148 ICE AND GLACIERS. 



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 imril 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 the atmosphere. As the water thus pre- 

 pared can be cooled considerably below 0° C. before the first ice 

 is formed, while when ice is in the flask it freezes at 0° 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 temperature 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 0° 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 beautifully 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, explains the slowness of the 

 frerzing process. 



Now as the pressure of one atmosphere on a square milli- 

 metre amounts to about ten grammes, a piece of ice weighing 

 ten grammes, which lies upon another and touches it in three 

 places, the total surface of which is a square millimetre, will 

 produce on these surfaces a pressure of an atmosphere. Ice will 



