NATURAL PHILOSOPHY. 267 



the cold winds. This was easily seen as the ice decayed from exposure to 

 the sun or south wind, when it breaks to pieces all small columns, the crys- 

 tals separating each from the other. When ice shrinks and cracks, the 

 edges fall down upon the water, forcing the latter up hetwcen them. This 

 water, in freezing, expands, and then finds the fissure too small to accom- 

 modate its increased bulk, the consequence of which is, that a ridge is 

 thrown up. Th.3 same effect is increased by the subsequent expansion con- 

 sequent on the occurrence of warm weather, crushing uie newly-formed ice 

 to heaps or mounds. 



ON THE PLASTICITY OF ICE. 



Mr. James Thomson, in a paper on the above subject before the British 

 Association, Dublin, commenced by stating that to Professor James Forbes 

 is to be attributed the discovery that the motion of glaciers down their 

 valleys depends on a plastic or viscous quality of the ice. He (Mr. Thom- 

 son) had formed a theory to explain the nature of this plasticity, and the 

 manner in which it originates. He had been led to his speculations on this 

 subject from a previous theoretical deduction at which he had arrived, 

 namely, that the freezing point of water, or the melting point of ice, must 

 vary with the pressure to which the water or the ice is subjected, the tempe- 

 rature of the freezing point being lowered as the pressure is increased. His 

 theorv on that matter led to the conclusion that the lowering of the frcezin^ 



^y o 



point for one additional atmosphere of pressure must be 0'0075 centigrade, 

 and that the lowering of the freezing point corresponding to other pressures 

 must be proportional to the additional pressure above one atmosphere. The 

 phenomena which he thus predicted, in anticipation of direct observations, 

 were afterwards fully established by experiments made by his brother, Prof. 

 William Thomson, of which an account was published in the " Proceedings 

 of the Royal Socieiy of Edinburgh for Feb. 1850." Having thus laid clown 

 as a basis the principle of the lowering of the freezing point of water by 

 pressure, Mr. Thomson proceeded to offer his explanation, derived from it, 

 of the plasticity of ice at the freezing point, as follows : If to a mass of ice at 

 centigrade, which maybe supposed, for the present, to be slightly porous, 

 and to contain small quantities of liquid water diffused through its substance, 

 forces tending to change its form be applied, whatever portions of it may 

 thereby be subjected to compression will instantly have their melting point 

 lowered so as to be below their existing temperature of centigrade-. 

 Melting of those portions will therefore set in throughout their substance, 

 and this will be accompanied by a fall of temperature in them, on account 

 of the cold evolved in the liquefaction. The liquefied portions being sub- 

 jected to squeezing of the compressed mass in which they originate, will 

 spread themselves out through the pores of the general mass, by dispersion 

 from the regions of greatest to those of least fluid pressure. Thus, the fluid 

 pressure is relieved in those portions in which the compression and lique- 

 faction of the ice had set in, accompanied by the lowering of temperature. 

 On the removal of this cause of liquidity, the fluid pressure, namely, the 

 cold, which had been evolved in the compressed parts of the ice and water, 



