EBULLITION. 



thus be made : A flask half-full of water is maintained at ebullition for some 

 minutes. It is removed from the source of heat, corked, inverted, and placed 

 in one of the rings of a retort stand. If cold water is poured on the upturned 

 bottom of the flask, the fluid will start into violent ebullition. The upper 

 portion of the flask is filled with steam, which maintains a certain pressure 

 on the water. By cooling the upper portion of the flask some of this is 

 condensed, and the pressure reduced. The temperature at which water boils 

 varies with the pressure. When it is reduced, water boils at a lower heat. 

 By pouring the cold water over the flask we condense the steam so that the 

 water is hot enough to boil at the reduced pressure. To assert that water 

 boils by the application of cold is a chemical sophism. 



Ebullition and Evaporation may be now considered, and these are the 

 two principal modes by which liquids assume the gaseous condition. The 

 difference is, when water boils we term it ebullition (from the Latin ebullio, I 

 boil) ; evaporation means vapour given out by water not boiling (from 

 cvaporo, I disperse in vapour). 



There are two operations based upon the properties which bodies 

 possess of assuming the form of vapour under the influence of heat, which 

 are called Distillation and Sublimation. These we will consider presently. 



Ebullition then means a bubbling up or boiling; and when water is 

 heated in an open vessel two forces oppose its conversion into vapour ; viz., 

 its own cohesive force and atmospheric pressure. At length, at 212 Fahr., 

 the particles of water have gained by heat a force greater than the opposing 

 forces ; bubbles of vapour rise up from the bottom and go off in vapour. 

 This is ebullition, and at that point the tension of the vapour is equal to the 

 pressure of the atmosphere, for if not, the bubbles would not form. All this 

 time of boiling, notwithstanding any increase of heat, the barometer will not rise 

 above 212 (Fahr.), for all the heat is employed in turning the water to steam. 

 We have said the ebullition takes place at 212 Fahr. (or 1 00 C), but 

 that is only at a certain level. If we ascend 600 feet high we shall find that 

 water will boil at a less temperature ; and on the top of a mountain (say 

 Mont Blanc) water will boil at 185 Fahr. ; so at an elevation of three miles 

 water boils at a temperature less by 27 Fahr. An increase of pressure 

 similarly will raise the boiling point of water. The heights of mountains 

 are often ascertained by noticing the boiling point of water on their summits, 

 the general rule being a fall of one degree for every 530 feet elevation at 

 medium altitudes. We append a few instances taken at random : 



