72 Professor Tyndall, [March 4' 



WEEKLY EVENING MEETING, 



Friday, March 4, 1859. 



Sir Roderick I. Murchison, D.C.L. F.R.S. Vice-President, 

 in the Chair. 



John Tyndall, Esq. F.R.S. 



MlOFEfiSOE OF NATURAL PHILOSOPHY, ROYAL INSTITtlTION. 



On the Veined Structure of Glaciers, 



If a transparent colourless solid be reduced to powder, the powder is 

 white: familiar illustrations of this fact were brought forward at the 

 conimencement of the discourse : specimens of transparent rock crystal 

 and of the substance pounded, of solid glass and of glass in powder, of 

 rock salt and common culinary salt were exhibited. A glass jar, par- 

 tially filled with a solution of carbonate of soda, with a little gum 

 added to give it tenacity, presents, on the addition of a little tartaric 

 acid, the appearance of a tall white column of foam. In all these 

 cases, the whiteness and the opacity were due to the intimate and irregu- 

 lar admixture of a solid or a liquid with air ; in like manner the white- 

 ness of snow was due to the mixture of air and transparent particles of 

 ice. 



This snow falls upon mountain eminences, and above the snow 

 line each year leaves a residue ; the substance thus collects in layers, 

 forming masses of great depth. The lower portions of such masses are 

 squeezed by the pressure of the mass above them, and a gradual ap- 

 proach to ice is the consequence. The air is gradually expelled 

 and the transparency of the substance augments in proportion 



Nevertheless, the ice in the upper glacier region always contains a 

 large amount of the air originally entrapped in the snow; this air is 

 now distributed through the solid in the form of bubbles, which give 

 the ice a milky appearance. Thus at the upper part of a glacier the 

 ice is white and more or less opaque ; while, at its lower extremity, as 

 almost every tourist knows, it is blue and transparent. The transition 

 from one state to the other is, in most cases, not a gradual change 

 which takes place equally throughout the entire mass ; the white ice, 

 on the contrary, of the middle glacier region is usually striped by 



