Mr. J. Gill on Revelation. 119 



of carbon is far less transparent than glass to the more refran- 

 gible rays of the spectrum ; and lastly, a slight change of tempe- 

 rature in the observing-chamber produces at once currents in the 

 liquid which are fatal to good definition. Nevertheless, under 

 the best conditions, we have found that sulphide-of-carbon 

 prisms define as well or even better than glass. The instrument 

 described in this paper, with the exception of the glass prisms, 

 was made by Messrs. Clark and Sons of Cambridgeport ; and we 

 would here especially express our indebtedness to Mr. George 

 Clark for his great ingenuity in planning and executing the me- 

 chanical details. 



XVI. On Regelation. By J. Gill, Esq* 



PROFESSOR TYND ALL'S ingenious explanation of glacier- 

 motion, in referring it to fracture and regelation of the ice, 

 is satisfactorily proved by his experiments of moulding solid ice 

 into the most varied forms by pressure only, and it seems to be 

 sanctioned by the opinions of Helmholtz and De la Rive ; but 

 various opinions still prevail as to the immediate cause of regela- 

 tion, or the manner in which the action takes place. Tyndall 

 imagines that the particles at the surface of a mass of ice, being 

 free on one side, have greater liberty of motion than the particles 

 in the interior of the mass, and consequently will melt at a lower 

 temperature. He proved by experiment that in ice containing 

 air and water-cells, the ice in the interior of the mass can become 

 liquid by heat which is conducted through the solid substance 

 without dissolving it, but which melts the ice forming the walls 

 of these little chambers where the particles are free on one side, 

 and therefore can liquefy more easily. In this respect the solid 

 particles forming the sides of the cells are in a condition similar 

 to that of the particles on the outside of the block ; and as regards 

 temperature, the interior of the solid mass is supposed to be 

 virtually colder than the surface; that is to say, its particles have 

 less liberty of motion, consequently less motion. When two 

 pieces of ice at 82° with moistened surfaces are placed in contact 

 with each other, their touching portions are, as it were, transferred 

 to the centre of the mass. " Before being brought together the 

 surfaces had the motion of liquidity, but the interior of the ice 

 had not this motion ; and as equilibrium will soon set in between 

 the masses on each side of the liquid film and the film itself, 

 the film will be reduced to- a state of motion inconsistent with 

 liquidity. In other words, it will be frozen, and will cement the 

 two surfaces of ice between which it is enclosed/' — Tyndall. 



* Communicated by the Author. 



K2 



