EFFECTS OF HEAT ON FLUIDS. 421 



were made to touch, respectively the upper and the lower 

 plate, the platinum wire would be heated, and the boiling 

 continued indefinitely in the vacuum of a very excellent air- 

 pump. The effect was very curious : the water did not boil, 

 but at intervals a burst of vapour took place, dashing the 

 water against the sides of the flask, some escaping into the 

 receiver. (There was a projection at the central orifice of 

 the pump plate, to prevent this overflow getting into the 

 exhausting tube.) 



After each sudden burst of vapour the water became per- 

 fectly tranquil, without a symptom of ebullition until the next 

 burst took place. These sudden bursts occurred at measured 

 intervals, so nearly equal in time that, had it not been for the 

 escape from the flask at each burst of a certain portion of 

 water, the apparatus might have served as a timepiece. 



I showed this experiment at one of my lectures at the 

 time, as affording an illustration of the action of the geysers, 

 of which it seemed to me to afford a rational explanation. 

 Supposing a source of heat at a certain depth beneath the 

 earth's surface, and subterranean wells whose only communi- 

 cation with the air was a narrow tube, probably formed by the 

 issue of vapour, the air, if any, dissolved in the water of these 

 wells would be expelled, and the boiling would take place 

 at intervals and by sudden bursts, instead of in the ordinary 

 way. 



This experiment, though instructive, did not definitively 

 answer the question I had proposed, as I could not of course 

 ascertain whether there was some minute residuum of gas 

 which would form the nucleus for each ebullition, and I pro- 

 ceeded with others. A tube of glass, five feet long, and -f^ths 

 inch internal diameter, was bent into a V shape ; into one end 

 a loop of platinum wire was hermetically sealed with great 

 care, and the portion of it in the interior of the tube was pla- 

 tinised. When the tube had been well washed, distilled water, 

 which had been purged of air as before, was poured into it to 

 the depth of eight inehes, and the rest of the tube filled with 

 olive-oil ; when the V was inverted the open end of the tube 

 was placed in a vessel of olive-oil, so that there would be 

 eight inches of water resting on the platinum wire, separated 



