42 FLUIDITY. 



water, indeed, but produce no rise of temperature in the mix- 

 ture above the 32 degrees originally possessed by the snow. 



Dr. Black proved that the heat which disappears in this man- 

 ner is not extinguished or destroyed, but remains latent in the 

 water so long as it is fluid, and is extricated again when it freezes. 



In water that has been cooled below its usual freezing point, 

 when the congelation is once determined, quantities of icy spi- 

 culse are produced in proportion to the depression of tempe- 

 rature, whilst at the same instant the temperature of ice and 

 water starts up to 32. The heat which thus appears was pre- 

 viously latent in that portion of the water which is frozen. The 

 same disengagement of latent heat may be conveniently illus- 

 trated by means of a supersaturated solution of sulphate of soda, 

 formed by dissolving, at a high temperature, three pounds of the 

 salt in two pounds of water. When this liquid is allowed to 

 cool undisturbed and with a few drops of oil on its surface, it 

 remains fluid, although containing a much greater quantity of 

 salt in solution than the water could dissolve at the temperature 

 to which it has fallen. But the suspended congelation of the 

 salt being determined by the introducion of any solid substance 

 into the solution, the temperature then often rises 30 and even 

 40 degrees, while crystals of sulphate of soda shoot rapidly 

 through the liquid. 



Wax_, tallow, sulphur and all other solid bodies are melted 

 in the same manner as water, by the assumption of a certain 

 dose of heat. The latent heat which the following substances 

 possess in the fluid form was, with the exception of water, de- 

 termined by Dr. Irvine. 



Latent heat. 



Water . . . . 140 degrees 



Sulphur . . . . 145 



Lead .... 162 



Bees' wax . . . 175 



Zinc .... 493 



Tin . . . . . 500 



Bismuth . . . . 550 



Even in the solid form certain bodies admit of a variation in 

 their structure and properties from the assumption or loss of 

 latent heat. Dr. Black made it appear probable that metals 

 owe their malleability and ductility to a quantity of latent heat 



