'62 Lieut. Hunt on CoJieslon 0/ Fluids. 



great variety of formal phenomena exhibited by drops, bub- 

 bles, and fluid surfaces. 



About four years since, I conceived the idea of directly 

 measuring fluid cohesion by rupturing a pure fluid column 

 in a cylinder with a moving piston. By filling the cylinder 

 with the fluid to be tested, and immersing the piston by the 

 aid of a valve closing at will, the force requisite for starting 

 the piston will be the cohesion of the column, on allowing for 

 atmospheric pressure. Of course, the fluid must adhere to 

 the cylinder more strongly than it coheres in itself, else 

 the adhesion only would be measured. Nor must it contain 

 any air-bubbles, as the presence of one such, however small, 

 will give a start to the break, by presenting a weak surface. 

 This is the great difficulty of the proposed experiment. In 

 May last, I had just begun such an experiment, on mercury, 

 in an amalgamated cylinder, but the requisite precautions 

 for excluding air could not be taken for lack of time, as I was 

 obliged to leave my station before the apparatus was com- 

 plete. The rapidity with which the mercury rushed past 

 the piston, in the rough trials made, shewed that some pack- 

 ing will probably be requisite in a deliberate measurement, 

 and this again will present the difficulty of introducing an 

 unamalgamated surface in the mass to be broken. The pre- 

 cautions requisite for a perfect trial of the experiment are 

 quite numerous. I anticipate that exceedingly small air- 

 bubbles will have the effect of making the indications irregu- 

 lar, as the smallest bubbles will only start a break on the ap- 

 plication of very considerable force. 



I will now apply this discussion to steam-boiler explosions. 

 The condition requisite for ebullition in boiling water is 

 simply that air-bubbles in the heated portions shall present 

 on their boundaries the weakly coherent surfaces requisite 

 for evaporation to be established. Perfectly deaerated water, 

 with a limited surface, would not boil at all, but would 

 steadily heat up until it reached that point at which it would 

 flash explosively into steam. Now, one chief cause of steam- 

 boat explosions is clearly of this description. The boat stops 

 at a wharf; "the doctor," or pump supplying water to the 

 engine, being worked by the engine itself, stops its water 



