130 CORRELATION OF PHYSICAL FORCES. 



material atoms of the substance, as constituting together a 

 compound medium denser than pure ether, but not more elas- 

 tic. Ether might thus be viewed as performing the functions 

 which oil does with tracing paper, giving continuity to the 

 particles of gross matter, and in the interplanetary spaces 

 forming itself the medium which transmits the undulations. 



Since the period when Huyghens, Euler, and Young, the 

 fathers of the undulatory theory, applied their great minds to 

 this subject, a mass of experimental data has accumulated, 

 all tending to establish the propositions, that whenever matter 

 transmitting or reflecting light undergoes a structural change, 

 the light itself is affected, and that there is a connection or 

 parallelism between the change in the matter and the change 

 in the affection of light, and conversely that light will modify 

 or change the structure of matter and impress its molecules 

 with new characteristics. 



Transparency, opacity, refraction, reflection, and colour 

 were phenomena known to the ancients, but sufficient attention 

 does not appear to have been paid by them to the molecular 

 states of the bodies producing these effects ; thus the trans- 

 parency or opacity of a body appears to depend entirely upon 

 its molecular arrangement. If strise occur in a lens or glass 

 through which objects are viewed, the objects are distorted : 

 increase the number of these striae, the distortion is so in- 

 creased that the objects become invisible, and the glass ceases 

 to be transparent, though remaining translucent ; but alter 

 completely the molecular structure, as by slow solidification, 

 and it becomes opaque. Take, again, an example of a liquid 

 and a gas : a solution of soap is transparent, air is transpar- 

 ent, but agitate them together so as to form a froth or lather, 

 and this, though consisting of two transparent bodies, is 

 opaque ; and the reflection of light from the surface of these 

 bodies, when so intermixed, is strikingly different from its re- 

 flection before mixture, in the one case giving to the eye a 

 mere general effect of whiteness, in the other the images of 

 objects in their proper shapes and colours. 



