n8 CORRELATION OF PHYSICAL FORCES. 



acid, a liquid, undergoes a chemical change and deposits a 

 solid carbonaceous compound by the action of light. Chlorine 

 and hydrogen gases, when mixed and preserved in darkness, 

 do not unite, but when exposed to light rapidly combine, 

 forming hydrochloric acid. 



The above facts and many others might have been ad- 

 duced go far to connect light with motion of ordinary matter, 

 and to show that many of the evidences which our senses 

 receive of the existence of light result from changes in matter 

 itself. When the matter is in the solid state, these changes 

 are more or less permanent ; when in the liquid or gaseous 

 state, they are temporary in the greater number of instances, 

 unless there be some chemical change effected, which is, as it 

 were, seized upon during its occurrence, and a resulting com- 

 pound formed, more stable than the original compound or 

 mixture. 



I might weary my reader with examples, showing that, in 

 every case which we can trace out, the effects of light are 

 changed by any and every change of structure, and that light 

 has a definite connection with the structure of the bodies 

 affected by it. I cannot but think that it is a strong assump- 

 tion to regard ether, a purely hypothetical creation, as changing 

 its elasticity or density for each change of structure, and to 

 regard it as penetrating the pores of bodies of whose porosity 

 we have in many cases no proof; the which pores must, more- 

 over, have a definite and peculiar communication, also assumed 

 for the purpose of the theory. 



Ether is a most convenient medium for hypothesis : thus, 

 if to account for a given phenomenon the hypothesis requires 

 that the ether be more elastic, it is said to be more elastic ; 

 if more dense, it is said to be more dense ; if it be required by 

 hypothesis to be less elastic, it is pronounced to be less elastic ; 

 and so on. The advocates of the ethereal hypothesis certainly 

 have this advantage, that the ether, being hypothetical, can 

 have its characters modified or changed without any possibility 

 of disproof either of its existence or modifications. 



It may be that the refined mathematical labours on light, 

 as on electricity, have given an undue and adventitious 

 strength to the hypotheses on which they are based. M. 



