386 Dynamic Theory. 



interpreted to mean that the molecules of the ( body when In the state 

 of an incandescent vapor, are more alike in form than when the body 

 begins to solidify. 



Kecurring now to the fact stated above, that while sound does not 

 pass through a vacuum both heat and light do, it may be asked, what is 

 the medium through which they are conveyed? In the first instance, 

 the origin of heat and light appears to be the vibratory movement of 

 the molecules of bodies under the excitement of energy transferred to 

 them from some other. The conveyance of this energy through the 

 molecules of ponderable bodies at least the lower forms of it which we 

 call heat is obvious enough. But . it is not obvious how any form of 

 energy passes across a vacuum, or how light passes through a solid 

 body like glass. But the facts being undeniable that they do, there 

 are only two ways of accounting for them. One is that impalpable, im- 

 ponderable, invisible, infinitesimal bodies are emitted and hurled from 

 the luminous bodies with inconceivable rapidity, from one end of the 

 universe to the other, and pass with little hindrance through the pores 

 of transparent solids, such as glass, &c. This is called the emission 

 theory. The other is that all space, including the pores and intermole- 

 cular spaces of all bodies, is permeated with an elastic medium almost 

 incompressible, capable of being set in vibration by the molecular agita- 

 tion of a hot or luminous body, and that the vibrations of this medium 

 when thus set going, propagate themselves in diverging rays until ar- 

 rested by a ponderable body. This medium is called ether, and the the- 

 ory which adopts it is the undulatory theory. This theory accounts sat- 

 isfactorily for all the phenomena connected with the transmission of 

 radiant light and heat, which the emission theory does not, and it is 

 now universally accepted by physicists. Assuming the correctness of 

 the theory, mathematicians have, by several various ingenious methods 

 which verify each other, measured the wave-length of the various 

 colored rays as they occur in the ethereal medium. The table of wave- 

 lengths given above, refers, therefore, to the waves in ether. And the 

 theory is that when a body is subjected to a violent molecular agitation, 

 the motion is imparted to the surrounding ether and propagated through 

 it in the form of undulatory vibrations at the rate of 186,000 miles per 

 second. Light comes from the sun to us in about 8 minutes. In the 

 case of sound the length of the wave differs according to the medium 

 through which it passes. So the wave-lengths of light of the same color 

 are different in different mediums, shorter in the denser mediums, and 

 longer in the rarer. 



The particles of ether enclosed in the intermolecular spaces of glass, 

 water, mineral crystals, &c. , are supposed to exist in a more condensed 

 state than in the air or above it, so that the light takes a reduced ve- 



