428 
Letters to the Editor. 
[The Editor does not hold himself responsible for 
opinions expressed by his correspondents. Neither 
can he undertake to return, nor to correspond with 
the writers of, rejected manuscripts intended for 
this or any other part of NATURE. No notice is 
taken of anonymous communications. | 
The Nature of the Liquid State. 
In his recent lecture to the Chemical Society on 
“The Significance of Crystal Structure,’’ Sir William 
Bragg has described and discussed the extremely 
important results obtained in his laboratory by the 
X-ray analysis of various carbon compounds, notably 
those belonging to the aromatic series. The special 
feature brought to light by the investigations is that 
the ultimate unit of crystal structure or elementary 
parallelepiped is not the chemical molecule, but, 
Wi TURE 
generally speaking, is a complex formed by the union | 
of two, three, or four molecules. Further, the sym- 
metry of the crystal tends to increase with the 
number of molecules in the unit, and also with the 
symmetry of the molecule itself. In fact, there are 
simple quantitative rules, first stated by Shearer, 
connecting these quantities. 
The question naturally arises whether when a 
crystal is melted and passes into the liquid state, the 
units in the latter condition are the same as in the 
crystal, or whether these break up further into the 
individual molecules. 
this very fundamental point is furnished by studies 
on the molecular scattering of light. If the units in 
the liquid state are the chemical molecules, that is, 
the same as in the condition of vapour, there should 
be a simple quantitative relation between the amount 
of unpolarised light (due to optical anisotropy) 
scattered by equal volumes of liquid and vapour and 
the densities in the two states of aggregation. This 
relation was indicated in my letter in NATURE of 
July 1, 1922, but the method of calculation there | 
given has to be amended to make allowance for the 
fact that the electric polarisation within a fluid is, 
according to the Lorentz-Mosotti formula, greater 
than in free space. When this correction is made, it 
is found that the amount of unpolarised light actually 
scattered is considerably smaller than that indicated 
by the calculation. The conclusion thus appears to 
be forced upon us that the ultimate unit in the liquid 
state is not the same as in the state of vapour. On 
the other hand, if we adopt the view that the ulti- 
mate unit is the same in the liquid state as in the 
crystalline state, a way is opened for a satisfactory 
explanation of the observed result. For, according 
to Shearer’s rule, the symmetry of the unit is always 
greater than that of the molecule, and hence the 
amount of unpolarised light scattered by it should be 
diminished, as is actually observed. 
A further consideration which suggests that the 
ultimate unit in the liquid state is the same as in the 
crystalline solid is the existence of those remarkable 
substances, known as liquid crystals, studied by 
Lehmann and others. If a liquid be conceived of as 
a collection of elementary crystal parallelepipeds 
which are ordinarily prevented from thermal agita- 
tion from forming regular arrays, it is easier to under- 
stand how in favourable circumstances, such arrays 
come into existence temporarily and as quickly dis- 
appear. This conception appears to fit in very well 
with the mathematical framework of the kinetic 
theory of liquid crystals recently developed by 
Oseen (Stockholm Academy, Handlingar, 1921). 
The same conception also appears to furnish a 
satisfactory explanation of the tendency shown by 
NO. 2787, VOL. 127] 
A method of investigating | 

[Marcu 31, 1923 2 
many liquids to refuse crystallisation and to pass 
into a highly viscous or glassy condition when super- 
cooled. We have only to suppose that the units 
gradually join up, but in an irregular way, and form 
an optically heterogeneous structure. This concep- 
tion of the constitution of vitreous solids is supported 
by the results of an extensive¥series of observations 
on the scattering of light in optical glasses and in 
supercooled organic liquids carried out under the 
writer’s direction. ; 
Finally, it may be remarked that the conception 
suggested does not, so far as the writer can see, 
appear to be inconsistent with any other known facts — 
regarding the physical properties of liquids. 
C. V. RAMAN. 
210 Bowbazaar Street, 
Calcutta, India, 
February 22. 

ProF. RAMAN’S very interesting explanation of his 
observations on the scattering of light by liquids is 
not affected if a slight change is made in his sug- 
gestion as to the appearance of the crystal unit in 
the liquid phase. be 
The crystal unit is a parallelepiped of minimum 
volume, the corners of which are occupied by molecules 
alike in all respects, including orientation. The de- 
finition allows the unit to be delimited in an indefinite 
number of ways. It is improbable that any one of 
these occurs as the only kind of unit in the liquid. 
For Prof. Raman’s purpose it is sufficient, I think, to 
suppose that association, when it occurs, is ordered, 
the molecules joining up as if they were beginning 
to build a crystal. Let us suppose, for example, 
that the crystal belongs to the monoclinic prismatic 
class, in which there are four types of molecular 
arrangement. Any molecule of one type possesses 
with any molecule of the other three types, a plane, 
a digonal axis, or a centre of symmetry, respect- 
ively. 
Groups of mutually arranged molecules may well be 
expected to form under suitable conditions, but it is 
not likely that the group will always consist of four, 
or be put together the same way. The group could 
always, however, be incorporated into a complete 
crystal: possibly some redressing of the boundary 
might be required. as a i 
An ordered association or incipient crystallisation 
has been suggested by Astbury (Proc. Roy. Soc., 102, 
Pp. 527) as the cause of the variability of the optical 
activity of tartaric acid with the strength of solution. 
The Debye-Scherrer photographs of colloidal gold 
show that each particle is essentially an association 
of gold atoms in crystalline array. It is possible that 
on the surface there is disorder which affects the 
further growth of the particle. Pe ; 
The point is that whenever association takes place, 
it tends to do so in the ordered fashion of the appro- 
priate crystal. W. H. Braaae. 
The Wegener Hypothesis and the Great Pyramid. 
In the discussions on the Wegener hypothesis I have 
not yet seen an allusion to the direct evidence given 
by Flinders Petrie (‘‘ Pyramids and .Temples of 
Gizeh,” second edition, 1885, pp. 11 and 41) of a 
change in azimuth at Gizeh amounting to four or 
five minutes since the erection of the Great Pyramid. 
Petrie’s account of the high accuracy used in the 
construction of the pyramid seems to render quite 
impossible an error of 4’ in the laying down of a 
meridian line 700 feet long, from which other base- 
lines were set off during 30 or 40 years. As my 

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