606 THE PRINCIPLES OF SCIENCE. [CHAP. 



of space so far as observation 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. 1 



Leaving gravity, it is somewhat difficult to discover 

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

 liaave 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 ; all substances begin to give off rays of 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 Frankland. Grove considers it 

 to be universally true that all bodies in combining produce 

 heat : with the doubtful exception of sulphur and selenium, 

 all solids in becoming liquids, and all liquids in becoming 

 gases, absorb heat ; but the quantities of heat absorbed 

 vary with the chemical qualities of the matter. Carnot's 

 Thermodynamic Law is held to be exactly true of all matter 

 without distinction ; it expresses the fact that the amount 

 of mechanical energy which might be theoretically obtained 

 from a certain amount of heat energy depends only upon 

 the change of the temperatures, so that whether an engine 

 be worked by water, air, alcohol, ammonia, or any other 

 substance, the result would theoretically be the same, if 

 the boiler and condenser were maincained at similar 

 temperatures. 



Variable Properties of Matter. 



I have enumerated some of the few' properties of matter, 

 which are manifested in exactly the same manner by all 

 substances, whatever be their differences of chemical or 

 physical constitution. But by far the greater number of 



1 Professor Loverring has pointed out how obscure and uncertain 

 the ideas of scientific men about this ether are, in his interesting 

 Presidential Address before the American Association at Hartford, 

 1874. Silliman's Journal, October 1874, p. 297. Philosophical 

 Magazine, vol. xlviii p. 493. 



