NATURAL PHILOSOPHY. 207 



remedy ; also, an explanation of the heating of fluids to high tempera- 

 tures, as observed by Donny, and of the entire agency of contained 

 air in ebullition. 



Several years have now elapsed since, in tracing out the results of a 

 highly general theory of molecular mechanics, it occurred to me to 

 call in question the commonly received views as to the amount and 

 character of fluid cohesion. Regarding all cohesion directly a func- 

 tion of the distance between adjacent molecules, it was quite impossi- 

 ble to imagine that the exceedingly small difference of the intermole- 

 cular distances, corresponding to the fluid and solid forms respectively 

 in any given substance, could produce that very great difference 

 of cohesive strength, so generally conceived to exist. The slight dif- 

 ference of volume, for instance, between a solid and fluid pound of 

 iron, would not lead us to anticipate any marked difference of cohe- 

 sion, so long as we regard this cohesion as any tolerably simple function 

 of the intermolecular distance. 



The ordinary experiments, professing to measure fluid cohesion, are 

 by no means cases of direct rupture, and indeed, furnish no measure 

 whatever of actual cohesive strength. The common experiment of 

 separating, by counterpoising weights, a disc from a fluid which wets 

 it, furnishes no indication of the cohesion in the mass of fluid, but mere- 

 ly shows the force required to break the fluid surface. Donny's ex- 

 periments show positively that the yielding is here entirely at the 

 surface, progressing through the mass by the successive breaking of 

 the successively-formed surfaces, only a mere fluid filament being at 

 last broken by direct rupture. It is truly a case of capillary action 

 between a horizontal fluid surface and a horizontal circular solid sur- 

 face, and like all other capillary action, exists primarily at the surfa- 

 ces only. Except in the frequently-observed adhesion of well-boiled 

 mercury in barometer tubes, to heights far above the true barometric 

 level, we have in fact no record of any experiments exhibiting the re- 

 sistance offered by a fluid mass to direct rupture, which only ought to 

 be taken as a true measure of cohesion. All the common views of a 

 slight fluid cohesion are based on erroneous interpretation, in which 

 the effects of the easy mobility of parts in fluids are very loosely impu- 

 ted to a low value of cohesion. Once clearly understanding that sur- 

 face yielding gives no measure of cohesion or direct resistance to rup- 

 ture, we can readily see that the prevalent ideas on this subject are 

 without support. 



If we study the phenomena attending the condensation of gases 

 and vapors into fluid, it is apparent that while contiguous molecules 

 are still at distances many times as great as that characterizing the 

 fluid state, the cohesive attraction manifests itself appreciably. Steam 

 instantly condensing at the rate of a foot of steam to an inch of water, 

 shows that in water the cohesive action of a molecule extends effective- 

 ly through a sphere whose diameter is at least twelve times- the dis- 

 tance between adjacent molecular centers in the fluid. Hence in wa- 

 ter, the radius of effective cohesive action must be so great as to in- 

 clude several molecular layers. The moment a gas ceases to follow 



19 



