1888.] On the Plasticity of Glacier and other Ice, 347 



Fig. 4. 



the bar be not attached accurately at right angles to the plates, it 

 will take up a vertical position and the plates will be tilted. This 

 contrivance was successful, for the icicle, which owing to its symme- 

 trical formation would probably under uniform tension stretch 

 equally on both sides, showed but small signs of, bending. So we 

 think it fair to conclude that in the later specimens the bending 

 was due to their unsymmetrical structure. 



In the later experiments (6, 8, 9 and 10) the apparatus was en- 

 clossd in a single box of wood about 3 cm. thick. The box was 

 jacketed on the outside with a layer of hay about 5 cm. thick, 

 covered with paper or felt. The cords, leading to the support and 

 the weight, passed through holes in the top and bottom well plugged 

 with cotton- wool. In all cases, except when the contrary is expressly 

 mentioned, the bar of ice was wrapped in gutta-percha tissue to check 

 the evaporation. 



The polariscope was of the simplest possible form. The light 

 transmitted by a sheet of thin paper was reflected at the polarising* 

 angle by a pile of three glass plates towards a Nicol prism supported 

 in the same framework. With its aid it was easy to see the 

 boundaries of the various crystals in a plate or bar of glacier ice, 

 though not a trace of division could be detected with the naked eye r 

 and with some difficulty the direction of the optic axes of a few of 

 the larger crystals could be made out. In the bath ice the homo- 

 geneousness of the crystal could be readily tested, by watching the 

 unchanged position of the rings and cross while the bar was moved 

 across the field. In lake ice a half -inch plate, cut at right angles to the 

 columns and viewed in the polariscope, showed a series of irregular 

 polygons black, white, or grey, when the empty field was black. The 

 almost invariable absence of colour proved that few or none of the 



vol. xliv. 2 D 



