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NATURE 
iti 2-19 us 

fluorescence at all, while the inside which had cooled 
more slowly, and was found to be crystalline, gave 
a very brilliant green glow. 
Calcium metaborate can by very sudden cooling be 
obtained in a vitreous state showing no fluorescence, 
while the crystalline variety gives a moderate blue 
glow. The presence of manganese induces no 
fluorescence in the vitreous form, but a very brilliant 
green in the crystalline modification. Evidence has 
been obtained that many substances which, in their 
crystalline form, fluoresce brightly, exhibit no glow 
in the amorphous or vitreous state. 
From numerous experiments it is concluded that 
the green fluorescence of X-ray tubes is associated 
with the presence of a notable quantity of calcium 
and a relatively small amount of manganese, that a 
truly vitreous glass exhibits little, if any, fluorescence, 
and that a glass containing manganese can only be 
kept in this condition by extremely sudden cooling. 
Without suggesting that ordinary X-ray glass has 
any definitely crystalline structure, the evidence would 
indicate that something akin to this is more readily 
obtained when manganese is present. 
A tendency to crystallisation may be so slight in a 
glass as not to interfere with its use under ordinary 
working conditions, but there is no advantage in 
fostering this tendency. The presence of manganese 
in any appreciable amount introduces other defects of 
minor importance. I mention them only to emphasise 
the point made before, that if the intensity of green 
fluorescence seen in many foreign X-ray tubes is 
considered imperative it can only be obtained by 
sacrificing some of the good working qualities of the 
new and purer English glass. 
HERGERT JACKSON. 
University of London, King’s College, June 23. 

The Magnetic Storm and Solar Disturbance of 
June 17, 1915. 
THE magnetic storm described in Nature of June 24 
by the Rev. A. L. Cortie seems to have been larger 
at Stonyhurst than at Kew. The extreme westerly 
position of the declination needle at Kew occurred 
about 1.30 p.m., and the extreme easterly position 
about 5.37 p.m., the total range being about 72’. 
Between 5 and 6 p.m. the movements had a range of 
61’. I am not clear which of the two corresponds to 
the 91-5’ mentioned by Father Cortie, but either js 
substantially less, even allowing for the fact that the 
strength of the horizontal field is about 6 per cent. 
higher at Kew than at Stonyhurst. This is, of 
course, quite in accordance with the usual tendency 
for disturbance to be greater in higher latitudes, but 
it helps to illustrate the fact that whatever the ulti- 
mate source may be, terrestrial position counts for a 
good deal. The total range shown by the horizontal 
force at Kew was about 460y (1y=1x10-° C.G.S.), 
the maximum occurring about 5.42 p.m., and the 
minimum about 9.30 a.m. : 
Father Cortie’s remarks on the absence of the rapid 
oscillations sometimes characteristic of magnetic storms 
refer, I presume, to the time 4 to 6 p.m., when the 
largest movements occurred. Earlier in the day, for 
instance, near 6 a.m., the oscillatory character was 
fairly prominent at Kew. One would, in fact, have 
expected to hear of telegraphic interruptions. 
Father Cortie seems to associate the magnetic 
storm with a particular spot or group of spots on the 
sun. That at least is what one would naturally infer 
from his remark: ‘‘Such a close approximation of the 
position of the spot and the earth referred to the 
sun’s central meridian during a magnetic storm is 
very unusual.”” Whether magnetic storms are directly 
due to the emission of electrons from the sun, and, if 
NO. 2383, VOL. 95] 



so, whether the emission is localised in sun-spots, are 
questions on which there is a diversity of opinion. 
The present case seems a good example of the diffi- 
culties in the way of a final decision. Father Cortie 
tells us that ‘‘on June 17-18 there were no fewer than 
seven groups of spots visible.’’ On the other hand, 
the disturbance of June 17, though much the largest,, 
was by no means the only magnetic disturbance 
about the time. There was, as Father Cortie men- 
tions, what is usually termed a ‘sudden commence- 
ment” about 1.50 a.m.—I make it a minute or two 
later—on June 17, but there was another—somewhat 
larger—about 1 p.m. on June 16. The subsequent 
disturbance on June 16 was not large, and after 6 p.m. 
the conditions were fairly normal. We should natur- 
ally regard the disturbances on June 16 and 17 as 
distinct. Then on June 21, about 3.10 p.m., there was 
yet another ‘‘sudden commencement,”’ the largest of 
the three, and this was followed by a considerable 
disturbance lasting to the end of June 22. ‘‘ Sudden 
commencements,’’ even when small, are usually recog- 
nisable over at least the greater part of the world. 
Of all types of disturbance they seem to have the 
best claim to a cosmic origin. The subsequent dis- 
turbances, when there are any worth mentioning 
—which is not always the case—show much more 
rapid local variation. 
We have here, then, three disturbances with sudden 
commencements in the course of about five days, so that 
even accepting the sun-spot emission theory, in the 
absence of special identification marks, the association 
of one particular disturbance with one particular spot 
or group of spots would seem to be arbitrary. 
Another aspect of the case is that magnetic disturb- 
ances sometimes occur when sun-spots are con- 
spicuously few or wholly absent. For instance, there 
was a ‘‘sudden commencement”’ of considerable size 
on June 7, at a time when Father Cortie tells us the 
sun was almost free of spots! 
The fact that magnetic disturbances occur at inter- 
vals of from twenty-six to twenty-eight days more 
frequently than is accountable for by pure chance is 
obviously consistent with the sun-spot emission theory ; 
but it does not necessarily favour it. Quiet magnetic 
conditions show the ‘‘twenty-seven-day”’ period to 
practically the same extent as disturbed conditions. 
Sometimes disturbance is the rule, and quiet condi- 
tions the exception, and it is not clear that the one 
phenomenon is more fundamental than the other. 
There seems a difficulty in associating quiet condi- 
tions with some limited area, some ‘‘anti-spot,”’ on 
the sun. C. CHREE. 
June 26. 

The Names of Physical Units. 
May I point out that Dr. Guillaume is wrong in 
suggesting, in his letter in Nature of June 17, that 
the adjective “ specific,” employed in connection with 
physical magnitudes, has no constant and definite 
meaning? ‘‘Specific’” is the adjective of ‘‘ species,” 
and the ‘‘specific resistance of iron”’ is that function 
of the resistance of a piece of iron and the other 
physical magnitudes characteristic of the piece which 
is the same for all pieces which belong to the species 
“iron.” The statement made in the last sentence is 
true if for ‘‘resistance’’ be substituted any other mag- 
nitude to which ‘specific’? is attached, and for ‘‘iron’” 
any other form of matter which is recognised as a 
“species.” 
I am not urging that the retention of the term 
““specific’’ is desirable; on that matter I offer no 
opinion. I am only urging that the word has a per- 
fectly definite and constant meaning. 
Teddington, June 22. Norman R. CAMPBELL. 
