EXPANSION AND CONTRACTION OF SOLIDS BY HEAT. 5 



the glass be heated gradually to the temperature of the water, danger 

 need not be apprehended, because the vessel undergoes a uniform and 

 gradual increase of bulk. 



There are exceptions to the law of uniform expansion by heat with 

 respect to solids. Lavoisier and Laplace, by experiment, showed that 

 tempered steel, from the temperature of 32° to 150°, decreased in expan- 

 sibility ; but they accounted for this exception to the law, by the 

 change in the temperature of tempered steel during the process of 

 annealing, which changed its nature, and converted it into untempered 

 steel. An apparent exception to this law of expansion by heat, mani- 

 fests itself with respect to aluminous clay. When this substance is 

 exposed to intense heat, it contracts, by reason of the extrication of 

 moisture ; it does not belong to the class of ordinary expansion by heat 

 any more than the case of tempered steel, as stated above. These sub- 

 stances, after the change of temperature which they suffer, cease to 

 consist of the constituent parts which originally composed their mass. 

 Exceptions, also, are observed in animal and vegetable matter ; they 

 are accounted for in the same manner as the case of heated clay, viz* 

 by their aqueous parts being dismissed in vapour, causing their volume 

 to contract. Hence it appears that the abstraction of heat from a 

 solid body is equivalent to compression or condensation ; the volume of 

 the substance acted upon being lessened without diminishing its parti- 

 cles of matter, proving that solid bodies are porous. 



The porosity of gold was proved by an experiment instituted at the 

 Accademia del Cimento, in Florence. A hollow ball of gold was filled 

 with water, and the aperture closed. The globe was submitted to pres- 

 sure until its figure was changed, when the water oozed through the 

 pores of the metal, and covered the surface of the globe, having the 

 appearance of dew. The pores of a solid body are occupied by air, 

 provided that the solid is exposed to the atmosphere, and that its pores 

 are larger than the particles of air. This is the case with limestone, 

 charcoal, dried fruit, and many kinds of wood, and particularly a sili- 

 ceous substance called hydrophane ; the proof of which is manifest by 

 plunging each substance under the surface of water, when the air occu- 

 pying their pores will escape at the surface in bubbles. 



The diminution of density produced in solids by the application of 

 heat, and the increase of density by the abstraction of heat, must be 

 evident to all persons who do not look upon objects around them with 

 a passive and indifferent gaze. Expansion and contraction of solids 

 force themselves into notice during our daily vocations, either to frus- 



