278 PROCEEDINGS OF THE COTTESWOLD CLUB 



In the first place, a liquid is found to arrive at that 

 critical point in its thermal history which we call "the 

 boiling state," at a temperature which depends entirely 

 upon the pressure to which it is subjected. " Boiling" is 

 that condition of a liquid in which bubbles are formed, or 

 capable of being formed, in all parts of the mass, and not 

 merely at the free surface, as in the case of simple evapor- 

 ation. In order that this should occur, the tension or 

 pressure of the vapour must be at least equal to the 

 pressure existing at the place where the bubble is to be 

 formed ; otherwise the cohesion of the molecules of the 

 liquid prevents their disruption. Now, the communication 

 of energy in the form of heat increases the pressure of a 

 vapour, and if we raise the temperature of a liquid exposed 

 to ordinary atmospheric pressure, we eventually reach a 

 point at which the pressure of the vapour becomes as 

 great as the pressure of the air upon it ; and then boiling 

 begins. If we now increase the pressure acting upon the 

 liquid, either by forcing more air into the vessel containing 

 it, or (more simply) by preventing the free escape of the 

 vapour and thus making its pressure act in addition to 

 that of the air, boiling cannot take place at the same 

 temperature as before. But we can cause it to go on 

 again by raising the temperature of the liquid still 

 further, until the pressure of the vapour becomes so 

 much greater that it counterbalances the existing pressure, 

 great as the latter is. If, while the liquid is at this 

 temperature and pressure, the supply of heat is stopped, 

 boiling ceases ; but it can be caused again to take place 

 without supplying more heat, bv simpiv diminishing the 

 the pressure on the liquid. 



E.\periment. — Water, nearly boiling, is [)laced in a globular flask 

 connected with a pressure gauge and having a stopcock opening into 

 the air. Heat is applied until the water boils, the pressure gauge then 

 showing a pressure within the flask equal to the pressure of the 

 external air. The sto]icock is then closed ; the gauge shows increase 



