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- 
- 
121, p. 2766). 
oi 
‘— 
923] 
June 2,1 

unit, or, in other words, is the number of identical 
or enantiomorphously related asymmetric parts into 
which it is subdivisible, if m is the number of mole- 
cules it contains and p the es number of each 
molecule, then »=mp. . Barker believes that 
Fedorov failed to prove his case, that the first paper 
referred to above contains an unconscious repetition 
of Fedorov’s argument, which, though new evidence 
is brought forward, is still unconvincing, and that 
the suggested structure for tartaric acid is against 
the principle and not, as we have said, in its favour. 
In the first place, Fedorov’s statement was surely 
unexceptionable in the form in which he made it. 
If one of the molecules, or groups of molecules into 
which the unit is divided, possesses a plane of sym- 
metry, this‘can mean only that it has similar relations 
with its neighbours on either side of the plane and 
through them with the rest of the crystal. That is 
to say, the plane of symmetry of the molecule is also 
a plane of symmetry of the crystal. On the other 
hand, we must be ready to allow, as Sir William Bragg 
has pointed out, that a molecule as built into a crystal 
may not have the same form as the freer molecule 
of a liquid or a gas. Such a difference seems to occur 
in the case of tartaric acid, on which account the 
crystal and its solution differ in their optical pro- 
perties. The molecule may have a plane of sym- 
metry in one case and not in the other. It is a 
task of the future to correlate the forms and the 
symmetries of the molecule in its different con- 
ditions. It is by no means improbable that the 
differences are small (Journ. Chem. Soc., 1922, vol. 
Fedorov was quite aware of this 
ey himself. If Fedorov’s statement is taken 
refer to the molecule as built into the crystal, it 
seems to require no further defence. 
In the next place, the rules or principles set out in 
the first of the two papers referred to do contain 
Fedorov’s statement, no doubt. If the author had 
been aware of the paper he would have referred to it. 
But the essence of the statement which is criticised 
is not an enunciation of a law of crystal symmetry 
which could not have been and was not overlooked 
by the searching examination of the crystallographers. 
It was an attempt to codify certain results of X-ray 
analysis. Fedorov could say, rightly as we think, 
that a crystal of the monoclinic prismatic class could 
be formed of four groups, A, B, C, and D: of which 
B was obtained from A by reflection across a plane, 
C by digonal rotation about an axis, and D by in- 
version through a centre of symmetry. He had no 
direct evidence to carry him further. The X-rays do 
go further: they show that in the crystal unit of 
nzoic acid, for example, there really are four groups 
so related to one another, and they give their relative 
positions. Moreover, they show that each of these 
groups is, in substance at least, the chemical mole- 
cule. This is new knowledge, which could not be 
proved by Fedorov. If it had been in his power to 
do so, the crystallographic tables would have con- 
tained the dimensions of the unit cell of each crystal ; 
and not merely, as they do now, the topical ratios. 
We may point out that Mr. Barker is in error also 
in peupoetng that nothing can be said about the 
symmetry of the molecule until the position of every 
atom in it is accurately determined. The X-rays 
show that the molecules of benzoic acid, for example, 
are divisible into two groups, which present exactly 
the same aspect when viewed along the axis of the 
. crystal and different aspects when viewed in any 
other direction. This is in agreement with the hypo- 
thesis that the two are the reflections of each other 
across the plane of symmetry, and that each is by 
itself asymmetric with respect to that plane. 
NO. 2796, VOL. 111] 
NATURE 
741 
Lastly, Mr. Barker refers to the structure of tar- 
taric acid, described in the second of the two papers, 
as an infringement of the principles set out in the 
first, because, as he says, it has an ‘‘ unobtrusive dyad 
axis,’’ which does not coincide with the axis of the 
crystal. The only answer is that it has not, as may 
be seen from Figs. 8, 12, 14, 15 of the paper, or more 
easily from the model itself. There is no such axis, 
and, therefore, no infringement. 
G. SHEARER. 
W. T. AstTBurRyY. 
Physics Department, 
University College, London. 
The Mechanism of the Cochlea. 
In Mr. Wilkinson’s letter in Nature of May 12, 
p: 636, three points are raised upon which I wish to 
comment. 
For the sake of simplicity I described the mechanical 
conditions occurring when sound waves reach the 
cochlea in the normal manner by the chain of ossicles. 
In the case of bone conduction the mechanism of 
analysis ought to be the same as under other condi- 
tions. Bone conduction is the response to a continu- 
ous series of uniform waves from a tuning-fork which 
would produce a corresponding series of vibrations ina 
resonating system. I cannot agree that the move- 
ment “ originates at the basilar membrane,” because 
the movement depends on the whole resonating 
ae including the inertia and friction of the 
uids. 
Damping is the decrease in amplitude due to 
resistance, and I believe that by using that term Mr. 
Wilkinson intends to deny any influence due to liquid 
friction in affecting the note to which the system 
resonates. In White’s ‘‘ Handbook of Physics” 
(Methuen and Co., first edition, p. 305), I find “ partly 
closing the mouth [of a resonator] lowers the note.” 
This is an example of friction in a gas affecting the 
frequency of resonance, which is also seen in the well- 
known method of tuning organ-pipes. If such an 
effect is shown with a gas, surely it must be much 
greater with a liquid in such narrow tubes as those of 
the cochlea. 
With reference to the spiral ligament, I think that 
the point is unimportant. I merely pointed out the 
danger of deducing from the size of the ligament the 
tension on the membrane at rest. To make the 
point clearer I would suggest the analogy of the size 
of a pair of hooks supporting a cable. The size of 
the hooks may not be designed with reference to the 
tautness of the cable. The cable may be slack, so 
that the only pull may be that due to its weight ; but 
large hooks may be used, because the cable may have 
to sustain heavy weights from time to time. I am 
quite willing to believe that the fibres of the basilar 
membrane near the fenestra ovalis may be more 
tightly stretched than those near the apex of the 
cochlea, but that does not necessarily follow from the 
dimensions of the spiral ligament. 
Finally, I wish to emphasise that this correspond- 
ence arose in relation to the dimensions of the cochlea 
and the possibility of such a small structure acting as 
a resonating mechanism. The point that I wished 
to bring out was that, on account of its small size, 
liquid friction will be very great and that this 
friction may be one of the factors in the analysis. 
H. E. Roar. 
London Hospital Medical College, 
Turner Street, Mile End, E.1, 
May I5. 
¥, 2 
