1871.] Molecules, Ultimates, Atoms, and Waves. 355 



only about a seventh of an inch in a second, during which 

 period of time the wave motion travels 185,000 miles. The 

 rate of motion of the particles is the same for all the waves, 

 and the amazingly short period of time in which the vibra- 

 tion is accomplished results merely from the extreme 

 minuteness of the motion performed. 



The foregoing estimates apply to only that portion of the 

 spectrum which is visible. But the thermal effects of the 

 invisible ultra-red waves can be traced to a distance beyond 

 A, which is equal to nearly five-eighths of the visible spectrum. 

 As the wave-lengths increase by a geometrical progression, 

 this distance would give for the longest wave-length whose 

 existence can be ascertained nearly 4797 hundred millionths 

 of an inch. Although this is only a rough approximation, it 

 may for the present purpose be assumed as correct. Owing 

 to the constitution of the ether, no particle can by a vibra- 

 tion be moved from its own point of rest so far as to become 

 nearer to that of any other particle. There is thus a natural 

 limit to the excursion of each individual particle — conse- 

 quently to the length of the wave. Assuming the above to 

 be the longest possible wave, the ratio which its length bears 

 to the departure of the individual particles involved in it 

 from their points of rest is' 208,926 millions to 1 ; and about 

 half this quantity, or, say, in round numbers, 100,000 mil- 

 lions to 1, will be the ratio which the longest wave-length 

 will bear to the normal distance between the particles, the 

 number of which in the longest wave will also be about 

 100,000 millions. This estimate may serve to convey some 

 notion of the extreme smallness of the intervals between the 

 particles of the luminiferous ether. What, then, must be 

 the minuteness of the particles themselves ? 



From the nature of the wave motion and of the vibrations 

 of the individual particles, the direction of which is across 

 that in which the wave motion is propagated, it appears pro- 

 bable that, in any given line of propagation, or ray, the 

 individual particles must depart wholly out of the line, so 

 as to produce a complete interruption of its continuity, in 

 order that the existence of the movement may be traceable 

 by any physical effects ; — in other words, the departure of 

 the particles from their points of rest in all likelihood slightly 

 exceeds their own radius. Could we, therefore, ascertain the 

 probable length of the shortest wave whose existence can be 

 traced by any physical effects, and the corresponding pro- 

 bable smallest departure of any particle from its point of rest, 

 we might obtain a rough approximation to the probable 



