PHYSICS OF NINETEENTH CENTURY 173 



clear the existence of some relation between the atoms 

 themselves, it leads us back once more to the old idea 

 that different substances are made up of the same 

 fundamental stuff, with its ultimate particles differ- 

 ing in arrangement or in number in the different 

 atoms. Here the required penetration into natural 

 phenomena grows too intense for the methods proper 

 to chemical research, and the actual discovery of an 

 ultimate particle, common to all kinds of matter, is 

 one of the latest triumphs of physics to be described 

 below. 



Meanwhile a vast extension of chemical knowledge 

 was made by optical methods. As we have already 

 The seen, Newton preferred the emission to the 

 Wave Theory undulatory theory of light because of the 

 of Light, difficulty of explaining the phenomena of 

 straight rays and shadows on any hypothesis of wave 

 motion. But in 1801-3 Thomas Young revived the 

 wave theory to account for his newly discovered 

 phenomena of the interference of two rays of light to 

 form coloured fringes, as well as the colour of thin 

 plates studied by Newton. Young's ideas were de- 

 veloped in mathematical form by Fresnel, who also 

 showed that rectilinear propagation would be a pro- 

 perty of waves which were very small compared with 

 the dimensions of the obstacles or the distances con- 

 cerned, and thus met Newton's objection to the theory. 

 Moreover, the phenomena of polarization, which indi- 

 cate that a ray of light possesses different properties 

 on different sides of the ray, were shown by Young to 

 prove that the direction of the vibrations which con- 

 stitute the wave is at right angles to the direction 



