Ice and its Natural History 239 



a solid ; and its melting-point to be the temperature at 

 which it, as a solid, passes into itself as a liquid. 



Under " substance " I understand a single substance, com- 

 pletely defined as a chemical individual. A good example of 

 what I mean is afforded by the substance, of which eighteen 

 parts, by weight, consist of two parts of hydrogen and six- 

 teen parts of oxygen, which have combined with the liberation 

 of a quantity of heat sufficient to raise the temperature of the 

 product by some 2000 C. In chemistry this substance is 

 expressed by the symbol H 2 O. In the solid state it is called 

 ice, in the liquid state water, and in the gaseous state steam. 



If we have a quantity of this substance, partly in the 

 solid and partly in the liquid state, and in such conditions 

 that, if ever so little heat be removed from the mixture, the 

 quantity of ice is increased, and, if ever so little heat be added 

 to the mixture, the quantity of ice is diminished and that of 

 the water correspondingly increased, the temperature of the 

 mixture is the freezing and melting temperature of the sub- 

 stance H 2 O. 



When ice is melting in a mixture of ice and water, im- 

 mersed in a melting-bath, the temperature of the water must 

 be a little higher than that of the ice, else there would be no 

 inducement for heat to pass from the water to the ice ; simi- 

 larly, when ice is being formed in a mixture of ice and water, 

 immersed in a freezing-bath, the temperature of the water 

 must be a little lower than that of the ice produced. There- 

 fore, the observed freezing temperature of water, and melting 

 temperature of ice must always be different, if quite exactly 

 determined. But it is found that, by supplying heat at as 

 low a temperature as possible in the melting experiment, and 

 removing it at as high a temperature as possible in the freezing 

 experiment, the observed melting and freezing temperatures 

 of the substance H 2 O approach each other more and more 

 closely. Therefore, as a matter of experiment alone, it is 

 legitimate to conclude that the limiting values of these 

 temperatures are identical. It is called o on the thermo- 

 metric scales of Reaumur and Celsius, and 32 on that of 

 Fahrenheit. 



