86 Scientific Proceedings, Royal Dublin Society. 
which is the 127,000th of an inch ; whereas sub-section Cw extends 
twice as far, 7.e. down to one tenth of a micron. ‘This brings us 
within the border of the next group, the group of molecular inter- 
vals, almost all of which lie farther beyond the reach of the micro- 
scope than microscopic objects lie beyond the grasp of the naked — 
eye. 
Group D (MoLecuLarR QuaNTITIES). 
On the borderland between Groups C and D, we find the 
lengths of waves of light, all of which can be represented by 
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 seventh-metrets to a little less than 8 seventh-metrets. The 
ultra-violet light which reaches the Harth 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 one seventh-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 
agit is at the surface of the Marth, 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 73 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 fol- 
lowing 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 /arge molecular magnitudes. 
When we attempt to penetrate farther, we find that we can 
only obtain a glimpse of those more fundamental events in Nature, 
