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FEBRUARY 10, 1923] 
NATURE 
187 

at Mt. Wilson has shown that the opacity of the 
vapour of an exploded iron wire under certain 
conditions is such that light is cut off in a distance 
not greater than a few centimetres. Application 
of the above equation for K indicates that the 
absorption of radiant energy by the free electrons 
in the doubly ionised iron vapour produces an opacity 
of this order of magnitude. Thus, estimating T as 
20,000 degrees absolute, i as 2, A as 5x10~® cm. 
(doubtful), and p/(1 +7), the partial pressure of the 
iron ions, as 20 atmospheres (doubtful), K comes 
out as 1-7 for \=6x10-§ cm. The electrical con- 
ductivity of the vapour theoretically is 1/1500th 
that of metallic copper. 
Application of the equation for K to conditions 
in the outer regions of the sun, employing Saha’s 
theory to calculate the ionisation as a function of 
the unknown gas pressure, makes it seem probable 
that at a depth in the sun where the pressure is as 
great as o-or atmosphere the ionised gas is sufficiently 
opaque to cut off radiation from farther down. 
This is, then, indicated as the approximate pressure 
in the solar photosphere ; and pressures in the solar 
atmosphere are much lower. Thus the sharpness 
of the Fraunhofer lines may be explained. I hope 
soon to publish these results in detail. The astro- 
“ree importance of the matter is obvious. 
aturally it will require a great deal of study to 
develop more than a rough theory of the opacity 
of an ionised gas. Radiation is selectively scattered 
by bound electrons; it is non-selectively scattered 
by free electrons ; and it is absorbed by free electrons. 
The part played by bound electrons in absorbing 
radiation (that is, in transforming it to heat) seems 
at present far from understood. Prof. Eddington’s 
recent discussion (Observatory, December 1922) of the 
absorption of radiation by quanta in the deep interior 
of stars perhaps opens a new line of attack on the 
general problem. JouHN Q. STEWART. 
Princeton University Observatory, 
Princeton, New Jersey, 
January 8. 

The High Temperature of the Upper Atmosphere 
as an Explanation of Zones of Audibility. 
Tue work of Lindemann and Dobson on the theory 
of meteors,’ with the remarkable conclusion that 
the temperature of the atmosphere at heights such 
as 80 kilometres is about the same as that near the 
earth’s surface, will be far-reaching in its influence. 
May I be allowed to point out that one of the pheno- 
mena for which an explanation will probably be 
provided is the occurrence of zones of audibility and 
zones of silence, surrounding the scenes of great 
explosions. 
If, as Lindemann and Dobson find, temperature 
increases rather rapidly at about 60 kilometres, then 
sound waves penetrating that region will be refracted 
back to earth, the comparatively rapid curvature 
of the sound rays making the phenomenon almost 
equivalent to reflection as is the case with the light 
rays which occasion mirage. 
If we assume a sharp transition of temperature 
from 220° A. to 280° A. we find a refractive index 
for sound rays passing from the lower level to the 
upper of ,\/280/220 or 1-13. Total reflection takes place 
with an angle of incidence 62°, and if the reflection 
is at 60 kilometres the minimum radius for the outer 
zone of audibility is 2 x 60 x tan 62° or 155 kilometres. 
This rough estimate is of the right order of magnitude, 
as may be seen by comparison with the most recent 
2 4 Theory of Meteors, and the Density and Temperature of the Outer 
Atmosphere to which it leads. F. A. Lindemann and G. M. B. Dobson 
(Royal Society Proceedings, vol. 102, 1922, p. 411). 
NO. 2780, VOL. 111] 
example, the Oldbroek Explosion of October 28, 1922, 
for which the corresponding limit is stated to have 
been ‘“‘ about 180 or 200 km.” (NATURE, January 6, 
P- 33). 
There should be no great difficulty in adapting the 
theories worked out by von dem Borne and de Quervain 
to the new hypothesis. The drift of meteor trails 
shows that there is considerable horizontal motion 
of the atmosphere at such heights as 60 kilometres, 
and this motion will have to be taken into account. 
It is not unlikely that monsoonal changes in the upper 
winds produce the seasonal variation in the direction 
of audibility which was so noticeable during the war. 
The number of known observations of meteor trails 
is too small (cf. Meteorological Magazine, vol. 56, 
Pp. 292, 1921) to throw any light on this question. 
Further progress in our knowledge of the temperature 
of the outer atmosphere and of its motion would be 
made if Prof. Goddard could send up his rockets. 
The times of passage of the sound waves from the 
bursting rockets would give immediate information 
as to the temperature of theair. Perhaps it would be 
more practicable to use a “‘ Big Bertha”’ to send upa 
bursting shell. Mr. Denning could say, no doubt, 
whether there are any instances in which the disrup- 
tion of a meteor has been heard and the time interval 
between sight and sound has been recorded. 
With regard to the theory suggested by Lindemann 
and Dobson in explanation of the high temperature 
of the outer atmosphere, it should be pointed out 
that the atmosphere is only exposed to solar radia- 
tion during the day-time. It would seem that the 
equation by which the authors determine the steady 
temperature should be modified considerably. Annual 
variation in the temperature of these outer layers of 
the atmosphere is to be anticipated ; it is not unlikely 
that examination of the statistics regarding meteors 
will reveal it. According to the theory meteors 
should reach much lower levels in winter than in 
summer. F. J. W. WHIPPLE. 
6 Addison Road, Bedford Park, W.4. 

Fixation of Nitrogen by Plants. 
In NATURE of January 20, p. 95, reference is made 
to an announcement in Science by Lipman and Taylor 
that they have proved conclusively the fixation of 
atmospheric nitrogen by the wheat plant. Should 
the detailed evidence, when available, show that their 
claim is well founded, it should not be forgotten that 
similar results on other plants were obtained in this 
country some little time ago by the late Prof. Ben- 
jamin Moore and his co-workers. In two com- 
munications to the Royal Society (Proc. Roy. Soc., B, 
vols. 91 and 92, 1920), he argues strongly in favour 
of such fixation, supporting his views by convincing 
experimental proof on both fresh-water and marine 
alge. The work was incorporated in his book 
“ Biochemistry ’’ (1921), and in the Hugo Miiller 
memorial lecture delivered before the Chemical 
Society in June of that year—one of his last public 
utterances—he reiterates in the strongest language 
his belief, founded upon no inconsiderable amount 
of experimental work, “ that both the lower and 
higher plants do build up nitrites and nitrates and 
form organic nitrogenous compounds from the free 
nitrogen of the atmosphere.”’ : 
I may say that in their article in Science Messrs. 
Lipman and Taylor give references to Moore’s work 
as to that of other previous observers. 
EpwarD WHITLEY. 
Biochemical Department, 
University of Oxford, 
January 24. 
