50 Messrs. G. J. Stoney and J. E. Reynolds on the Cause 



17. We have endeavoured by modifying this simple hypothesis, 

 or substituting another, to gain a closer approach to the actual 

 phenomena, and we have in this way been able to fit the hypo- 

 thesis to the phenomena ; but it has been by assumptions which 

 are as yet too arbitrary to warrant our placing the somewhat 

 complicated details before the public. The simple case given 

 above will sufficiently explain the method we employ ; and we 

 think we have received sufficient encouragement from the results 

 of our discussion to hope that this method may elicit in some 

 cases really valuable information about the nature as well as the 

 periodic times of molecular motions. 



Section IV. On the perturbations the motion suffers. 



18. In conclusion we wish to advert to one other phenomenon 

 which appears to us worthy of note. Every line in a spectrum, 

 in order to be visible, must have a certain physical as well as in- 

 strumental breadth. By the physical breadth of a line we mean 

 that breadth which it has because the light that constitutes it 

 is not restricted to one wave-length, but extends between certain 

 limits of wave-length; by the instrumental breadth we mean 

 that appearance of breadth which is given to a line by the width 

 of the slit of the spectroscope. A line becomes invisible if either 

 its physical or its instrumental breadth dwindles to zero*. Now 

 the lines of chlorochromic anhydride have a very considerable 

 physical breadth. Hence the original disturbance communicated 

 to the aether by the motion in the vapour consists of waves of 

 corresponding physical breadth. This must be occasioned either 

 by a property inherent in the aether, whereby it can expand over 

 a certain range of wave-length a disturbance which it receives 

 from a strictly isochronous source ; or it is due to real differences 

 in the periodic times of the motions in the molecules of the va- 

 pour f. Now the variety of the phenomenon in the spectra of 

 different gases forbids our accepting any general explanation, 

 such as that which alleges a property of the Bather ; and we are 

 therefore compelled to admit that the motions in the molecules 

 of the vapour are not strictly isochronous, but that the periodic 

 times of some of them slightly exceed, and of others fall short of 

 the mean periodic time. The presumption appears to be that 

 the motions within the molecules have naturally a definite pe- 



* Phil. Mag. vol. xxxvi. (1868) § 5. p. 136. 



t The supposition that it mny be attributed to the irregular journeys of 

 the molecules amongst one another, which must in some cases lengthen 

 and in other cases shorten the intervals at which the light-waves reach the 

 eye, is excluded (1) by the amount of the effect (which is beyond what this 

 cause could produce), and (2) by the circumstance that lines situated in the 

 same region of the spectrum are variously expanded. 



