1871.] Molecules, Ultimates, Atoms, and Waves. 467 



energy to set them in motion. The effects of very high 

 pressure would be explained by supposing the pressure to 

 drive the atoms constituting the ultimate into greater 

 mutual proximity than that in which they subsist under 

 ordinary atmospheric pressure. The widening of the lines 

 would result from an adventitious inertia of position de- 

 veloped in the atoms by their being thus forced together — 

 those at the surface of the ultimate being more easily 

 movable, and vibrating at slower rates, and with greater 

 amplitudes, than those in its interior, exactly as in the case 

 of a solid incandescent body, from which the ultimate of 

 hydrogen would differ only in this respect, that, when freed 

 to a certain extent from extraneous pressure, its constituent 

 atoms are at liberty to vibrate in their own peculiar times, 

 without being embarrassed in their movements by each 

 other. The relief from pressure narrows the lines ; so 

 that, when the gas becomes much attenuated, the lines 

 acquire an exceeding degree of fineness. The atoms then 

 vibrate more precisely at their own definite rates, unaffected 

 by the proximity of their neighbours. The pre-eminence of 

 hydrogen over other bodies, in its power of compressing the 

 ether and retarding the progress of the wave-motion, in 

 proportion to its specific gravity, would, according to this 

 hypothesis, be due to the higher proportion which the atomic 

 force bears to the gravitating force in hydrogen than in 

 other substances having heavier ultimates. 



Thus the whole of the phenomena presented by hydrogen, 

 in its relations to the luminiferous ether, receive an easy 

 explanation from the supposition that its ultimates are not 

 simple and homogeneous masses, but compound bodies con- 

 sisting of numerous atoms of four diverse kinds — each sort 

 having a different intrinsic inertia of its own, in virtue of 

 which it has a tendency to vibrate at a particular rate. This 

 hypothesis explains not only the four bright lines and the 

 appearances they exhibit, but also the remarkable property 

 of hydrogen in exerting on the ether a compressing power 

 far exceeding in proportion what is due to its specific 

 gravity. 



Having thus established the probability of the compound 

 nature of the ultimate of hydrogen — the lightest of all 

 known ultimates, it appears unnecessary to enter minutely 

 into the evidence tending to prove the same fact with 

 respect to the other chemical ultimates. There is, how- 

 ever, one piece of evidence which seems to deserve special 

 notice as bearing on the general question of the compound 

 nature of the ultimates of the chemical elements. It is the 



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