ICE AND GLACIERS. 133 



physics, and which constitute what is known as the 

 'mechanical theory of heat. Among a great number of 

 deductions as to the relations of the diverse natural 

 forces to each other, the principles of the mechanical 

 theory of heat lead to certain conclusions as to the 

 dependence of the freezing-point of water on the pressure 

 to which ice and water are exposed. 



Everyone knows that we determine that one fixed 

 point of our thermometer scale which we call the freez- 

 ing-point or zero, by placing the thermometer in a 

 mixture of pure water and ice. Water, at any rate 

 when in contact with ice, cannot be cooled below zero 

 without itself being converted into ice; ice cannot be 

 heated above the freezing-point without melting. Ice 

 and water can exist in each other's presence at only one 

 temperature, the temperature of zero. 



Now, if we attempt to heat such a mixture by a flame 

 beneath it, the ice melts, but the temperature of the 

 mixture is never raised above that of so long as some 

 of the ice remains unmelted. The heat imparted changes 

 ice at zero into water at zero, but the thermometer in- 

 dicates no increase of temperature. Hence physicists 

 say that heat has become latent, and that water contains 

 a certain quantity of latent heat beyond that of ice at 

 the same temperature. 



On the other hand, when we withdraw more heat from 

 the mixture of ice and water, the water gradually freezes ; 

 but as long as there is still liquid water, the temperature 

 remains at zero. Water at has given up its latent 

 heat, and has become changed into ice at 0. 



Now a glacier is a mass of ice which is everywhere 

 interpenetrated by water, and its internal temperature 

 is therefore everywhere that of the freezing-point. The 

 deeper layers, even of the fields of neve, appear on the 

 heights which occur in our Alpine chain to have every- 



