682 LECTURE LX. 



bled the fictions of poetry much more than the truths of science. He 

 maintained, for example, that ideas existed independently of the human 

 mind, and of the external world, and that they composed beings of different 

 kinds, by their union with an imperfect matter. It is observed by Bacon, 

 in his essay on the opinions of Parmenides, that the most ancient' philo- 

 sophers, Empedocles, Anaxagoras, Anaximenes, Heraclitus, and Demo- 

 critus, submitted their minds to things as they found them ; but that Plato 

 made the world subject to ideas, and Aristotle made even ideas, as well as 

 all other things, subservient to words ; the minds of men beginning to be 

 occupied, in those times, with idle discussions and verbal disputations, and 

 the correct investigation of nature being wholly neglected. Plato enter- 

 tained, 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 col- 

 lection of real information contained in his works on natural history. 

 Aristotle attributed absolute levity to fire, and gravity to the earth, consi- 

 dering air and water as of an intermediate nature. By gravity the ancients 

 appear in general to have understood 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 Plutarch. 



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 mea- 

 sure 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 arrange- 

 ments, all the varieties of natural bodies. He considered both heat and 

 cold as material ; the heat emitted by the sun he thought not absolutely 

 indentical 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 vision. 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 

 observations 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 in Theophrastus, Dioscorides, and Pliny, as well as in 

 some of the historical writers of antiquity. Protagorides of Cyzicum, who 

 is quoted by Athenaeus, relates that in the time of king Antiochus, it was 

 usual, as a luxury, to cool water by evaporation ; and it is not impossible 

 that the custom may have been introduced from the east, where even ice 



