ICE AND GLACIERS 215 



are first in contact, there will be a greater difference between 

 the temperature of the ice and water, and the bridges will 

 be more rapidly formed, and be of greater extent. 



In order to show the slow action of the small differences 

 of temperature which here come into play, I made the 

 following experiments. 



A glass flask with a drawn-out neck was half filled with 

 water, which was boiled until all the air in the flask was driven 

 out. The neck of the flask was then hermetically sealed. 

 When cooled, the flask was void of air, and the water within 

 it freed from the pressure of tire atmosphere. As the 

 water thus prepared can be cooled considerably below o C. 

 before the first ice is formed, white when ice is in the flask it 

 freezes at o C, the flask was in the first instance placed in a 

 freezing mixture until the water was changed into ice. It 

 was afterwards permitted to melt slowly in a place the temper- 

 ature of which was + 2 C, until the half of it was liquefied. 



The flask thus half filled with water, having a disc of ice; 

 swimming upon it, was placed in a mixture of ice and water, 

 being quite surrounded by the mixture. After an hour, the 

 disc within the flask was frozen to the glass. By shaking 

 the flask the disc was liberated, but it froze again. This 

 occurred as often as the shaking was repeated. 



The flask was permitted to remain for eight days in the 

 mixture, which was kept throughout at a temperature of o 

 C. During this time a number of very regular and sharply 

 defined ice-crystals were formed, and augmented very slowly 

 in size. This is perhaps the best method of obtaining beauti- : 

 fully formed crystals of ice. 



While, therefore, the outer ice which had to support the 

 pressure of the atmosphere slowly melted, the water within 

 the flask, whose freezing-point, on account of a defect of 

 pressure, was 0.0075 C. higher, deposited crystals of ice. 

 The heat abstracted from the water in this operation had, 

 moreover, to pass through the glass of the flask, which, 

 together with the small difference of temperature, explaini 

 the slowness of the freezing process. 



Now as the pressure of one atmosphere on a square 

 millimetre amounts to about ten grammes, a piece of ice 

 weighing ten grammes, which lies upon another and touches 



