CHANGE OF STATE LIQUID VAPOUR. 



167 



v volume X 



FIG. 92. Relation between Volume and Pressure 

 of Gas contained in a Bubble in a Liquid. 



with the nucleus left behind, the growth of the new bubble being delayed 

 till the condition for its growth obtains, when there is another sudden 

 swelling out. The intro- 

 duction of sand or iron 

 filings puts an end to this 

 state of delayed boiling, 

 since it presents a large 

 number of air-bubbles as 

 nuclei for evaporation, 

 and then ordinary boiling 

 occurs. 



Stability of Bubbles 



in a Liquid. The stabi- 

 lity of bubbles formed on 



the side of a vessel may be 



discussed by the aid of 



diagrams. The pressure 



within a bubble is that 



of the vapour and that of 



the contained gas. This 



may be represented by 



Fig. 92, the different 



hyperbolas representing 



the relation between pressure and volume for different quantities of 



contained gas. Lifting the curves above the zero pressure a distance 



equal to the vapour pressure, then the relation between the internal 



pressure and the volume 

 of a bubble containing 

 a given quantity of gas 

 will be represented by the 

 abscissa and ordinate of 

 the corresponding curve. 



The external pressure 

 is that of the atmosphere 

 + the hydrostatic pressure 

 due to depth below the 

 surface + that due to sur- 



2T 



face tension - We shall 

 r 



include the hydrostatic in 

 the atmospheric pressure. 

 Supposing the bubble 

 spherical, the latter term 

 FIG. 93.-Relation between Volume and External Aversely proportional 

 Pressure on a Bubble. The surface tension curve to tne cube root ot \ ne 

 slopes down more gradually than the hyperbolas volume. Hence, for in- 

 in Fig. 92. stance, to halve this part 



of the pressure, we in- 

 crease the volume eight times. Then the pressure curve representing 



2T 



slopes down much more gradually than any of the hyperbolas in 



T 



