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We have, for example, estimated the size and form 
of the molecules of naphthalene and anthracene on 
the assumption that they consist, respectively, of two 
benzene rings, and three benzene rings, in line; in 
accordance with chemical evidence. We have found, 
by X-ray analysis, the size and form of the unit 
cells (Fig. 6); a simple calculation shows that two 
molecules have to be packed into the cell in each case ; 
the crystals, it should be observed, are isomorphous. 

Fic. 4.—Showing mutual relations of three naphthalene molecules and 
parts of others. Letters are attached to all the carbon atoms of one 
molecule ; eae atoms completing this molecule are attached at 
ABCDGHIJ. 
It is found that the molecules pack into place very 
well, if they are arranged as in the sketch of Fig. 4. 
The figure refers to naphthalene, but the modification 
required for anthracene is readily conceived; and, 
indeed, it appears that the cell in one case is exactly 
as much longer than in the other as would be expected 
considering that the anthracene molecule contains one 
more ring than the naphthalene. Here again we may 
see in the structure of this little unit of naphthalene, 
which contains two double rings, everything that fore- 
shadows the properties of the whole crystal. Why is 
the substance so light? Because the structure is so 
lace-like and there are so many empty spaces. Why 
does it break up so easily into thin flakes? Because 
the molecules lie side by side somewhat like corn bent 
by the wind, and their side-to-side attachments differ 
from those that are end to end: the latter break more 
easily and the substance naturally splits up into layers, 
each of which is like a velvet pile, the fibres of which 
represent the molecules. Why does the substance 
melt so easily ? Because all the attachments of mole- 
cule to molecule are feeble and break up under dis- 
turbances due to heat. And so we may goon. If we 
attach a hydroxyl group to the side of the molecule 
we see the fibres of the pile open out sideways. If 
we attach it at the ends, we find the fibres grow longer ; 
the two substances formed in this way are well known 
in the dye industry. 
We have recently been examining the crystalline 
form of a number of the organic substances, and have 
learnt something of a very interesting system which 
Supplement to “ Nature,” June 9, 1923 
fied or not, occupy in the organic crystal, and we get | governs the packing. 
a very satisfactory fit (Fig. 4). 
* 
vil 

It holds for all crystals ap- 
parently, but is very plain in the organic field. There 
are two stages in the process of the formation of the 
‘crystal from the original atoms. First of all, the 
‘atoms are grouped into companies which the chemist 
calls “ molecules.” Chemistry has concerned itself 
largely with the study of the molecule, and particularly 
with the molecule in the free state, as in a liquid or a 
gas. In the second stage the molecules, retaining their 
composition if not their exact form, are packed 
together to make the crystal pattern: it is this stage 
which is the subject of our present considerations, 
and can be analysed by the X-ray methods. Take 
a simple example :—Two atoms of hydrogen and 
one of oxygen make up the water molecule. — It is 
a company of atoms strongly tied together in an 
alliance which stands much rough treatment. The 
molecules can exist in a state of independence as 
steam or water vapour: in a condition of semi- 
independence they associate themselves together as 
water. We know how much care has been given to 
the study of the water molecule in both these states. 
Now, in the second stage the molecules are arranged 
side by side and end to end to form the crystal of ice. 
It has been necessary to take away much of their 
motion in order to induce them to take the new form. 
They are no longer running hither and thither, twisting 
and spinning with the energy of their motion. They 
lie more quietly now, still quivering, no doubt, but 
tied together so that they can no longer change ap- 
preciably their relative positions. They are now the 
crystal to be investigated by the new methods: the 
result is shown in Fig. 3, already described. 
We find that when the molecules are packed into 
the crystal pattern—and they do not seem to suffer 
much in the process—they 
are put together just as any- 
one would try to pack a box 
with objects all alike in shape 
but individually of irregular 
or, one might say, of awk- 
ward form. How would 
you pack a box full of 
boots? You would natur- 
ally put them in pairs, the 
right boot over the left in 
the familiar way. It is just 
such methods of packing that are followed in a 
crystal. It is convenient to illustrate by means of 
models. Here are a number of wooden “ shoes ” 
which are to represent molecules without symmetry 
in their form. Take four of them and put them to- 
gether in the manner illustrated (Fig. 5). The result isa 
pattern which possesses a certain amount of symmetry, 
Fic. 5.—Arrangement of the 
four “shoes” showing the 
mutual orientation but not the 
mutual positions of the four 
molecules usually found in a 
monoclinic prismatic crystal 
such as benzoic acid or 
phthalic acid. 
