APRIL 28, 1923] 





































A Static or Dynamic Atom? 
SoME writers still contrast the static atom of 
_ Lewis and Langmuir with the dynamic atom of Bohr, 
as if the two alternatives were mutually exclusive. 
It does not seem to be realised generally that any 
‘inconsistency there may have been between them has 
vanished completely with the publication of Bohr’s 
later views on atomic orbits; speculations about 
chemical constitution based on the static atom can 
be translated directly into the language and con- 
_c2ptions of the dynamic atom. 
The fundamental idea of Lewis is that non-polar 
combination consists in the sharing of electrons 
between atoms in such a way as to complete stable 
electronic configurations. If the sharing of an elec- 
tron means the sharing of an orbit, and if the stable 
electronic configurations are those in which the 
groups of highest quantum number are completed, as 
they are in the rare gases: then the Lewis-Langmuir 
theory, expressed in terms of Bohr’s conceptions, 
‘states that such compounds are formed when some of 
the electronic orbits, instead of surrounding one 
nucleus only, surround both, and therefore help to 
complete the quantum groups of both atoms. With 
this principle as a guide, it is merely a matter of 
linguistic alteration to interpret on the basis of a- 
dynamic atom the conclusions which have been 
reached on the basis of the static atom. 
Of course the question remains whether the theory 
cap be true and whether such shared orbits are 
i This is a matter for quantum theory to 
My last letter to Nature on this subject 
(November 25, 1920, vol. 106, p. 408) succeeded in 
eliciting from Prof. Bohr the first statement of the 
later and most exciting developments of his theory ; 
perhaps this one will be equally fortunate ! 
_ Until the question is settled, it would be waste of 
time to make the necessary translation, even in a few 
examples. But it may be well to point out that, if 
his interpretation of the ‘‘ sharing of electrons” 
be accepted, the task of explaining chemistry 
according to the Lewis theory will probably be 
facilitated. For it seems likely that some limitations 
at present imposed upon the forms of sharing and 
upon the stable configurations could be removed. So 
far as I can see, Lewis’s principle that only pairs of 
electrons are shared, and Langmuir’s principle (in 
she original statement) that the stable configuration 
is always an octet, are based not so much on definite 
facts as on the need of some guiding principle if 
eculation is to be limited. The limitations sug- 
gested by the identification of stable configurations 
with the completion, or partial completion, of quan- 
tum groups are not exactly those which are usually 
ado; at present ; but once more, while the whole 
basis of the theory is so uncertain, the attempt to 
decide the constitution of particular compounds is 
‘premature. NorMAN R. CAMPBELL. 

The Zwartebergen and the Wegener Hypothesis. 
Critics of the Wegener hypothesis have made a 
deal of capital out of the northward deflexion of 
the folds of the Zwartebergen on approaching the 
west coast of Africa, but their failure to point out the 
cause of this deflexion seems to me to lay them open 
_to the charge of advocacy which they so freely lay at 
Wegener's feet. 
e deflexion is produced by the incidence of the 
chain on a massif of older rocks of the Swaziland 
System with a core of granite trending north-west. 
On nearing this resistant axis, the folds bend north- 
west and then north forming the Cederbergen. 
Finally, they flatten and die out northwards. 
NO. 2791, VOL. 111 | 
“NATURE 

569 
clear that the existence of the granitic axis has inter- 
fered with the direct westerly continuation of the 
folds. The interference, however, is only local, for 
the Cederbergen do not continue for any distance to 
the north. 
An exactly analogous deflexion occurs in the case 
of the Armorican folds in Ireland where they impinge 
on the highly resistant north-easterly trending 
Wicklow chain with its massive granitic axis. The 
folds turn north-east in Tipperary as they approach 
the granite and then north in Kilkenny and Queen’s 
County, where they flatten out and finally disappear. 
The analogy is very perfect in that the final deflexion 
from the general trend of the folds is greater than 
would be brought about by a mere falling into line 
with the Wicklow chain. 
Now, as every one is aware, the interruption along 
the line of the Wicklow granite does not stop the 
Armorican folds. They are renewed on the other 
side of St. George’s Channel in southern Wales, where 
they once more assume their normal direction. If, 
therefore, we imagine the supposed Atlantic rift 
valley to have opened up along St. George’s Channel, 
so as to leave Ireland attached to Newfoundland, we 
may profitably consider what would have happened 
when one of those irrepressible Germans had come 
along and announced that it once formed part of the 
British Isles, basing his argument on the fact that the 
Irish and Welsh folds, as well as other geological 
structures, fitted one another when the countries were 
placed in juxtaposition. The critics would at once 
have objected that the Armorican folds in Ireland on 
the west side of the Atlantic turned up northwards 
before they reached the coast, and therefore could 
not be regarded as a continuation of those of Wales. 
It is clear that the objection would have no force in 
this instance, so one may well ask whether it has any _ 
in the actual case of Africa and South America, 
W. B. WriGcuHrt. 

Manchester, March 3r. 

Egyptian Water-Clocks. 
Permit a brief correction to the paragraph in 
Nature of April 7, p. 479, on the casts presented to 
the Science Museum. The variable divisions of the 
water-clocks are not for different lengths of day, but 
compensations for the changes of viscosity of water, 
over 9° and 12° F. respectively. This is proved by 
the extremes being nearer to the equinoxes than to 
the solstices, to harmonise with the slow passage of 
heat through massive temples. Further, the conical 
form of the clepsydra of 1400 B.c. was to compensate’ 
for the greater flow under fuller pressure, the form 
being a near approach to a portion of a parabola. 
Thus the variation of pressure was as I : 3-7, and the 
water varied as I : 2:9 to meet this. 
W. M. FLINDERS PETRIE. 

A Permanent Image on Clear Glass. 
AFTER silvering an ordinary clock glass (about 5 in. 
diameter) on the convex side, I noticed on removing 
the wax, with which the concave side had been pro- 
tected, that a perfectly distinct image of a small 
child’s head had been rendered visible. The image is 
a photographic “ positive.” 
It occurred to me that the clock glass had possibly 
“been a photographic plate at some earlier time. Pre- 
sumably it was a plane surface then. In giving the 
glass the curvature requisite to a clock glass it would 
be expected that any silver which may have been 
deposited while the plate was flat would have been 
disturbed when the glass had been moulded in a 
molten condition to its present shape. The image, 
It is | however, is not distorted in the slightest degree. 
R 2 
