GENERA LIZA TIOX. 257 



Light and heat are conveyed by it with equal velocity in 

 all directions, and in all parts of space so far as observa 

 tion informs us. But the presence of gravitating matter 

 modifies the density and mechanical properties of the 

 so-called ether in a way which is yet quite unexplained. 



Leaving gravity, it is somewhat difficult to discover 

 other laws which are equally true of all matter. Boer- 

 haave was considered to have established that all bodies 

 expand by heat, but not only is the expansion very dif 

 ferent in different substances, but we now know positive 

 exceptions. Many liquids and a few solids contract by 

 heat at certain temperatures. There are indeed other 

 relations of heat to matter which seem to be universal 

 and uniform ; thus all substances begin to give off rays of 

 heat or light at the same temperature, according to the 

 law of Draper ; and gases will not be an exception if 

 sufficiently condensed, as in the experiments of Frank- 

 land. Grove considers it to be universally true that all 

 bodies in combining produce heat ; all solids, with the 

 doubtful exception of sulphur and selenium, in becoming 

 liquid, and all liquids in becoming gases, absorb a certain 

 quantity of heat ; but the quantities of heat absorbed 

 vary with the chemical qualities of the matter. On the 

 other hand, Carnot s Thermodynamic Law is held to be 

 exactly true of all matter without distinction ; it ex 

 presses the fact that the amount of mechanical energy 

 which miolit be theoretically obtained from a certain 



O V 



amount of heat energy depends only upon the tempera 

 tures between which a substance is made to change, so 

 that whether an engine be worked by water, air, alcohol, 

 ammonia, or any other substance, the result would theo 

 retically be the same, if the boiler and condenser were 

 employed at similar temperatures. 



VOL. II. 



