464 Dr. G. Johnstone Stoney : Survey of that 'part of 



numbers inserted in the column which is the extreme right- 

 hand column of Group C and the extreme left-hand column 

 of Group D. The wave-lengths of visible light extend from 

 a little less than 4 seven th-metrets to a little less than 8 seventh- 

 metrets. The ultra-violet light which reaches the Earth from 

 the Sun carries us down to about 3 seventh-metrets ; the light 

 which has been explored by Professor Hartley extends the 

 range nearly down to 1^ seventh-metrets ; and Professor 

 Schumann has got down to light whose wave-length is about 

 1 seven th-metret. Thus the wave-lengths of light come all 

 of them upon the column which, in our table, is on the border 

 between microscopical magnitudes and molecular. Almost 

 the only true molecular length long enough to be measured 

 in this column is the average free path in attenuated air, or in 

 some other gases. On the other hand, when air is as dense 

 as it is at the surface of the Earth, the average length of these 

 free paths has to be recorded in the next column (the column 

 of eighth-metrets), and may be considered as about the longest 

 of legitimate molecular intervals. According to Maxwell's 

 determinations, it seems to be about 1\ eighth-metrets. The 

 wave-lengths of Rontgen rays perhaps extend into this 

 column. 



One or two units in the next column, the column of ninth- 

 metrets, may be taken as about the average interval at which 

 the molecules of ordinary air are spaced ; and a unit or two 

 in the following column, that of tenth-metrets, is about the 

 average spacing of the chemical atoms of which solids and 

 liquids consist. It will be seen that none of these intervals 

 extend beyond Du, the sub-section of large molecular mag- 

 nitudes. 



When we attempt to penetrate farther, we find that we can 

 only obtain a glimpse of those more fundamental events in 

 Nature, the size of which or the range of which has to be 

 measured in the next three columns, i. e. in tenthets of the 

 decimetre, of the centimetre, of the millimetre. These all 

 come into sub- section v, the sub-section of medium molecular 

 magnitudes. That there are events of this kind going on 

 unremittingly within every chemical atom is indicated to us 

 by the lines in the spectra of the chemical elements ; for 

 these are caused by such events. Here, at present, human 

 knowledge stops : the whole of the work which Nature is 

 carrying on at still closer quarters, although we are well 

 aware that it must lie at the basis of all the rest, is totally 

 hidden from our view, except so far as the speculations of 

 mathematicians may doubtfully attempt to probe it ; and in 

 all such conjectures the speculator has to substitute something 



