B.—CHEMISTRY 43 
group. Here one has to deal with configurational or spatial arrangement 
and no one model can be selected as typical for the whole group. ‘The 
classical methods of organic chemistry will always be given first place 
among those available for the elucidation of constitution in this as in other 
series of compounds. A great many other factors must, however, enter 
into the discussion and interpretation of these results. Among the more 
important are the considerations which apply to the observations of optical 
rotatory power. It would be difficult to exaggerate the value this property 
possesses for the more intimate explorations of the carbohydrates. 
Occasionally too much has been claimed for the significance of optical 
rotatory power in the sugar group and numerous empirical rules designed 
to correlate structure with this property are valid only within narrow limits. 
During the subsequent discussion, Dr. E. L. Hirst, F.R.S., will speak on 
this subject, so that I need not pursue it further in this address. 
Other problems of the spatial arrangement which carbohydrates assume 
are bound up with such considerations as the conformation of the pyranose 
ring. In this the atoms may be all co-planar, in which case strain will be 
introduced by deflexion of valency direction or the various atoms con- 
stituting the ring may be staggered, in which event the structure will be 
strainless. This question of the different modes of packing the atomic 
assemblages may find some solution by the application of X-ray methods, 
which have already rendered great service in the carbohydrate field. 
Mr. E. Gordon Cox will deal with some experimental results which have 
followed from this line of inquiry. 
In the cell wall of many lichens occurs the polysaccharide lichenin, 
which resembles cellulose except in its property of being water-soluble. 
Recently its investigation has been resumed and many of its properties 
show that it is closely allied in structure to cellulose. Its chain-length 
is, however, considerably shorter, corresponding to about 80 glucose 
units. A pentosan constituent of certain grasses such as esparto, and also 
of wood, is recognised as the polysaccharide xylan. Its recent investiga- 
tion has shown that, although 93 per cent. of xylose may be isolated from 
its hydrolysis products, yet it is by no means the case that the whole of 
this polysaccharide is constituted on the basis of xylose. It is found 
that the deficiency of 7 per cent. is entirely made up by the presence of 
this exact percentage of arabofuranose residues which occur as terminal 
groups in the xylan chain. This observation is a sufficient commentary 
on the need of an inquiry into the finer structure of carbohydrates. 
That the xylose residues are linked as pyranose units through the 1 : 4 
positions as in cellulose has been made clear. The chemical molecule 
of xylan consists of 18 or 19 xylopyranose residues united in this way and 
terminated at one end by an arabofuranose unit. It is probably aggre- 
gated with two or three other such chains by co-ordination at the reducing 
terminal group. The progressive break-down of xylan gives rise to a 
shortened chain consisting only of xylopyranose residues and these xylo- 
dextrins are invariably terminated by a xylopyranose group isolated as 
trimethyl xylopyranose after the loss of the arabofuranose residue. This 
is confirmatory evidence of the occurrence of xylose units in the body of 
the chain in the pyranose form. It is believable that of the numerous and 
