352 Prof. Tyndall on some Physical Properties of Ice. 



effect upon the ice-cylinder was that shown in fig. 5, the sur- 

 faces being developed along that side which had suffered the 

 pressure. 



Fig. 5. 



y////////7/////////////////^ 



Fig. 4. 



V///////////////////AW''' ■ 



• ///y/////My>/>/////^y'/Wi^/// 



54. Sometimes the surfaces commence at the centre of the 

 cylinder. A dim small spot is first observed, which, as the 

 pressure continues, expands until it sometimes embraces the 

 entire transverse section of the cylinder. 



55. On examining these surfaces with a pocket- lens, they ap- 

 peared to me to be composed of very minute water parcels, like 

 what is produced upon a smooth cold surface by the act of 

 breathing. Were they either vacuous plates, or plates filled with 

 air, their aspect would, on optical grounds, be far more vivid 

 than it really was. 



56. A concave mirror was so disposed, that the diffused light 

 of day was thrown full upon the cylinder while under pressure. 

 Observing the expanding surfaces through a lens, they appeared 

 in a state of intense commotion ; this was probably due to the 

 molecular tensions of the little water parcels. This motion fol- 

 lowed closely on the edge of the surface as it advanced through 

 the solid ice. - Once or twice I observed the hazy surfaces pio- 

 neered through the mass by dim offshoots apparently liquid. 

 They constituted a kind of negative crystallization, having the 

 exact form of the crystalline spines and spurs produced by the 

 congelation of water upon a surface of glass. I have no doubt, 

 then, that these surfaces are produced by the liquefaction of the 

 solid in planes perpendicular to the direction of pressure. 



57. The surfaces were developed with great facility where 

 they corresponded to the surfaces of freezing. Wherever the 

 liquid discs before described were observed, the surfaces were 

 always easily developed in the planes of the discs. By care I 

 succeeded, in some cases, in producing similar effects in surfaces 

 at right angles to the planes of the discs, but this was very dif- 

 ficult and uncertain. 



