166 HEAT. 



them = atmospheric pressure. Now, when the temperature rises to the 

 point at which the pressure of the vapour equals the atmospheric 

 pressure, there can no longer be equilibrium, since the internal pressure 

 exceeds the external by the pressure of the gas or air in them. The 

 bubbles grow, their buoyancy increases, and finally they break away and 

 float to the top. 



The small portion of each still remaining serves as a fresh nucleus, 

 and the process is repeated indefinitely, as we see from the constant 

 stream of steam-bubbles from the same point in the containing vessel. 

 The heat supplied to the liquid is taken up as the latent heat of the 

 steam formed. 



If the heat is supplied very rapidly the temperature of the liquid 

 tends to rise above the normal boiling-point, and the evaporation into 

 any bubble present tends to increase. Thus the bubble grows rapidly. 

 It is true that evaporation and condensation are always going on together. 

 But suppose that we are considering water at 101. The vapour-pressure 

 at 101 is about 787 mm., and only with that pressure of vapour in a bubble 

 would the evaporation and condensation balance. The growth of the 

 bubble, however, keeps the pressure within down at 760 mm., so that the 

 rate of condensation is hardly affected by the rise of temperature, while 

 the rate of evaporation has grown considerably. The unbalanced evapora.- 

 tion increasing the size of the bubble increases the evaporating surface. 

 Steam is more readily formed, more latent heat is taken up, and if the 

 evaporating surface is sufficient the temperature may be brought down 

 to the normal boiling-point. If, however, the points at which steam is 

 formed decrease in number if there are not sufficient bubbles the 

 steam given off may not be sufficient to carry away all the heat supplied, 

 and the temperature may rise appreciably above the normal boiling-point. 

 In fact, with glass vessels some rise above it almost always takes place. 



The presence in steam bubbles of gas other than steam an observa- 

 tion due to Grove supports this explanation. 



After a time, through the removal of the foreign gases, the portions 

 of the bubbles left behind probably get smaller so small that their 

 surface tension seriously affects the pressure within them. 



The surface tension alone exerts a pressure p = T( - + V where r 



and r' are the principal radii of curvature of the stretched surface, and 

 T is the tension per centimetre. In order, therefore, that a bubble may 

 grow, we must have (the vapour-pressure + the pressure of the contained 

 gas) greater than (p+ the atmospheric pressure), and if p becomes 

 sensible, through the diminution of the residual bubble, the temperature 

 must rise sensibly above the normal boiling-point before this condition 

 will hold. When the bubble once begins to grow, r and ?' increase, and 

 p diminishes, so that the pressure within the bubble diminishes ; but the 

 evaporation into the bubble is still at the rate corresponding to the 

 higher temperature of the liquid, while the condensation is only at the 

 rate corresponding to the diminished pressure, now tending rapidly 

 towards the atmospheric pressure. The bubble, therefore, grows with 

 very great rapidity, almost explosively ; much latent heat is taken up, 

 and the temperature of the liquid falls, though not necessarily to the 

 normal boiling-point. As the bubble rises up, the process is repeated 



