Existence of the Luminiferous ^Ether. 487 



equally difficult of explanation on the aether theory, but are 

 natural consequences of the molecular theory. All show 

 either that the addition of more atoms to the molecule or the 

 arrangement of the atoms in different ways causes a difference 

 in the ability of those molecules to oscillate, and therefore 

 causes a difference in the refractive powers. 



2. Colour of Organic Compounds. — Most organic compounds 

 are colourless, as is well known; but those which are 

 coloured are neither few nor unimportant. The connexion 

 between the chemical constitution and the colour of the 

 carbon-compounds has not been well worked out. It is, 

 however, a most remarkable fact that most, if not all, of these 

 coloured bodies contain certain elements (as, for example, 

 nitrogen and oxygen) united in a peculiar manner. ISTow that 

 there should be the remotest connexion between the colour of 

 a body and the atoms of which that body is made up is a cir- 

 cumstance that cannot be explained upon the aether theory. 

 The presence of the atoms is only important on this theory in 

 so far as they affect the aether which occupies the spaces be- 

 tween them. Why the presence of particular atoms should 

 affect this aether more than others is not clear. But if we 

 assume that the molecules take up and transmit the vibrations 

 of the light-waves, then it is evident that these molecules must 

 be made different by the presence of different elements; and 

 this difference gives rise to the colours of the substances. 



3. Circular Polarization. — The relation between the chemi- 

 cal composition of substances and their rotary power has re- 

 cently been studied by Landolt. In his paper upon the sub- 

 ject (Liebig's Annalen, clxxxix. pp. 241-337) he divides active 

 bodies into three classes: — 



(1) Those active only when in the crystalline form. 



(2) Those active only when in solution or when fused. 



(3) Those active when crystalline as well as when in 

 solution. 



The bodies belonging to the first class we have already con- 

 sidered. Only two bodies are known belonging to the third 

 class. The second is therefore the most important. Con- 

 cerning the second class, Landolt considers it proved beyond 

 doubt that the activity of these bodies is a property of the 

 molecules, and depends on the arrangement of the atoms in 

 those molecules. In support of this latter statement he quotes 

 the experiments of Biot on camphor and turpentine-oil (M4- 

 moires de V Acad. vol. ii. p. 114). In fact, Biot has applied to 

 the rotation produced by these bodies the term " molecular 

 rotation," to distinguish it from crystalline rotation. Again, 

 Van t'Hoff has found, from a comparison of all known active 



