534 
NATURE 
[APRIL 21, 1923 

Wegener’s hypothesis, and emphasises the fact that 
supporters of that hypothesis must look elsewhere 
than to the facts of animal and plant distribution for 
positive evidence in its support. Zoologists and 
botanists are dependent on the geologist and geo- 
physicist for the correct interpretation of the palzo- 
geographical changes which have taken place in the 
earth, and must be guided by them in selecting the 
basis on which the known geographical distribution of 
living forms can be explained. 
THE WRITER OF THE ARTICLE. 

Use of the Triode Valve in Spectrometry. 
THE three-electrode valve offers a very simple 
and trustworthy method of amplifying the small 
currents produced in the thermopile of an infra-red 
spectrometer. Bright lines are more readily picked 
up and the limits of absorption bands determined 
with greater certainty with a valve and telephone 
than with a galvanometer. Moreover, the valve is in- 
stantaneous in action, while a sensitive galvanometer 
takes an appreciable time to give a trustworthy 
indication—so much so that the fainter lines are apt 
to be missed when using a long-period galvanometer. 
In the thermopile circuit an interrupter is necessary : 
this may take the form of a steel wire maintained 
in vibration electrically to which is attached a 
small wire dipper making contacts through a cup 
containing mercury. The interrupted thermopile 
current is passed through the primary of a small 
step-up transformer the secondary of which is 
connected to the grid of the valve. 
For quantitative work the thermopile current is 
balanced by a potentiometer, a minimum of sound 
in the telephones indicating the point of balance. 
The valve has a further advantage over the galvano- 
meter in that it is unaffected by vibration or stray 
magnetic fields. The use of a valve for such work 
would seem to have many other applications, and 
to this end further experiments are being carried 
out. L. BELLINGHAM. 
71 Hornsey Rise, 
London, N.19, 
March 22. 

The Release of Electrons by X-rays. 
In his interesting article, “‘ Recent Advances in 
Photographic Theory,” in Nature of March 24, Dr. 
Mees touches upon the nature of X-rays and the 
mechanism of their production, and quotes Sir William 
Bragg’s analogy of the plank of wood dropped into 
the sea. 
I believe that Sir William Bragg put forward this 
analogy in a Robert Boyle Lecture, rather with a 
view towards successfully visualising the electron- 
X-ray process than of proving an individual relation- 
ship between them. One is tempted to say that an 
analogy never proves anything, although it may be 
thoroughly illuminating. 
It is an extraordinary fact that a beam of X-rays 
will release electrons from an object which they hit, 
with just the velocity of the stream that originates 
the rays ; it appears probable from energy considera- 
tions that this relation cannot hold down to the 
individual electron, so it might not be unprofit- 
able if experiments were directed towards finding the 
limiting strength of the stream of electrons for the 
production of X-rays. S. Russ. 
Physics Department, 
The Middlesex Hospital, W.1, 
March 26. 
NO. 2790, VOL. IIT], 
The Magnetic Disturbance of March 24-25. 
A CONSIDERABLE magnetic disturbance occurred 
on March 24-25, as recorded by the Stonyhurst 
magnetographs. There was no marked sudden com- 
mencement of the disturbance, but the declination 
magnet began to move steadily towards the W., 
accompanied by a decrease in horizontal force, be- 
tween the hours 8 and 9 G.M.T. The declination 
magnet attained the extreme limit of its westerly 
movement at 13 h. 14 m., when it began to move 
gradually towards the E. At 17h. 12 m. a period of 
rapid oscillations commenced on the declination 
magnet. On the horizontal force magnet the de- 
crease in force was succeeded, at 12 h. 24 m., by an 
increase. A very rapid oscillation of increase and 
decrease occurred between 17 h. 2 m. and 17 h. 18 m., 
the range being 88 y (1 y=10-° C.G.S. unit). 
A quieter period ensued on the declination magnet 
between 18 h. 24 m. and 21 h. 12 m., while the hori- 
zontal force magnet, after the rapid oscillation at 17 h._ 
2 m., showed a gradual decrease in force, which 
reached its limit at 18 h. 48 m. A remarkable rapid 
oscillation, to E. and return to W., occurred on the 
declination magnet, between 21 h. 12 m. and 21 h. 
26 m., the range of the oscillation being 38’. —This was 
accompanied on the horizontal force magnet by an 
even more noteworthy rapid oscillation of increase 
and decrease of force, of range 189 y, between 21 h. 
24 m. and 21 h. 50m. 
The only other notable feature of the disturbance 
was a bay in both elements, between March 25, 2 h. 
12 m., and 3 h. o m., the range in declination being 
16’, and in horizontal force 97 y. The more violent 
phases of the storm had ceased by March 25, 8 h. 
The extreme ranges were, in declination 1° 6’, and in 
horizontal force 238 y. The vertical force magnet 
showed a general movement, with oscillations, of 
increase and decrease of force between the hours 
March 24, 17 h. 12 m., and March 25,4 h.24m. This 
would indicate the period of the greatest activity of 
the disturbance. The sensibility of the magnet is 
uncertain, but the extreme range was about 80 y. 
A disturbed period in magnetic activity occurred 
on February 25-28, so that this storm follows after an 
interval of 27 days, the synodical rotation period of 
the sun. But the solar surface has been unusually 
quiet during the past two months, at least so far as 
spots, which have been very few and of small area, 
and facule, which have been very faint, are con- 
cerned. The connexion of these disturbances with 
solar phenomena will require further elucidation. 
Father Dechevrens, S.J., recorded strong earth- 
currents at his observatory, St. Louis, Jersey, during 
the February disturbance. It will be interesting to 
hear of any observations of aurora borealis. 
A. L. Corte, Safe 
Stonyhurst College Observatory, 
April 3. 
Pressure of Fluidity of Metals. 
Mr. Hucu O'NEILL, in his letter in NaTuRE of 
March 31, p. 430, gives what he calls H,, the ‘‘ ultimate 
hardness ’’ of tin, zinc, and steel. On referring to 
my letter at p. 17 of Nature of January 6, it will be 
seen that H is there used for H,, and that the pressure 
of fluidity =twice the ultimate hardness. The units— 
of H, given at p. 430 are evidently kilograms per 
sq. mm. Expressing these in kilos per sq. cm. and 
multiplying by 2 we obtain the following values of 
the pressures of fluidity as calculated by Mr. O’Neill 

