ON THE HISTORY OF TERRESTRIAL PHYSICS. 581 



of a thunderstorm that was passing by. If, however, the fact was such, 

 the experiments must probably have been suggested rather by an accidental 

 discovery of the light on the point of a spear, than by any rational opinions 

 respecting the nature of the ethereal fire. 



Thales is the most ancient of the Grecian philosophers, who appear to 

 have seriously studied the phenomena of nature. He supposed water to be 

 the general principle from which all material things are formed, and into 

 which they are resolved ; an opinion which was without doubt suggested 

 to him by the obvious effects of water in the nutrition of plants and of 

 animals. He particularly noticed the properties of the magnet, which had 

 been before observed to attract iron, as well as the effect of friction in ex- 

 citing the electricity of amber ; and he attributed to both of these substances 

 a certain degree of animation, which he considered as the only original 

 source of motion of any kind. 



Anaximander appears to have paid some attention to meteorology ; 

 he derived the winds from the rarefaction of the air, produced by the 

 operation of heat : thunder and lightning he attributed to the violent 

 explosion or bursting of the clouds, which he seems to have considered as 

 bags, filled with a mixture of wind and water. The same mistaken notion 

 was entertained by Anaximenes, who compared the light attending the 

 explosion, to that which is frequently exhibited by the sea, when struck 

 with an oar. 



Pythagoras, great as he was in some other departments of science, rea- 

 soned respecting physical effects in a manner too mathematical and vision- 

 ary, to allow him much claim to be ranked among those who have studied 

 to investigate the minute operations of nature. 



Anaxagoras was so far from confining himself to the supposition of four 

 elements, which was most generally received by the philosophers of anti- 

 quity, that he imagined the number of elements nearly if not absolutely 

 infinite. He conceived that the ultimate atoms, composing every sub- 

 stance, were of the same kind with that substance, and his system was 

 thence called the homoeomeria ; it erred perhaps less from the truth than 

 many of the more prevalent opinions. Democritus, adopting the senti- 

 ments of Leucippus, proposed a still more correct theory of the consti- 

 tution of matter, supposing it to be ultimately so far homogeneous, that 

 the weight of its atoms was proportional to their bulk. He asserted that 

 the forms of these atoms were different and unalterable ; that they were 

 always in motion, and that besides their primitive difference of form, they 

 were also susceptible of a variety in the mode of their arrangement. The 

 space not occupied by the atoms of matter, he considered as a perfect 

 vacuum. 



As Thales had supposed water to be the first principle of all things, 

 and Anaximenes air, so Heraclitus fixed on fire as the foundation of his 

 system, attributing to it the property of constant motion, and deriving 

 all kinds of grosser matter from its condensation in different degrees. 

 This Doctrine was wholly unsupported by any thing like reason or obser- 

 vation. 



Plato introduced into philosophy a variety of imaginations, which resem- 



