132 ICE AND GLACIERS. 



In order to show the slow action of the small differences of tempera- 

 ture 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 the atmosphere. As the water thus prepared can be cooled con- 

 siderably below C. before the first ice is formed, while when ice is 

 in the flask it freezes at C., the flask was in the first* instance 

 placed in a freezing mixure 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 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 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 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 therefore be formed more 

 rapidly in the surrounding water than it was in the flask, where the 

 side of the glass was interposed between the ice and the water. 

 Even with a much smaller weight the same result will follow in the 

 course of an hour. The broader the bridges become, owing to the 

 freshly formed ice, the greater will be the surfaces over which the 

 pressure exerted by the upper piece of ice is distributed, and the 



