SOME PHYSICAL PROPERTIES OF ICE 627 



MICROSCOPIC EVIDENCES 



The theory that an ice crystal is composed of thin laminae 

 parallel to its basal plane gives rise to the conclusion that when 

 the crystal is bent by force perpendicular to its optic axis, it will 

 show simultaneous extinction at the bent and unbent portions 

 under the petrographic microscope.' Tarr and Rich^ have described 

 the case in which this optical property of the bent bar was not 

 changed, as well as when it was changed. In the latter case the 

 original optic axis was either parallel or perpendicular to the 

 length of the bar, and if we assume the latter as the case, it is in 

 contradiction to the idea that movement along the gliding planes 

 controlled the bending. 



The present writer examined the bent parts of an ice bar and 

 the results were very suggestive, showing facts which seem not 

 to agree with former ideas. A bar of ice with crystals parallel to 

 its length was bent under certain stress. When the bent portion 

 was thinned down and examined under the microscope with the 

 Nicols crossed, it clearly showed an extinction strip across the bar, 

 which moved along the bent portion as the stage was turned. 

 Wanting to be sure about this, the writer asked Professor R. T. 

 Chamberlin to see it and he recognized the same fact with cer- 

 tainty. The test piece in this case consisted of one main crystal 

 with small portions of other crystals on either side. The same 

 fact was observed in one more case but with the oncoming of 

 warm weather these two were the only trials which it was possible 

 to undertake in the present investigation. 



"Examinations without crossed Nicols revealed another impor- 

 tant fact. The test specimen consisted of parallel crystals whose 

 optical axes were horizontally transverse to the length. The bent 

 portion was examined under microscope so as to see the side of 

 the bar, i.e., the basal planes of the crystals. In the field of the 

 microscope, it was found that faint but distinct straight lines 

 nearly parallel to each other had developed on the sections of the 

 crystals. The boundaries of the crystals were zigzag and some- 

 times the straight lines developed in the crystals were observed to 



' R. S. Tarr and O. D. von Engeln, Zeits.f. Glets., Vol. IX (1915), p. iii. 

 = R. S. Tarr and J. L. Rich, Zeits.f. Glets., Vol. VI (1911-12), p. 235. 



