Stoney — Cause of Double Lines in Spectra. 565 



is one of the three fundamental fixed units of nature.* This standard velocity I 

 propose to call the Maxivell. It is a velocity of almost exactly 30 quadrants of 

 the earth per second, meaning by a quadrant the distance along the earth's 

 meridian from the equator to the pole — in other words, it is a velocity of 300 

 millions of metres per second. It is a velocity that pervades all nature and 

 establishes a natural relation, which exists everywhere, between time and length. 

 Accordingly lengths, such as metres, millimetres, &c., do naturally represent 

 definite periods of time, viz. the times occupied by light in advancing over those 

 distances in the open aether. But as perhaps it might be thought too great a 

 departure from usage to speak of metres, millimetres or tenthet-metresf of time, 

 I shall for our present purpose select one definite period, viz. the time that light 

 takes to advance the tenth part of a millimetre, and will call it the jot of time. 

 This little fragment of time, which is one-third of the billionth of a second, will 

 be found a very convenient standard in which to measure the periods which 

 present themselves in the study of molecular events. Thus light takes one deka- 

 jot to advance a millimetre ; it advances a tenthet-metre in the micro- jot, meaning 

 by the deka-jot, ten jots, and by the micro-jot, the millionth part of a jot. The 

 periodic times of the oscillations that present themselves in the spectra of gases 

 range from a little less than two milli-jots up to about twelve milli-jots, meaning by 

 the milli-jot the thousandth part of a jot. This range extends from the limit in the 

 ultra-violet explored by Professor Hartley to the farthest ultra-red reached by 

 Captain Abney. In ordinary air the flight of a molecule between its encounters 

 lasts on the average about 420 jots,$ during which time there can, accordingly, 

 take place upwards of 200,000 of the swiftest and 30,000 of the slowest of the 

 oscillations§ spoken of above — oscillations which we must bear in mind are set up 

 in the surrounding aether by the events that occur in the molecules. 



* See a Paper by the author in the Proceedings of the Royal Dublin Society, February 16, 1881, and in 

 the Phil. Mag., May, 1881, p. 384. 



f A tenthet means a unit in the tenth place of decimals ; in other -words it is 1/10'°. 



I The rate of diffusion of gases and the amount of their viscosity depend on the length of the excursion 

 of the molecules. Maxwell made three determinations founded on this consideration, the mean of which 

 is that the average length of the free path in air is a little more than seven eighthet-metres (7/10* of a 

 metre). See Phil. Mag. for August, 1868, p. 138, where Maxwell's results are collected. Taking 

 seven eighthet-metres as the length of the excursion, and 500 metres per second (the "velocity of mean 

 squares" in air at 18° temperature) as the speed of the molecule, the mean duration of its flights will 

 be 420 jots. It is probably a little more than this in air, and will vary in different gases and in the same 

 gas under different circumstances ; but for the pv/rposes of this Paper a very rough approximation is suffi- 

 cient, and accordingly we shall iise 420 jots when we want to indicate the sort of interval of time that 

 elapses while a molecule is on one of its journeys. 



§ The -word oscillation is used throughout this Paper in a generalized sense to include revolutions in an 

 orbit as well as vibrations in a straight line. 



4L2 



