EBULLITION. 793 



The liquid is then said to i boil/ and the formation of 

 vapour while the liquid boils is usually denoted by the 

 term ' ebullition.' 



As in the case of solids the cohesion is diminished by 

 heat, and at the same time the expansive force of the 

 particles increased, so is the evaporation and ebullition 

 in a liquid body caused by the fact that heat continues 

 to diminish the cohesion and to increase the expansive 

 force until the expansive force considerably exceeds the 

 cohesion, and it will be seen further on that even at 

 common temperatures liquids receive heat during the 

 progress of ordinary evaporation. 



Steam,, the vapour of water, is, like most other gases, 

 perfectly colourless arid transparent and therefore in- 

 visible. This is easily proved by boiling some water in 

 a glass retort or flask with a narrow neck ; the space 

 above the boiling water is after a little time filled with 

 steam, but it appears perfectly clear and transparent. 

 But as the steam issues from the vessel it becomes 

 visible and at the same time ceases to be true steam, 

 being converted into water, which in the form of small 

 drops suspended in the air appears as fog. If a liquid 

 which contains solid bodies in solution be evaporated, 

 the solids are left behind, and since the vapour may by 

 suitable means be condensed and the liquid collected, 

 we have thus a means of separating liquids from im- 

 purities which arise from admixture of solids in solution. 

 This process of evaporation and re -condensation of a 

 liquid is called distillation. The apparatus in which the 

 vapour is condensed, the 'condenser/ consists usually 

 of a tube which is surrounded by a wider vessel filled 

 with cold water ; as this water soon becomes heated by 



