CHANGE OF STATE LIQUID VAPOUR. 



183 



and the galvanometer remains unaffected. On removing the source of 

 heat, and allowing the plate to cool, a point is reached at which contact 

 takes place, the galvanometer is deflected and the liquid boils violently. 

 When in the spheroidal state, the liquid never reaches the temperature 

 of boiling, the highest temperature for water being at ordinary pressure 

 about 98. The plate must be above 140. 



Perhaps we may give a general explanation of this somewhat as 

 follows : If the two surfaces were both non-volatilising solids, the air 

 between them would tend to get into a steady state at a pressure equal 

 to the atmospheric, and the upper solid would settle down into contact 

 more or less rapidly. But even here some little time would be taken in 

 the adjustment. This may be illustrated by allowing a small, very hot 

 plate of glass to fall flat on a smooth, cold surface, when it moves freely 

 about for a short time, evidently on a cushion of hot air. The layer 

 of air between the two becomes heated, its pressure is increased, and it 

 only slowly escapes out through the 

 narrow space round the edge of the 

 heated glass. The excess of pressure, 

 meanwhile, sustains the weight of 

 the glass. 



In the case of the drop, evapo- 

 ration comes in to maintain the 

 excess of pressure. For by the heat 

 received from the plate the surface 

 of the drop is rapidly heated, and 

 evaporation takes place at the rate 

 corresponding to this higher tem- 

 perature. Let us imagine that the 

 drop is a large flat one, and that it is 

 held in position in some way a short FIG. 106. Experiment to show that in 

 distance above the plate. First the Spheroidal State a Drop is not in 

 suppose both at the same tempera- Contact with the Plate, 

 ture. The space will tend to be 



filled with vapour at the pressure corresponding to that temperature, and 

 evaporation from the drop will be balanced by condensation on to it if 

 we neglect the escape round the edges. Evidently in this case the drop 

 will have to be held up otherwise than by the pressure of the vapour. For 

 at the maximum temperature of the liquid, the boiling-point, the vapour- 

 pressure only equals that of the atmosphere. But now make the plate 

 much hotter than the boiling-point of the liquid. Let us suppose that 

 the pressure of the vapour between it and the drop is still the vapour- 

 pressure at the temperature of the liquid. The contact of the vapour 

 with the hot plate superheats it, i.e. increases the momentum of the 

 molecules, and if they are as a whole still exerting the same pressure on 

 their return to the liquid there must be a diminution in the number 

 returning to compensate the more violent impacts. Hence the number 

 condensing is lessened, or the value of the pressure is not an equilibrium 

 value, since the condensation is not equal to the evaporation. The 

 pressure will, therefore, increase until the escape round the edges balances 

 the excess of evaporation over condensation. Now this pressure will, in 

 general, be above the atmospheric pressxire, for the drop is itself near 



