n6 CORRELATION OF PHYSICAL FORCES. 



so increased that the objects become invisible, and the glass 

 ceases to be transparent, though remaining translucent ; but 

 alter completely the molecular structure, as by slow solidifi- 

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

 liquid and a gas : a solution of soap is transparent, air is 

 transparent, but agitate them together so as to form a froth 

 or lather, and this, though consisting of two transparent bodies, 

 is opaque : the light being dashed about by minute reflections 

 and refractions in different directions. From the same cause 

 the reflection of light from the surface of these bodies, when 

 so intermixed, is strikingly different from its reflection 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. 



To take a more refined instance : nitrogen is perfectly 

 colourless, oxygen is perfectly colourless, but chemically 

 united in certain proportions they form nitrous acid, a gas 

 which has a deep orange-brown colour. I know not how the 

 colour of this gas, or of such gases as chlorine or vapour of 

 iodine, can be accounted for by the ethereal hypothesis, with- 

 out calling in aid molecular affections of the matter of these 

 gases. 



Colour in many instances depends upon the thickness of 

 the plate or film of transparent matter upon which light is 

 incident ; as in all those cases which are termed the colours 

 of thin plates, of which the soap-bubble affords a beautiful 

 instance. 



When we arrive at the more recent discoveries of double 

 refraction and polarisation, the effects of light are found to 

 trace out as it were the structure of the matter affected, and 

 the crystalline form of a body can be determined by the effects 

 which a minute portion of it exercises on a ray of light. 



Let a piece of good glass be placed in what is called a 

 polariscope (i.e. an instrument in which light that has under- 

 gone polarisation is transmitted through the substance to be 

 examined, and the emergent light is afterwards submitted to 

 another substance capable of polarising light, or, as it is 

 termed, an analyser), no change in effect will be observed. 

 Remove the glass, heat it and suddenly or quickly cool it so 



