MISCELLANEOUS PAPERS. 407 



CAPILLARITY. 



Water is the vehicle by which mineral foods of all necessary kinds 

 are carried even to the tops of the tallest trees. There is a power 

 connected with water that is greater within certain limits than the at- 

 traction of gravitation. That is capillarity, or, as it is often called, 

 capillary attraction. This depends on two powers — first, adhesion, 

 or the attraction of wood or vegetable fiber for water ; second, cohe- 

 sion, or the attraction of the particles of water for each other, so that 

 the surface never becomes broken. Every housewife knows that if a 

 wet cloth be left hanging over the edge of a tub the water will creep 

 over and run out. In the case of the cloth there is a great deal of 

 surface for the water to cling to with little space between the various 

 thread surfaces ; so that in that case capillarity is stronger than gravi- 

 tation. I have seen an old cottonwood stump four feet high, its 

 roots in the water during a freshet, two years after the tree had been 

 cut, with the soured water creeping out over the top of the stump, 

 being fed upon by numerous insects, and the water flowing over in 

 such quantity as to trickle down the sides of the stump after some of 

 it had been evaporated. 



I do not know the limit where there is a balance between gravity 

 and capillarity ; but the diameter of such a capillary tube must be 

 greater than the largest sap duct in wood and much less than half the 

 diameter of a drop of water. The size of the largest capillary tube 

 must depend upon the nature of the material of which the tube is 

 composed and its power of adhesion to water. Of several substances 

 the relative power of attraction for water is as follows, the one having 

 the greatest attraction being first : Salt, wood or carbon fiber, sand, 

 glass, loam, chalk, iron filings, shale, unburnt clay, tin. Clay and 

 shale may even repel water and be impervious to it if they contain 

 oil in any quantity and no sand. A small percentage of salt in earth 

 greatly increases its capacity to admit water. 



The power of capillarity also depends much on temperature. Thus, 

 water that will scarcely enter or flow in capillary tubes just above the 

 freezing-point will flow readily at 50° F., and still more rapidly at 80°. 



STRUCTURE OF WOOD IN TREES. 



The wood of trees is constructed in the form of cells. Wood cells 

 are from a fraction of a millimeter up to several inches in length, and. 

 very small, even microscopic, being 10 to 50 microns in diameter, 

 equivalent to 500 to 2500 transversely to the linear inch. They 

 usually taper to points and lap over so as to break joints ; but some- 

 times they end in the same plane so as to form a joint, in stems, as 

 in grasses and polygonums ; at the ground, as in tumbleweeds ; at the 

 insertion of a leaf, leaflet, stem of a flower, fruit, etc., so as to break 



