474 PHYSIOLOGICAL PHYSICS. [Chap, xxxvn. 



is supposed to be due to the force of adhesion exer- 

 cised by the particles of the vessel on the water. In 

 a glass vessel the boiling point is usually 101 C., in 

 one of iron 100 C. The introduction, however, of 

 fragments of platinum into a glass vessel will lower 

 the boiling point to 100. 



Pressure influences the melting and boiling points, 

 an increase of pressure raising both temperatures. 

 The effect on the boiling point is very marked. A 

 fall of pressure from the normal 760 rnin. of mercury to 

 730 lowers the temperature of the boiling point by 1. 



Evaporation. Yapour is given off' by liquids 

 and solids through a wide range of temperature ; and 

 the evaporation is accompanied by the disappearance 

 of heat ; but, as in the case of ebullition, evaporation 

 occurs only from the surface of the body. It is by 

 evaporation that the volume of a liquid exposed to 

 the air gradually diminishes. Some liquids pass into 

 vapour more quickly than others, alcohol more 

 quickly than water, and ether than alcohol, the 

 abstraction of heat being more marked as the rapidity 

 of evaporation is greater. The vapour of ether is 

 thus used to produce a freezing temperature. This 

 fact is applied in surgery for producing local anaes- 

 thesia. By means of a spray producer a fine spray of 

 ether is directed on the part of the body to be rendered 

 insensible. Its rapid evaporation abstracts the heat 

 from the part to such an extent, that intense local 

 cold is produced, accompanied by loss of sensibility in 

 the part, so that an incision may be made without 

 causing pain. A similar method is employed in his- 

 tology, for cutting sections of tissues. The tissue is 

 placed on a metal plate on the under surface of which 

 a current of ether spray is directed. The plate and 

 the tissue it supports are thus reduced to the freezing 

 temperature, and sections of the tissue may then be 

 readily cut in the frozen condition. 



