456 



ice is subjected, the temperature of freezing or melting being lowered 

 as the pressure is increased. My theory on that subject is to be 

 found in a paper by me, entitled " Theoretical Considerations on the 

 Effect of Pressure in Lowering the Freezing Point of Water," pub- 

 lished in the Transactions of the Royal Society of Edinburgh, vol. xiv. 

 part 5, 1849 *. Tt is there inferred that the lowering of the freezing 

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

 centigrade ; and that if the pressure above one atmosphere be denoted 

 in atmospheres as units by n, the lowering of the freezing point, 

 denoted in degrees centigrade by t, will be expressed by the formula 

 = -0075 re. 



The phenomena which I there predicted, in anticipation of direct 

 observations, were afterwards fully established by experiments made 

 by my brother, Professor William Thomson, and described in a paper 

 by him, published in the Proceedings of the Royal Society of Edin- 

 burgh (Feb. 1850) under the title, "The Effect of Pressure in 

 lowering the Freezing Point of Water experimentally demon- 

 strated!." 



The principle of the lowering of the freezing point by pressure 

 being laid down as a basis, I now proceed to offer my explanation, 

 derived from it, of the plasticity of ice at the freezing point as 

 follows : 



If to a mass of ice at centigrade, which may be supposed at the 

 outset 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 cent. 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 

 subjected to squeezing of the compressed mass in which they origi- 

 nate, will spread themselves out through the pores of the general 



* The paper here referred to is also to be found in the Cambridge and Dublin 

 Mathematical Journal for November 1850 (vol. v. p. 248), where it was repub- 

 lished with some slight alterations made by myself. 



f The paper by Prof. William Thomson, here referred to, is also to be found 

 republished in the Philosophical Magazine for August 1850. 



