CHANGE OF STATE LIQUID VAPOUR. 



163 



Stirrer 



present as well, its chief effect is to lengthen out the process by 

 which the steady state is reached. The air molecules will hinder the 

 evaporation by knocking back into the liquid many of the molecules 

 trying to escape, but they will equally hinder the condensation by get- 

 ting in the way of those returning to the liquid. The two processes 

 being equally interfered with, the same number of molecules must be 

 present in the space for a balance, whether the air is there or not. We 

 may, in fact, compare the effect of the air to a screen perforated with 

 holes and laid upon the surface. Such a screen would equally interfere 

 with both processes, and merely lengthen out the time required to attain 

 the maximum vapour-pressure. 



Boiling. Besides the quiet transformation from liquid to gas, which 

 we have hitherto considered, there is the more rapid conversion which 

 occurs in boiling. If we heat some 

 water in an open glass vessel, so that 

 we can notice what occurs, evapora- 

 tion of course goes on from the top- 

 surface. This evaporation is indicated 

 by the clouds forming where the 

 vapour mixes with the colder air, 

 and condenses through cooling below 

 the temperature at which its pres- 

 sure is the vapour-pressure. As the 

 temperature rises, bubbles, which 

 gradually increase in size, appear on 

 the sides of the vessel. These consist, 

 partly, of gases driven from solution 

 the higher the temperature, the 

 less the quantity of gas which a 

 liquid will dissolve and, partly, of 

 water-vapour, for evaporation takes 

 place into the bubbles. Perhaps 

 these bubbles have been formed by 

 the swelling out of bubbles already 

 existing, but too small to be seen ; 

 or they may have been formed in 

 cavities free from liquid, for however smooth the surface is, there 

 are still probably minute irregularities. These cavities may serve 

 as nuclei into which the expelled gas and the water vapour can 

 pass. As the bubbles increase in size, their upward buoyancy at last 

 detaches them and they float to the top, and with this stage is associated 

 the well-known " singing," which occurs shortly before boiling. Soon 

 afterwards, boiling commences, that is, bubbles of steam form at 

 points on the containing-walls and rise up to the surface. These 

 bubbles always rise from definite points, each point supplying a constant 

 succession of bubbles, and as the boiling continues these points diminish 

 in number. The bubbles are never formed in the middle of the liquid. 

 Even if they appear to be thus formed, closer examination always shows 

 a particle of foreign matter forming a boundary to the liquid. The 

 temperature now ceases to rise and it is found that the boiling-point is 

 that temperature at which the maximum pressure of the vapour is equal 

 to the atmospheric pressure. This may be illustrated by the barometer 



FiG. 88. Vapour Pressure equals 

 Atmospheric Pressure at the Boiling- 

 Point. 



