Prof. Tyndall on some Physical Properties of Ice. 351 



and when the latter was looked at obliquely, these lines were 

 found to be the sections of dim hazy surfaces which traversed 

 the cylinder, and gave it an appearance closely resembling that 

 of a crystal of gypsum whose planes of cleavage had been forced 

 out of optical contact by some external force. 



Fig. 2 represents the cylinder looked at perpendicular to its 

 axis, and fig. 3 the same cylinder where looked at obliquely. 



Fig. 2. 



Fig. 3. 



52. To ascertain whether the rupture of optical contact which 

 these experiments disclosed was due to the intrusion of air be- 

 tween two separated surfaces of ice, a cylinder of ice, 2 inches 

 long and 1 inch wide, was placed in a copper vessel containing 

 ice-cold water. The ice-cylinder projected half an inch above 

 the surface of the water. Placing the copper vessel on a slab of 

 wood, and a second slab of wood upon the cylinder of ice, the 

 whole was subjected to pressure. When the hazy surfaces were 

 well developed in the portion of the ice above the water, the 

 cylinder was removed and examined. The planes of rupture 

 extended throughout the entire length of the cylinder, just the 

 same as if it had been squeezed in free air. 



Still the removal of the cylinder from its vessel might be at- 

 tended with the intrusion of air into the fissures. I therefore 

 placed a cylinder of ice, 2 inches long and 1 inch wide, in a stout 

 vessel of glass, which was filled with ice-cold water. Squeezing 

 the whole, as in the last experiment, the surfaces of disconti- 

 nuity were seen forming under the liquid quite as distinctly as 

 in air. 



53. The surfaces are due to compression, and not to any tear- 

 ing asunder of the mass by tension, and they are best developed 

 where the pressure, within the limits of fracture, is a maximum. 

 A cylindrical piece of ice, one of whose ends was not parallel to 

 the other, was placed between slabs of wood and subjected to 

 pressure. Fig. 4 shows the disposition of the experiment. The 



