Oy THE HISTORY OF TERRESTRIAL PHYSICS. 745 



those times, with idle discussions and verbal disputations, and the correct 

 investigation of nature being wholly neglected. Plato entertained, however, 

 some correct notions respecting the distinction of denser from rarer matter 

 by its greater inertia; and it would be extremely unjust to deny a very high 

 degree of merit to Aristotle's experimental researches, in various parts of 

 natural philosophy, and in particular to the vast collection of real information 

 contained in his works on natural history. Aristotle attributed absolute 

 levity to fire, and gravity to the earth, considering air and water as of an 

 intermediate nature. By gravity the ancients appear in general to have un- 

 derstood a tendency towards the centre of the earth, which they considered 

 as identical with that of the universe; and as long as they entertained this 

 opinion, it was almost impossible that they should suspect the operation of a 

 mutual attraction in all matter, as a cause of gravitation. The first traces 

 of this more correct opinion respecting it are found in the works of Plu- 

 tarch. 



Epicurus appears to have reasoned as justly respecting many particular 

 subjects of natural philosophy, as he did absurdly respecting the origin of the 

 world, and of the animals which inhabit it. He adopted in great measure the 

 principles of Democritus respecting atoms; but attributed to them an innate 

 power of affecting each other's motions, and of declining, in such a manner, 

 as to constitute, by the diversity of their spontaneous arrangements, all the 

 varieties of natural bodies. He considered both heat and cold as material; 

 the heat emitted by the sun he thought not absolutely identical with light, 

 and even went so far as to conjecture that some of the sun's rays might 

 possibly possess the power of heating bodies, and yet not affect the sense of 

 v^ision. In order to explain the phenomena of magnetism, he supposed a 

 current of atoms, passing, in certain directions, through the magnet and 

 through iron, which produced all the effects by their interference with each 

 other. Earthquakes and volcanos he derived from the violent explosions of 

 imprisoned air. 



Among all these opinions and conjectures, there is scarcely any one which 

 was scientifically established upon sure foundations. Some insulated observa- 

 tions had a certain degree of merit; and we find many interesting facts relating 

 to different departments of natural knowledge, not only in Aristotle, but also 



