166 M. A. de la Rive on Glaciers. 



one hand, producing a greater quantity of water than now 

 actually falls in the form of snow upon the high mountains, 

 and on the other, a smaller quantity than at present of the 

 snow melting by the action of the solar and the terrestrial 

 heat, — would necessarily produce an amount of ice more con- 

 siderable than that which constitutes our existing glaciers, and 

 consequently would enormously augment their actual extent. 



Moreover, direct experiment has convinced me that the 

 cold produced by evaporation is more intense where the water, 

 which is undergoing the process instead of forming a surface 

 perfectly liquid and uniform, is commixed with substances 

 which it holds in suspension, or between whose particles 

 it is lodged ;* a phenomenon which is produced by many 

 causes, and which has relation to the influence which the 

 presence of certain substances in the water exercises upon 

 the temperature of its ebullition, — its boiling point. — and in 

 diminishing cohesion. Hence it results that the cold pro- 

 ceeding from the evaporation eflfected upon the surface of 

 uprisen humid deposits, was much greater than that which 

 the same evaporation produced when the water entirely 

 covered the surface. 



I may now direct attention to a particular observation 

 which substantiates what I have said above concerning the 

 influence which prolonged humidity, and the cold thereby re- 

 sulting, exercises upon the extent of glaciers. In the year 

 1815, Professor Pictet ascertained, by precise measurement, 

 the horizontal distance of some of the prominent points of 

 the base of the glacier of the Bosson de Chamouni from cer- 

 tain other fixed points, which were marked by enormous 

 blocks of granite forced into the soil of the lower ground, to- 

 wards which the glacier seemed to be advancing. At the close 

 of the year 1816, the glacier had advanced in all directions, 

 and in particular not less than 50 feet towards one of the 

 marks ; and, at the close of the year 1817, all the marks had 

 disappeared under the glacier. Another glacier, that des 



* The cold produced by the alcarazaa cannot be accounted for, solely from 

 the greater surface presented to the water which is evaporated. There is in this 

 phenomenon a cause analogous to that which makes moist sand refrigerate 

 more under the process of evaporation than pure water. 



