FLUIDITY. 43 



combined with them. When hammered they become hot from 

 the disengagement of this heat, and at the same time become 

 brittle. Their malleability is restored by heating them again in 

 a furnace. Sugar, it is well known, may exist as a transparent 

 and colourless body, with the physical properties of glass, or as 

 a white and opaque, because a granular or crystalline mass. 

 The transition from the glassy to the granular state is attended 

 by a very remarkable evolution of heat, which appears to have 

 escaped the notice of scientific men. If melted sugar be allowed 

 to cool to about 100, and then, while it is still soft and viscid, 

 be rapidly and frequently extended and doubled up, till at last 

 it consists of threads, the temperature of the mass quickly rises 

 so as to become insupportable to the hand. Applying the 

 thermometer, I found the temperature of a considerable mass 

 to rise from 105 to 175 in less than two minutes. After this 

 liberation of heat, the sugar on again cooling is no longer a glass, 

 but consists of minute grains, and has a pearly lustre. The 

 same change may occur in a gradual manner, as when a clear 

 stick of barley-sugar becomes white and opaque in the atmo- 

 sphere ; but then we have no means of observing the escape of 

 the latent heat on which the change depends. It may be in- 

 ferred that glass itself, like transparent barley-sugar, owes its 

 peculiar constitution and properties to the permanent retention 

 of a certain quantity of latent heat. Of this heat glass can be 

 deprived, by keeping it long in a soft state ; it then becomes 

 granular, and passing into the condition of Reaumur's porcelain, 

 loses all the characters of glass. 



It is not unlikely that the dimorphism of a body, or its pro- 

 perty to assume two different crystalline forms, may likewise 

 depend upon the retention of a certain quantity of latent heat 

 by the body in the one form and not in the other. Thus sulphur 

 assumes two forms, one on cooling from a state of fusion by 

 heat, another in crystallizing at a lower temperature, and pro- 

 bably with the retention of less latent heat, from a solution of 

 sulphuret of carbon ; in charcoal and plumbago, again, we have 

 carbon which has assumed the solid form at a high temperature, 

 and possibly with the fixation of a quantity of latent heat which 

 does not exist in the diamond, another form of the same body. 



When a solid body is melted by the intervention of some 

 affinity, without heat being applied to it, cold is generally pro- 

 duced. Thus most salts occasion a reduction of temperature. 



