26 Lieut. Hunt on Cohesion of Fluids, 



On Cohesion of Fluids, Evaporation, and Steam-Boiler 

 Explosions. By Lieut. E. B. Hunt, Corps of Engineers, 

 U.S.A.* Communicated by the Author. 



I now wish to present a simple exposition of the mecha- 

 nical theory of cohesion in fluid masses, and from this to de- 

 duce the structure of a fluid surface, shewing that its cohesive 

 strength is much less than that of the interior layers. The 

 result furnishes a clear and direct explanation of the great 

 fact of evaporation, and shews why, in all cases, even in ebul- 

 lition, evaporation is a strictly surface phenomenon. Hence 

 follows an explanation of one of the chief causes of steam- 

 boiler explosions, and the easy suggestion of a very practical 

 remedy; also an explanation of the heating of fluids to high 

 temperatures, 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 a fluid cohesion. 

 Regarding all cohesion as directly a function of the distance 

 between adjacent molecules, it was quite impossible to ima- 

 gine that the exceedingly small difference of the intermolecu- 

 lar distances corresponding to the fluid and solid forms re- 

 spectively in any given substance, could produce that very 

 great difference of cohesive strength so generally conceived 

 to exist. The slight difference of volume, for instance, be- 

 tween a solid and fluid pound of iron, would not lead us to 

 anticipate any marked difference of cohesion, so long as we 

 regard this cohesion as any tolerably simple function of the 

 intermolecular distances. 



The ordinary experiments professing to measure fluid co- 

 hesion, 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 indi- 

 cation of the cohesion in the mass of fluid, but merely shews 



• Read before the American Association for the Advancement of Science, at 

 Cleaveland, Aug. 1853. 



