Prof. Tyndall on some Physical Properties of Ice. 335 



ice was pi'eseuted to me, by sending a sunbeam through it, I 

 could tell in an instant the direction in which it had been frozen. 

 Allowing the beam to enter the edge of a plate of ice, and 

 causing the latter to move at right angles to the beam, so that 

 the radiant heat traversed different portions of the ice in suc- 

 cession, when the track of the beam was observed through an 

 eye-glass, the ice, which a moment before was optically conti- 

 nuous, was instantly starred by those lustrous little bubbles, and 

 around each of them the formation and growth of its associated 

 flower could be distinctly observed. 



6. The maximum effect was confined to a space of about an 

 inch from the place at which the beam first struck the ice. In 

 this space the absorption, which resolved the ice into liquid 

 flowers, for the most part took place ; but I have traced the effect 

 to a depth of several inches in large blocks of ice. 



7. At a distance, however, from the point of incidence, the 

 spaces between the flowers became greater; and it was no un- 

 common thing to see flowers developed in planes a quarter of 

 an inch apart, while no change whatever was observed in the ice 

 between these planes. 



8. The pieces of ice experimented on appeared to be quite 

 homogeneous, and their transparency was very perfect. Why 

 then did the substance yield at particular points ? Were they 

 really weak points of crystalline structure ? or did the yielding 

 depend upon the manner in which the calorific wave impinged 

 upon the molecules of the body at these points ? However these 

 and other questions may be answered, the experiments have an 

 important bearing upon the question of absorption. In ice the 

 absorption of the rays which produce the flowers is fitful, and 

 not continuous ; and there is no reason to suppose that in other 

 solids the case is not the same, though their constitution may 

 not be such as to reveal it*. 



I have applied the term " bubbles " to the little bright discs 

 in the middle of the flowers, simply because they resembled the 

 little air-globules entrapped in the ice ; but whether they con- 

 tained air or not could only be decided by experiment. 



9. Pieces of ice were thei'efore prepared, through which the 

 sunbeams were sent so as to develope the flowers in considerable 

 quantity and magnitude. These pieces were then dipped into 

 warm water contained in a glass vessel ; and the effect, when the 

 melting reached the bright spots, was carefully observed through 

 a lens. The moment a liquid connexion was established between 



* Notwithstanding the incomparable diathcrmaneity of the substance, 

 M. Knoblauch finds that, when plates of rock-salt are thick enough, they 

 always exhil)it an elective absorption. Effects like those above described 

 may possibly be the cause of this. 



