1871J Molecules, Ultimates, Atoms, and Waves. 463 



Thus the retardation of the six media is equivalent to the 

 following additional spaces of the free ether : — 



Inches. Inches. 



For hydrogen . . . 138 For tabasheer . . . 111,450 

 „ oxygen . . . 272 „ water .... 336,000 

 „ atmospheric air . 294 ,, chromateof lead 1,974,000 



That is to say, the wave motion would pass through the 

 above additional spaces of the free ether during the addi- 

 tional time it would require to traverse a million inches of 

 the above refracting media, or, taking hydrogen as the 

 standard of unity, the following numbers will represent the 

 comparative retarding powers of these substances : — 



Hydrogen. . . . roooo Tabasheer . . . 812*69 

 Oxygen .... 1*9829 Water .... 2,450*40 

 Atmospheric air . 2*1367 Chromate of lead . 14,394*00 



If, however, we take into account the specific gravities of 

 the substances, the proportions will be very different. 

 Again, making hydrogen the standard of unity, they will 

 stand thus : — 

 Hydrogen .... rooooo Atmospheric air . 0*147360 



Water 0*20770 Oxygen . . . 0*123930 



Chromate of lead . 0*20323 Tabasheer . . . 0*028566 



Hydrogen and tabasheer thus stand at the head and foot 

 of the list, and, indeed, of all known substances ; the power 

 of the former in retarding the progress of the wave-motion 

 being about 35 times that of the latter, in proportion to 

 their respective specific gravities. 



It thus appears that the energy with which any substance 

 retards the speed of the wave-motion of light passing 

 through it, depends only to a partial extent on its specific 

 gravity, and in no inconsiderable degree on a certain special 

 power, in which hydrogen excels all other substances. 

 This power must obviously be due to some peculiar relation 

 subsisting between the particles of the ponderable sub- 

 stance and those of the ether in which they are immersed. 



The relation between the ether and the ultimates of any 

 permanent gas, like hydrogen, manifests itself to a certain 

 extent in a tendency to separate those ultimates one from 

 another, in opposition to their mutual attraction and their 

 gravitation towards the earth. It is only by a very con- 

 siderable amount of pressure that the ultimates of hydrogen 

 are kept in that degree of mutual proximity in which they 

 subsist at the earth's surface, when they exhibit that degree 

 of retarding force which is shown in the preceding tables. 



