OX THE ATOMIC GROUPING IN ORGANIC BODIES. 
895 
Effect of increase or diminution in the length of the absorbing medium. 
A natural question to ask is as to the effect of the increase or the diminution in 
length of the compound placed before the slit of the spectroscope. The answer is not 
difficult to give. Where an increase of length is used, one of two things occurs : either 
general absorption creeps up further towards the more refrangible end, or the absorption 
features are more marked. It may be supposed that in the latter case the bands should 
become more defined, but this is not so ; the bands, as the length of column of liquid 
increases, may spread out till they reach the locale of another hydrogen band, each line 
becoming, as it were, a stepping stone for a further advance of absorption. This 
usually takes place only on the more refrangible edge of a band, the less refrangible 
edge remaining, as a rule, constant. In some cases both edges of the band remain 
fixed (as example we may quote the ethyl alcohol band situated about 900) and 
neither increase nor diminution of length of fluid alters their relative positions. 
Where the length of column is diminished one of three things happens : the absorp¬ 
tion disappears altogether, the bands fade into the lines bounding one of their edges, 
or they become fainter and remain constant. At the edge of the band which is 
least defined the absorption gradually disappears till a line is left at the most defined 
edge, or if both are ill-defined the nucleus will probably be found to be central to it. 
When both edges are well-defined the band remains constant in width, but fainter. 
Oxygen combined in the radical. 
Hitherto we have only taken into account oxygen which is not contained in the 
radical; when it is so contained it appears to act differently, always supposing hydrogen 
to be present as well. We need only refer to the spectrum of aldehyde which is 
inclined to be linear rather than banded, or rather the bands are bounded by absolute 
lines, and are more defined than when oxygen is more loosely bonded. 
Detection of the radical. 
An inspection of our maps will show that the radical of a body is represented by 
certain well-marked bands, some differing in position according as it is bonded with 
hydrogen, or a halogen, or with carbon, oxygen, or nitrogen. There seem to be 
characteristic bands, however, of any one series of radicals between 1000 and about 
1100, which would indicate what may be called the central hydrocarbon group, to 
which other radicals may be bonded. 
The clue to the composition of a body, however, would seem to lie between X 700 and 
X 1000. Certain radicals have a distinctive absorption about X 700 together with 
others about X 900, and if the first be visible it almost follows that the distinctive mark 
of the radical with which it is connected will be found. Tiius in the ethyl series we find 
