loS On Capillary A&ion. 



to fpread over the internal cavity of the tube, and, confs- 

 quently, to mount upwards. 



But let us view the fubjeft a little more clofely. Suppofe 

 I put a drop of water upon a horizontal plate of glals : it 

 will quit its globular form, adhere to the glafs, and fpread 

 out till it has covered the plate with a tliin aqueous film. 

 What then is the caufe of this phasnomenon ? It is furely 

 not the mere incumbent weight of the water ; for that would 

 not have been fuflicient even to furmount the mutual adhefion 

 6f the particles of the tiuid, or their natural tendency to ag- 

 globulate. Befides, the fame precife effecl will take place if 

 the drop be applied to the under fide of the plate. The water 

 therefore diftuies itfelf on the glafs in the fame manner as if 

 it were urged by the preffure of a column afting againft that 

 furface. Its attraction to the glafs is equivalent to this fup- 

 pofed prefflire, and is productive of the fame confequences. 

 But why fliould the mere tendency of the water to the furface 

 of the alafs occafion a difpcrlive motion ? The reafon is, that 

 the exTernal particles could not approach without fpreading 

 themfelves and extending the film : and analogy will inrtru(it 

 u?, that the attraction of water to glafs mult increafe in pro- 

 portion to the proximity of its approach, till it has reached 

 the term of clofelt union. 



If the plate be held vertically the aqueous film will ftill 

 adhere, but only to a certain limited elevation, depending on 

 its thicknefs. I reckon the force of fufpenfion corrcfpondinw 

 to each inch of width, or of the extent of the horizontal 

 linear boundary, as equal to the weight of about the icoth 

 part of a cubic inch of water. Hence, if the film was only 

 the joooth part of an inch in thicknefs, it would maintain a 

 beiohtoften inches; but if its thicknefs amounted to the 

 icoth of an inch, it would fubfide to an elevation of one inch. 



Suppofe I now dip the plate perpendicularly in a bafin of 

 water, the film will fufler a very confiderable modification. 

 A new portion of water greedily attaches itfelf to the film, 

 and depvefles it by the load of additional weight. The curve 

 of protuberance feems nearly an inverted parabola, and con- 

 fequenilv the prcfliire to be fuftained is only the third part of 

 what would obtain if the column had been of uniform thick- 

 nefs. The relative force of attraction is therefore = r^-ri 

 and if we conceive the height to be equal to the breadth, 

 which appears nearly the cafe, either of them will be denoted 

 by Vxcr, or a little more than the fixth part of an inch; 

 a conclufion which correfponds fufficiently with" obferva- 



tion. 



If 



