On Capillary Adion. I97 



Tr two glafs plates, held near each other and parallel, be 

 dipped in water, the water will mount in the included fpace. 

 The aqueous protuberance is now confined, and the afcent of 

 the column is therefore greater; befides that the effect is 

 doubled by the united adion of both oppofite furfaces. Each 

 furface acls only upon a thin film ; but, fincc the force is 

 fpent in fupporting the particles which adhere to this, the 

 height of the column muft evidently be inverfely as the weight 

 fufpended, or the diftance between the glafs plates. Imagine 

 the interval between thofe plates to be the looth part of an 

 inch : then each furface may be confidered as ailing againd 

 9. column of the thicknefs of the 300th part of an inch ; and 

 fince the force of attraftion is equivalent to the weight of tlic 

 looth part of a cubic inch, the correfponding afcenl muft be 

 two inches. In like manner, if the interval were only the 

 200th part of an inch, the height of the column muft be 

 four inches. In general, put d = the diftance between the 



plates, and , = the height of the column. 



* 50.^ *= 



The fame reafoning is applicable in the cafe of capillary 

 tubes. The attra6lion of the internal furface is exerted on 

 a thin circular lining; but this force is diluted and attenu- 

 ated by the prefi'ure of the water which adheres to tlie film 

 and occupies the cavity of the tube. A circle, it is well 

 Jcnown, is equal to a rectangle which has the circumference 

 for its bafe and iialf the radius for its altitude: confequenlly 

 the altrattive power of the glafs will produce the fame eft'e£t 

 as if it aftcd fimjily againft a colunm whole thicknefs is one- 

 fourth part of the bore of the tube. But we have already feen 

 that the meafure of force is expreffed by y^tj ; and hence, if 

 d denote the width of the bore, the height at which the water 



will be fufpended in the tube will be z=. —, = — -. 



' 100 X ^d 25 J. 



Thus the altitude of fufpenfion in capillary tubes is the double 



of what obtains with parallel plates whofe mutual diftance is 



the diameter of tiie bore. 



The fufpenfion of water in capillary tubes muft depend 



entirely on the fmallncls of the iuperior orilice. Nor will 



the cfl'ect l)e in any decree altered, however much the lower 



part of (he lube be enlarged : for, by the laws of fluids, the 



preffure is proportional merely to the altitude of the colunm ; 



and this preftiire is balanced at the upper cxtremitv bv the 



adhefive force of the lilm, which attaches itfelf to the irfibc 



■•f the lube. 



N3 If 



