei 

 the substance of which the bladder consists, occupies the pores thereof, 

 and expels the alcohol. Nor would any of the latter substance find its 

 way in the opposite direction, back into the water, were it not so soluble 

 or diffusible in that liquid. Exosmosis therefore takes place through 

 the water, and constitutes a very subordinate or feeble current. 



Now it is precisely relations of this kind that are observed in the case 

 of the circulating and nutritive juices of all organic beings. 



The simplest instance is presented by the fluid contents of certain nu- 

 cleated cells, both among animals and plants, in which a cur- Circulation iu 

 rent moves toward, and then from, the nucleus, coming back nucleated 

 in a returning path. The fluid which the cell contains yields c 

 to the nucleus, in which seems to be concentrated all the activity of the 

 organism, the nutritive material it requires, and, this done, passes on to 

 make way for other portions. The act of nutrition, therefore, is followed 

 by motion, and this upon the above simple principle ; for the liquid, be- 

 fore it approaches to the nucleus, is charged with material which the nu- 

 cleus can attract ; but immediately after contact has taken place, and the 

 material has been removed, the liquid maintains no longer any relation with 

 the nucleus, the affinity or attraction is satisfied, and, so to speak, it loses 

 its hold thereupon, and is pressed off by new-coming portions. Before its 

 approach, and after its departure, the liquid has opposite relations to the 

 nucleus, and in this respect may be regarded as representing two liquids, 

 the one having a high affinity, and the other none, for the nucleus. The 



