PRINCIPLES OF THiNGfe\ 



35 



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

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



The Epicurean doctrine, moreover, of a fiux and reflux of elementary 

 i particles exterior to every material system, perpetually feeding and re- 

 plenishing it, and carrying off its dissolved and rejected rudiments, bears no 

 small resemblance 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." j It is a doctrine, also, peculiarly 

 coincident with Dr. Herschel's recent theory of nebulee, 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 mi- 

 nutely in due time, into masses of a luminous fluid, existing independently 

 of all stars or planets, though originally, perhaps, emitted from them ; ag- 

 gregated 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 the'ories 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 different laws, ail 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 consti- 

 tuent elements ; concerning the kind and number of which, however, we 

 have had almost as many opinions offered as concerning the origin and 

 nature of the primitive principles themselves. 



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

 sometimes air, sometimes earth, and sometimes water, has been con- * 

 sidered as the sole constituent element or source of things. Sometimes 

 two of these substances have been thus denominated, and sometimes 

 three ; but more generally 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 simple and indecomposable substances : 

 while, under the old Atomic system, and especially as improved by Epi- 

 curus, 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 pe~ 

 culiar modifications and combinations of the primitive or elementary prin- 

 ciples — the RERUM PRiMORDiA — and equally resolving into them upon de- 

 composition. 



Of these different 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 ch^mrstry, we are indebted to Empe- 



