42 PHYSICAL SCIENCE 



temperature. The pure substance Ice has a con- 

 stant melting-point. Similarly, if water be cooled 

 at constant pressure, it begins and finishes to 

 freeze at the same temperature. It has a constant 

 freezing-point, identical with the melting-point. 



When water boils, a still larger quantity of 

 heat is absorbed, but the temperature again 

 remains unaltered during the whole process. 

 When the barometer stands at 760 millimetres, 

 or just under 30 inches of mercury, the tempera- 

 ture of the boiling-point is taken as the second 

 fixed point on our thermometers, and called 100° 

 or 212' according as we use the Centigrade or 

 the Fahrenheit scale. If the barometer stands 

 higher or lower than the standard height, the 

 boiling-point of water is found to be above or 

 below 100° C, rising or falling through i°C. for 

 a change of 27 millimetres in the barometer. 

 The freezing-point also depends on the pressure ; 

 but the change is much smaller than in the case 

 of the boiling-point, and delicate experiments are 

 necessary to determine It. 



The variation with pressure of the points of 

 transition from one state of matter to another are 

 connected with the changes of volume which 

 simultaneously occur. Water expands on freez- 

 ing, for ice floats on the surface of a lake, and 

 pipes burst In a frost. If this increase in volume 

 be resisted by an external pressure, as by putting 

 the water Into a strong closed vessel, the act of 

 freezing Involves the performance of external 

 work in forcing outwards the walls of the vessel 

 to give room for the Ice to form. It Is therefore 

 more difficult to produce Ice under pressure, and 

 a greater lowering of temperature Is necessary. 



