CHANGE OF STATE LIQUID VAPOUR. 165 



On high mountain levels the low atmospheric pressure produces a 

 serious lowering of the boiling-point, that on the summit of Mt. Blanc 

 being about 84 C. At such a temperature, boiling water has not its 

 usual cooking qualities. 



On the other hand, increase of pressure is accompanied by rise of 

 boiling-point. In high pressure steam-boilers, the boiling-point rises 

 many degrees above the normal. This rise of boiling-point is employed 

 to obtain the gelatinous matter from bones, water above 100 dis- 

 solving this matter more freely than water at or below 100. The 

 bones are, therefore, heated in a closed vessel of water, fitted with a 

 safety-valve, so that the water is exposed to the pressure of its own 

 vapour, and this increases as the temperature rises. There is therefore 

 no definite boiling-point short of that corresponding to the pressure at 

 which the safety-valve is forced open. Such an arrangement is called a 

 Papin's digester from its inventor, the discoverer of the rise of boiling- 

 point with pressure. 



Delayed Boiling. When boiling has been going on for some time 

 in a glass vessel, the temperature of the water begins to rise above the 

 normal boiling-point, and at the same time the steam is given off in larger 

 bubbles and from fewei points. If the vessel be very clean (it may be 

 cleaned by rinsing with hydrofluoric acid and then with water) and the 

 water very pure, by alternate boiling and cooling it is quite easy to raise 

 the temperature of the water ultimately to 105 or 106 0. With still 

 greater precautions, the temperature may be raised many degrees higher. 

 In this state when boiling does occur it is almost explosive in its character, 

 and the phenomenon is termed " boiling with bumping." On putting 

 into the vessel sand, iron filings, or any rough material containing 

 crevices or air bubbles, boiling again becomes normal at the normal 

 temperature. 



A still more striking experiment was made by Dufour, who prepared 

 a mixture of oils of about the density of water and of high boiling-point, 

 and' then placed in it small drops of water, which remained suspended. 

 They could be heated without boiling to at least 178 0. 



The observation that normal boiling is resumed when air bubbles are 

 present gives the clue to the nature of boiling. It is probably always 

 associated with the presence of bubbles or cavities. 1 



We have seen that as the temperature rises towards the boiling- 

 point, the dissolved gases are expelled, and collect in bubbles. As these 



are detached, each bubble probably leaves a 



small portion behind, just as a drop of water in "\^/ 7T ^-^ 

 breaking off from a surface and falling down, 



always forms a neck at which rupture takes FIQ. 91. Formation of a 

 place and leaves behind the part above the neck drop by pinching in at 

 as in Fig. 91. These minute bubbles do not a neck, 

 grow so largely as before, since most of the dis- 

 solved gas has been expelled in their first formation. Evaporation, 

 however, goes on into them, and their size will be such that the vapour- 

 pressure for the existing temperature + the pressure of gas or air in 



1 An excellent account of the phenomenon of melting and boiling, with experi- 

 ments illustrating the explanation, is given by Aitken in the Transactions of the Royal 

 Scottish Society of Arts, vol. ix, 1874-75. 



