Ancient Extent of the Glaciers of Chamonix, 59 



changed into ice. This transformation takes place in the 

 following manner. Under the heat of the sun's rays, the 

 surface of the snow begins to melt ; the water thus produced 

 infiltrates into the lower beds, which are changed, under the 

 influence of the night frosts, into a granular mass, composed 

 of small flakes still in a state of disaggregation, but more ad- 

 herent than the flakes that produced them. This state of 

 the snow has been named by the Swiss Natural Philosopher 

 neve. Throughout the whole summer, this neve becomes 

 infiltrated with additional quantities of water, always pro- 

 ceeding from the supei^ficial melting, or from that of the 

 neighbouring snows, whose waters have united in the de- 

 pression which forms the cradle of the glacier. In these re- 

 gions, the thermometer falling every night below zero, even 

 in the middle of summer, the neve is frozen by many succes- 

 sive congelations. In consequence of these successive melt- 

 ings and congelations, it presents the appearance of a white 

 compact ice, but filled with an infinity of small spherical or 

 spheroidal bubbles of air ; this is the vescicular ice {glace hul- 

 leuse) of the authors who have written on this subject. The 

 infiltration and congelation of the mass becoming more and 

 more perfect, as the glacier descends towards the inhabited 

 regions, water at length replaces all the air-bubbles ; the 

 transformation is then complete ; the ice appears homogene- 

 ous, and presents those beautiful azure tints which have ex- 

 cited the admiration of travellers. Such, in a few words, is 

 the history of the formation of a glacier ; in reality, it is com- 

 posed, as is seen, of all the layers of snow accumulated dur- 

 ing a long series of years, and which is converted by degrees 

 into ice, more or less compact. 



If the summer heats did not set a limit to the increasing 

 glaciers, they would go on enlarging indefinitely in length 

 and momentum ; but every summer causes a considerable 

 decrease in the thickness of the glacial surface.* This is the 

 phenomenon which M. Agassiz has designated by the name 

 of ablation. At the same time, the lower extremity melts 

 rapidly, and the glacier would diminish every year, if an in- 



* Nearly three metres. 



