140 Mr. G. J. Stoney on the Internal Motions of Gases. 



spectral rays must lie between these limits, we are forced to con- 

 clude that, on the average, something- like from fifty to one hun- 

 dred thousand of these little* orbital revolutions are executed 

 between two consecutive collisions. It is now no longer surpri- 

 sing that the temporary perturbation caused while two molecules 

 are whirling past each other, has, in many instances, had abundant 

 time to pass away early in the interval between two collisions, 

 so as to leave the greater part of the minute internal motions to 

 be executed in the undisturbed manner which the detinitenessof 

 the spectral lines attests to us. We come also to see how the 

 motions with which the molecules dart about amongst one an- 

 other cannot produce or intercept light; in fact they are far 

 too sluggish — just as the motions of our fingers, or a very gentle 

 waving of the hand, do not produce sound. 



9. Again, Clausius has shownf that it is consistent with what 

 we know of the most perfect gases to suppose d, the mean dis- 

 tance between the molecules, to be about ^ of /, the length of 



* It is very difficult, and at the same time it is of importance, because it 

 guards us from error, to appreciate in some degree such amazingly minute 

 intervals of time as those with which we are here concerned. 



The double vibrations of visible light are executed in periods of time 

 which range from 1*3 to 2(> XVth-seconds. These swift little motions are 

 accordingly related to a fifteenth-second in somewhat the same way as the 

 motions of our limbs are to a second of time. Now the XVth-secoud is 

 the same portion of a second that a second is of upwards of thirty millions 

 of years. Hence the motions of light bear the same relation to one second 

 of time which the motions of our limbs bear to that almost inconceivable 

 cosmical period — a vast succession of geological ages, during which several 

 races of animals, culminating in man, have appeared, have lasted long, and 

 have finally perished upon our globe. 



When the study of nature has guided our thoughts into such a region as 

 this, our minds can scarce resist the impulse to wander outside the domain 

 of science into the adjoining precincts of imagination, and to speculate that 

 if there were sentient beings with bodies that move as deftly as this aether, 

 and with thoughts and perceptions as quick as their bodies are active, there 

 would be sufficient time for them within a small fragment of one second to 

 live the lives of all the generations of men that have dwelt upon this earth, 

 thinking all their thoughts and doing all their acts. The mind is almost 

 carried beyond itself in the contemplation of such periods. 



It is from the vastness of a second in reference to light, that with its little 

 waves light can travel such an immense distance as 298 millions of metres 

 in that period of time. It puts a strain upon the mind to form a concep- 

 tion of the mere moment of time that suffices for the light of a candle, at 

 this unheard-of pace, to travel one hand-breadth from the flame ; but brief 

 as this instant is, it corresponds to what several weeks would be in refer- 

 ence to our movements : the light has had time to execute hundreds of 

 thousands of its tiny vibrations ; and it is no wonder if it has already lost all 

 trace of everything that is fleeting or irregular in the disturbance originally 

 created by the incandescent matter of the flame. Accordingly man has 

 never detected any peculiarity in the vibration when examined in a situation 

 which he calls close to its origin. 



t Pogg. Ann. 1858, vol. iii. p. 251 ; or Phil. Mag. 1859, vol. xvii. p. 89. 



