Evaporation, and Steam-Boiler Explosions. 27 



the force required to break the fluid surface. Donny's expe- 

 riments shew 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 sur- 

 face and a horizontal solid circular surface, and like all other 

 capillary action exists primarily at the surfaces only. Ex- 

 cept in the frequently observed adhesion of well-boiled mer- 

 cury in barometer tubes, to heights far above the true baro- 

 metric level, we have in fact no record of any experiments 

 exhibiting the resistance offered by a fluid mass to direct rup- 

 ture, which only ought to be taken as a true measure of cohe- 

 sion. All the common views of a slight fluid cohesion are 

 based on erroneous interpretations, in which the effects of 

 the easy mobility of parts in fluids are very loosely imputed 

 to a low value of cohesion. Once clearly understanding that 

 surface yielding gives no measure of cohesion or direct re- 

 sistance in rupture, we can readily see that the prevalent 

 ideas on this subject are without support. 



If we study the phenomenon attending the condensation of 

 gases and vapours into fluids, it is apparent that while con- 

 tiguous 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, shews that in 

 water the cohesive action of a molecule extends effectively 

 through a sphere whose diameter is at least twelve times the 

 distance between adjacent molecular centres in the fluid. 

 Hence in water the radius of effective cohesive action must 

 be so great as to include several molecular layers. The mo- 

 ment a gas ceases to follow Mariotte's law, cohesive action 

 becomes appreciable ; and this is proof enough that in masses 

 many layers contribute their action in making up the total 

 cohesion. 



If we conceive any fluid mass to be distributed into layers, 

 then the correct measure of fluid cohesion will be the force 

 requisite to produce a direct simultaneous separation of all 

 the parts along a unit of the dividing surface between two 



