jQiy COHESION*- OF FttJIDS, 



Cohesive at- m«ans becR Sufficiently confirmed to afford an objection to a'ny 

 soiiclr" d^ theory. The principle, which has been laid down respecting 

 fluids, the contractile powers of the common surface of a solid and a 



ftuidj is confirmed by an observation which I have made on 

 the small drops of oil which form themselves on water. Theru 

 is no doubt but that this cohesion is in some measure indepen- 

 dent of the chemical affinities of the substances concerned : 

 tallow when solid has a very evident attraction for the water 

 out of which it is raised ; and the same attraction must operats 

 upon an unctuous fluid to cause it to spread on water, the 

 iluidity of the water allowing this powerful agent to exert itself 

 with an unresisted velocity. An oil which has thus been spread 

 is afterwards collected, by some irregularity of attraction, into 

 thin drops, which the slightest agitation again dissipates : their 

 surface forms a very regular curve, which terminates abruptly 

 in a surface perfectly horizontal : now it follows from the lawj 

 I hydrostatics, that the lower surface of these drops must consti- 

 tute a curve, of which the extreme inclination to the horizon is 

 to the inclination of the upper surface as the specific gravity 

 of the oil to the difference between its specific gravity and that 

 ©f water: consequently since the contractile forces are held in 

 equilibrium by a force which is perfectly horizontal, their mag- 

 nitude must be in the ratio that has been already assigned : and 

 it may be assumed as consonant both to theory and to obser- 

 tatior^, that the contractile force of the common surface of two 

 substances, is proportional, other things being equal, to the dif- 

 ference of their densities. Hence, in order to explain the ex^ 

 periments of Boyle on the effects of a combination of fiuic|s in 

 capillary tubes, or any other experiments of a similar nature, 

 %ve have only to apply the law of an equable tension, of which 

 the magnitude is determined by the diflcrencc of the attractive 

 powers of the fluids. 



1 shall reserve some further illustrations of tliis subject for 

 a work which I have long been preparing for tlie press, and 

 which I flatter myself will contain a clear and simple expla- 

 nation of the most important parts of natural philosophy, t 

 have only thought it right, in the present paper, to lay before 

 the Royal Society, in the shortest possible compass, the, parti* 

 culars of an original investigation, tending to explain some facts 

 and establish some analogies, which have hitherto been obscure 

 and unintelligible, 



X. 



