19.2 Prof. Reusch on some Properties of Ice. 



three such centres between the Sun and Mercury, at distances 

 originally yV^h, tV^hs, an d Atlis of that of Mercury. But of 

 these, according to Mr. Drach's calculation, the two first must 

 have been swallowed up by the Sun ; so that we can expect to 

 find one only, namely the third, which, though originally within 

 the limit, would have been driven beyond it by the great expan- 

 sive force arising from the heat generated in the later stages of 

 the process of condensation. It is interesting that M. Leverrier 

 should have recently expressed the belief that such a planet 

 exists, and has been observed, between Mercury and the Sun. 



XXXIII. On some Properties of Ice. 

 By Professor Reusch of Tubingen. 



To John Tyndall, Esq., F.R.S. fyc. 



Tubingen, Jan. 28, 1S64 

 SlR, (Wurtemberg). 



YOUR book has awakened my interest for that wonderful 

 body ice ; and I have devoted the recent cold weather to 

 a more accurate determination of its coefficient of refraction for 

 ordinary and for extraordinary light, as well as to the observa- 

 tion and measurement of the coloured rings of polarized light. 

 In particular your interesting observation upon the layer of snow 

 which sank away from the roof of the greenhouse and curved 

 itself below (page 202), led me to make a direct experiment upon 

 the deformation of ice — an experiment which answered perfectly, 

 and a description of which will perhaps be more interesting to 

 you than to anyone else. 



A plate of clean ice, A B, about 100 

 millims. in length, 12 to 15 millims. in 

 breadth, 3 to 4 millims. in thickness, 

 prepared by sawing and subsequent 

 rubbing with a warm metal plate, is 

 laid in the loops S S of a cotton or 

 silken string which pass above over 

 two pegs, N, N' ; a third loop, T, carries 

 a weight of about 200 grms. The 

 temperature of the circumambient space 

 should not much exceed 4° to 6° C. 

 After the lapse of 20 to 30 minutes 

 a bending may be plainly seen, which 

 is at first regular, but afterwards greater in the middle, in 

 consequence of the middle string be- 

 coming imbedded in the ice, and of the 

 diameter of the plate being diminished. 



