PRINCIPLES OF THINGS. 



47 



the changes of corporeal things are to be placed only in the various separa^ 

 tions, and new associations and motions of these permanent particles : com- 

 pound bodies being apt to break, not in the midst of solid particles, but where 

 those particles are laid together, and touch only in a few points." 



The Epicurean doctrine, moreover, of a flux and reflux of elementary par- 

 ticles exterior to every material system, perpetually feeding and replenishing 

 it, and carrying off" its dissolved and rejected rudiments, bears no small re- 

 semblance to the ethereal medium of Sir Isaac Newton ; and, in its law of 

 action, has been singularly revived within the course of the last six years by 

 Professor Leslie, in his principles of impulsion, as detailed in his " Inquiry 

 into the Nature of Heat." It is a doctrine, also, peculiarly coincident with 

 Dr. Herschel's recent theory of nebulae, or milky ways in the heavens, which, 

 contrary to his own earlier opinions, and those of former astronomers, who 

 ascribed such appearance to the mixed light thrown forth from clusters of 

 stars too remote to be reached by the best telescopes, he now resolves, as 

 we shall have occasion to show more minutely in diie time, into masses of a 

 luminous fluid, existing independently of all stars or planets, though origin- 

 ally, perhaps, emitted from them ; aggregated by a variety of causes that 

 tend to give its minute particles unity ; sometimes forming new stars 

 by its condensation, and often feeding and regenerating those that are 

 exhausted. 



Such is a brief survey of the chief theories of the primitive or elementary 

 substance of matter which have been offered in ancient or modern times ; 

 from a combination of the different particles of which, in different modes 

 and proportions, and under the operation of diff'erent laws, all sensible bodies 

 are supposed to have proceeded. 



Of sensible bodies thus produced, some, however, in direct repugnancy to 

 the Atomic philosophy, whether of ancient or more recent times, have been 

 very generally conceived to have been formed first; to be peculiarly simple 

 in their composition, indecomposable by any known powers in their structure, 

 and to be the basis of all other bodies, or those from which all other bodies 

 proceed, by different unions and modifications: and hence such substances 

 have been denominated constituent principles, or constituent elements; concern- 

 ing the kind and number of which, however, we have had almost as many 

 opinions off'ered as concerning the origin and nature of the primitive princi- 

 ples themselves. 



Thus, among both the ancients and the moderns, sometimes fire, some- 

 times air, sometimes earth, and sometimes water, has been considered as the 

 sole constituent element or source of things. Sometimes two of these sub- 

 stances have been thus denominated, and sometimes three; but more gene- 

 rally the whole. Occasionally, indeed, a fifth and even a sixth have been 

 added to the number, as cold and oil, each of these having at times been 

 considered as sim.ple and indecomposable substances ; while, under the old 

 Atomic system, and especially as improved by Epicurus, all such principles 

 were completely swept away, and no one sensible substance whatever was 

 conceived to be better entitled to the character of a constituent principle than 

 another; the whole equally flowing from peculiar modifications and combi- 

 nations of the primitive or elementary principles — the rerum primordia — and 

 equally resolving into them upon decomposition. 



Of these diff'erent theories, the greater number are scarcely worth exa- 

 mining; and I shall only therefore observe, that for that which supposes the 

 existence of four distinct elements, fire, air, earth, and water, and which for 

 ages has been in almost universal acceptation, and would have been so still 

 but for the recent discoveries of chemistry, we are indebted to Empedocles. 

 This celebrated philosopher, and very excellent poet, flourished about four 

 centuries before the Christain era. His opinions, like those of almost all 

 the earliest sages, were given in metre, in a didactic poem, " On Nature," of 

 which only a few fragments have descended to our own times. He was a 

 native of Sicily, and his talents and his country are celebrated by Lucretius, 

 who was, nevertheless, of a very different school of philosophy, in verses so 



