the Luminiferous Ether, 233 



(7) Inability to explain circular polarization produced by 

 magnetic action. 



(8) Incompleteness of explanation offered of the chemical 

 effects of light. 



Probably a more exhaustive survey will reveal other and 

 greater difficulties ; but these will be sufficient to show that the 

 acceptance of the theory is beset with difficulties and makes 

 such great calls upon our imaginations that it behoves the 

 least sceptical to " pause and consider." 



Now let us proceed with the second portion of our subject, 

 to endeavour to show that the particles of matter themselves 

 are capable of taking up and transmitting the wave-motion. 

 Dalton considered all bodies to be composed of atoms, which 

 atoms are all of the same size but of different weights. This 

 difference in weight is expressed in the atomic weight of the 

 elements. Atoms in the free state combine with each other and 

 form molecules. About the absolute size or density of these 

 atoms we know nothing, save that they are very very small. 

 Recent advances in scientific theory have but extended Dalton's 

 hypothesis. Thus, a high authority, stating the theory at pre- 

 sent held of the constitution of bodies, says : — ■ 



" All bodies consist of a finite number of small parts called 

 molecules. Every molecule consists of a definite quantity of 

 matter, which is exactly the same for all the molecules of the 

 same substance A molecule may consist of several di- 

 stinct portions of matter held together by chemical bonds, and 

 may be set in vibration, rotation, or any other kind of relative 



motion Tbe molecules of all bodies are in a state of 



continual agitation. The hotter a body is, the more violently 

 are its molecules agitated"*. Here, then, heat is produced 

 by the vibration of the molecules of bodies. If the particles 

 of a body are capable of such rapid vibration in the production 

 of heat, why may they not be capable of taking up this vibra- 

 tion and transmitting it ? It is more than probable that heat 

 is thus transmitted. Why may not light be also thus trans- 

 mitted ? We only have a difference in quantity between the 

 two, heat being produced by the less rapid and light by the 

 more rapid vibrations. Nor is the difference so great between 

 the vibration producing heat and that producing light. The 

 maximum heating effect of the spectrum occurs at a point 

 where the rate of vibration is about 400 millions of millions 

 of vibrations per second ; the maximum chemical effect occurs 

 where the rate is about 800 millions of millions per second. 

 As an effort of the intellect, it is as easy to endow matter with 

 the ability to vibrate at the latter rate as at the former ; and if 

 * Maxwell, Theory of Heat, p. 806. 



