OV TUE COHESION OF. FLUIDS. 



6'59 



<1ius ; liencc the force of the solid is repre- 

 sented by half the difference between the co- 

 sine and the radius, or by half the versed sine ; 

 •or, if the force of the fluid be represented by 

 the diameter, the whole vei'sed sine will iiidi- 

 ■cate the force of the solid. And the same result 

 follows when the angle of the fluid is acute. 

 Hence we may infer, that if the solid have 

 half the attractive force of the fluid: tiie sur- 

 faces will be perpendicular ; and this seems 

 in itself reasonable, since two rectangular 

 edges of the solid are equally near to the an- 

 gular particles with one of the fluid : and we 

 may expect a fluid to rise and adhere to the 

 surface of every solid more than half as at- 

 tractive as itself; a conclusion which Clai- 

 raut has already inferred, in a difi^erent man- 

 ner, from principles which he has but cur- 

 sorily investigated, in his treatise on the fi- 

 gure of the <;arth. 



The versed sine varies as the square of the 

 sine of half the angle : the force must there- 

 fore be as the square of the height to which 

 the fluid may be elevated in contact with a 

 horizontal surface, or nearly as the square of 

 the number of grains expressing the apparent 

 cohesion. Thus, according to the experiments 

 of Morveau, on the suppositions already pre- 

 mised, we may infer that the mutual attrac- 

 tion of the particles of mercury being unity, 

 that of mercury for gold will be 1. or more, 

 that of silver about .y4, of tin .90, of lead .81, 

 of bismuth .72, of zinc .21, of copper .10, 

 of antimony .08, of iron .07, and of cobalt 

 .0004. The attraction of glass for mercury 

 will be about one sixth of the mutual attrac- 

 tion of the particles of mercury : but when 

 the contact is perfect, it appears to be con- 

 siderably greater. 



Although the whole of this reasoning, on 

 tlic attraction of solids, is to be considered 



rather as an approximation than as a strict 

 demonstration, yet we are amply justified in 

 concluding, that all the phenomena of ca- 

 pillary action may be accurately explained 

 and mathematically demonstrated from the 

 general law of the equable tension of the sur- 

 face of a fluid, together with the considera- 

 tion of the angle of contact appropriate to 

 every combination of a fluid with a solid. 

 Some anomalies, noticed by Musschenbroek 

 and others, respecting in particular the eflfects 

 of tnbes of considerable lengths, have not 

 been considered : but there is great reason to 

 suppose, that cither the want of uniformity 

 in the bore, or some similar inaccuracy, has 

 been the cause of these irregidiuities, which 

 have by no means been sufliciently confirmed 

 to jiftbrd an objection to any theory. The 

 principle, which has been laid down respect- 

 ing the contractile powers of the common 

 surface of a solid and a fluid, is confirmed 

 by an observation which 1 have made on the 

 small drops of oil which form themselves on 

 water. There is no doubt but that this cohe- 

 sion is in some measure independent of the 

 chemical aftinities of the substances con- 

 cerned: tallow, when solid, has a very evident 

 attraction for the water out of which it is 

 raised ; and the same attraction must operate 

 upon an unctuous fluid, to cause it to spread 

 on water, tlie fluiditv of the water aiiowinjj 

 this powerful agent to exert itself with an un- 

 resisted velocity. An oil, which has thus been 

 sprea-d, is afterwards collected, by some irre- 

 gularity of attraction, into l];in drops, which 

 the slightest agitation again dissipaios ^ their 

 surface forms a very regidar curve, whici) 

 terminarcs abruptly in a surface perfectly ho- 

 rizontal : now it follows from the lawa oi' 

 hydrostatics, that the lower surface of these 

 drops must constitute a curvCj of which thr 



