On Capillary JSiion. 303 



of capillary aftion : it niun therefore obtain in other kindred 

 fubftances. I am inclined to refer it to the fame caufe, how- 

 ever apparently fubtle, which has fo forcibly Itruck me in 

 fome other refearches. It is well underftood that phyfical is 

 different from mathematical, contaft. In the former there 

 is a finite interval whofe extent is various, depending on 

 the mutual relation of the bounding furfaces. The degree 

 of approach is greater in fome cafes than in others, or 

 the conta6l is of a more perfect kind. I have proved 

 lately, I think to demonttration, that air is feparated from 

 the polidied furface of metal by a much wider interval than 

 from glafs or other fubftances. The air therefore, charged 

 with its humidity, approaches fo clofe to the glafs, that the 

 latter exerts moft powerfully its fpecific attraction, and de- 

 taches the minute globules of water, i'aper and other vege- 

 table fubftances, though faturated with moifture, and there- 

 fore capable only of difplaying capillary adlion, feem to pof- 

 ftfs the fame property as glafs. 



I confider the power of ahftrafting humidity from the at- 

 mofphere as of important confequence in the vegetable eco- 

 nomy. Plants may thus inhale liquid nutriment, when the 

 ambient air is ftill far from being difpofed to part with its 

 watery ftore. And we have feen that heat promotes the 

 efteft. Hence certain vegetable tribes, without feeling the 

 beneficial influence of rain or dews, are yet capable of ex- 

 iftiug in the fultry arid plains of Arabia. They extraft a 

 fcanty fupply of moifture from the air by a fort of chemical 

 proccfs. Perhaps the organic Urufture of the plant may con- 

 fpire with its phyfical quality in augmenting the efte6l. 



The celerity of the flow of liquids in capillary fvphons de- 

 pends chieily on a circumllancc which, I believe, ' has not 

 hitherto been noticed. If we confider with a little degree of 

 attention the motion of water through a narrow paflagc, we 

 fliall foon be convinced that it is not regular and uniform. The 

 obftrudtion at firft experienced occafions a certain accunuila- 

 tion, which at length overpowers refiftance and cauies a par- 

 tial acceleration. In this manner, a fort of continual re- 

 ciprocating tide is produced in a fcries of pulfes more or !c(s 

 intricate. This curious cfi'eft is diltm(Slly vifibic in artificial 

 jcls-d'eau. But the fame thing may be perceived in every 

 current (jf water. Hence the varving dimpled furface and 

 the gentle murnuir, of brooks. '1 he volume of water conti- 

 nues the fanicj but its contours and ilexurts are inccifantly 

 changing, and loolhe the fancy by exhibiting a jiitlure of ani- 

 niuion. After performing a certain cycle, however, the lame 

 imjiularilies ai^ again repeated 3 and hence the niulical ca- 

 dence 



