Prof. Tyndall on Ice and Glaciers. 399 



explanation before you which I consider to be the most satis- 

 factory." 



In the Appendix, Professor Helmholtz returns to the subject 

 thus handled in the body of his discourse. " The theory of the 

 regelation of ice/' he observes, " has given rise to a scientific 

 discussion between Faraday and Tyndall on the one hand, and 

 James and William Thomson on the other. In the text of this 

 lecture I have adopted the theory of the latter, and have there- 

 fore to justify myself for so doing/' He then analyzes the rea- 

 sonings on both sides, points out the theoretic difficulties of 

 Faraday's explanation, shows what a small pressure can accom- 

 plish if only sufficient time be granted to it, draws attention to 

 the fact that when one piece of ice is placed upon another the 

 pressure is not distributed over the whole of the two appressed 

 surfaces, but is concentrated on a few points of contact. He 

 also holds with Professor James Thomson, that in an experiment 

 devised by Principal Forbes even the capillary attraction exerted 

 between two plates of ice is sufficient, in due time, to produce 

 regelation. To illustrate the slow action of the small differences 

 of temperature which here come into play Professor Helmholtz 

 made the following experiment, to which reference has been 

 already made. 



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

 water, which was boiled until all the air above it was driven out. 

 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 

 considerably below 0° C. before the first ice is formed, while when 

 ice is in the flask it freezes at 0° C. [why ?], 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 -f-2° C, until the half 

 of it was liquefied. 



" The flask thus half filled with water with a disk of ice 

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

 quite surrounded by the mixture. After an hour the disk 

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

 the disk was liberated, but it froze again as often as the shaking 

 was repeated. The flask was permitted to remain for eight days 

 in the mixture, which was preserved throughout at a tempera- 

 ture 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 pres- 

 sure of the atmosphere slowly melted, the water within the flask 



