ii4 CORRELATION OF PHYSICAL FORCES. 



their peculiar character, as far as a judgment can be formed 

 from their effect on the electric spark, throughout any extent 

 of rarefaction which can experimentally be applied to them : 

 thus the electric spark in protoxide of nitrogen, however 

 attenuated, presents a crimson tint, that in carbonic oxide a 

 greenish tint. 



Without, however, entering on the metaphysical enquiry as 

 to the constitution of matter (or whether the atomic philo- 

 sophers or the followers of Boscovich are right), a problem 

 which probably human appliances will never solve ; and even 

 admitting that an ethereal medium, not absolutely imponder- 

 able, as asserted by many, but of extreme tenuity, pervades 

 matter, still ordinary or non-ethereal matter itself must exer- 

 cise a most important action upon the transmission of light ; 

 and Dr. Young, who opposed the theory of Euler, that light was 

 transmitted by undulations of gross matter itself, as sound is, 

 was afterwards obliged to call to his assistance the vibrations 

 of the ponderable matter of the refracting media, to explain why 

 rays of all colours were not equally refracted, and other diffi- 

 culties. One of his arguments in support of the existence of 

 a permeating ether was, ' that a medium resembling in many 

 properties that which has been denominated Ether does exist, 

 is undeniably proved by the phenomena of electricity.' This 

 seems to me, if I may venture to say so, of anything proceed- 

 ing from so eminent a man, scarcely logical : it is supporting 

 one hypothesis by another, and considering that to be proved 

 which its most strenuous advocates admit to be surrounded 

 by very many difficulties. 



If it be said that there is not sufficient elasticity in ordinary 

 matter for the transmission of undulations with such velocity 

 as light is known to travel, this may be so if the vibrations be 

 supposed exactly analogous to those of sound ; but that mole- 

 cular motion can travel with equal and even greater velocity 

 than light, is shown by the rapidity with which electricity 

 traverses a metal wire, where each particle of metal is undoubt- 

 edly affected. It has, moreover, been shown by the experi- 

 ments of Mr. Latimer Clarke upon a length of wire of 760 

 miles, that whatever be the intensity of electrical currents, 

 they are propagated with the same velocity, provided the 



