126 CORRELATION OF PHYSICAL FORCES. 



substances exposed to the sun, and which are believed to have 

 undergone some chemical change by this exposure. It is 

 desirable to await further experiment before forming a decid- 

 ed opinion. 



The analogies in the progression of sound and light are 

 very numerous : each proceed in straight lines, until inter- 

 rupted ; each is reflected in the same manner, the angles of 

 incidence and reflexion being equal ; each is alternately nulli- 

 fied and doubled in intensity by interference ; each is capable 

 of refraction when passing from media of different density : 

 this last effect of sound, long ago theoretically determined, 

 has been experimentally proved by Mr. Sondhauss, who con- 

 structed a lens of films of collodion, which, when filled with 

 carbonic acid, enabled him to hear the ticking of a watch 

 placed in one focus of the lens, the ear of the experimenter 

 being in the opposite focus. The ticking was not heard 

 when the watch was moved aside from the focal point, though 

 it remained at an equal distance from the ear. An experi- 

 ment of M*. Dove seems, indeed, to show an effect of polari- 

 sation of sound. 



The phenomena presented by heat, viewed according to 

 the dynamic theory, cannot be explained by the motion of an 

 imponderable ether, but involve the molecular actions of 

 ordinary ponderable matter. The doctrine of propagation by 

 undulations of ordinary matter is very generally admitted by 

 those who support the dynamical theory of heat ; but the 

 analogies of the phenomena presented by heat and light are 

 so close, that I cannot see how a theory applied to the one 

 agent should not be applicable to the other. When heat is 

 transmitted, reflected, refracted, or polarised, can we view 

 that as an affection of ordinary matter, and when the same 

 effects take place with light, view the phenomena as pro- 

 duced by an imponderable ether, and by that alone ? 



An objection that immediately occurs to the mind in 

 reference to the ethereal hypothesis of light is, that the most 



