424 
NATURE 
[AucusT 31, 1899 
diffused light of the sky caused the photographic reversal 
of the first image. 
Photography thus gives no support to the view that 
dark lightning has a real physical existence; and Lord 
Kelvin’s letter printed in NATURE of August Io (p. 341), 
together with that by Dr. W. J. Lockyer in last week’s 
number, show conclusively that when it is visually 
observed it is an effect due to fatigue of the retina. 
THE RECENT ERUPTION OF ETNA, 
psc A. RICCO, Director of the Etna Observatory, 
informs us that on July 19, at 8 a.m., Mount Etna 
threw out from its central crater an enormous mass of 
vapour, stones, lapilli, and cinders, which were lifted to a 
height of several kilonietres, and afterwards covered all 
the south-east slope of the volcano as far as Zofferana 
Etnae (altitude 600 m.), where the roads are covered by 
neatly a centimetre of volcanic ash. A number of stones 
struck the dome of the Etna Observatory (which is about 
a kilometre from the central orifice), so that about thirty 
holes were made in the iron plates, six millimetres in 
thickness, which cover this dome ; five of these holes have 
a diameter of 30 centimetres, and the stones causing them 
fell into the observatory containing the refractor. Two 
stones also pierced the floor, and embedded themselves in 
the basement ; and one broke three steps of the observ- 
ing chair. Another pierced the wooden base surrounding 
the foot of the refractor ; fortunately, this and the other 
apparatus of the observatory received no damage. Two 
other stones passed through the roofs of the side-rooms. 
Round the observatory there are about fifty holes, 
caused by the fall and penetration of the stones in the 
sandy soil. 
A heap of straw which was near the stables of the 
observatory was reduced to ashes, which proves the high 
temperature of the eruptive materials ; moreover, holes 
were also burnt in the wooden flooring where it had 
been pierced by stones. 
The steam of the eruption condensing in the air gave 
place to a warm and acid rain in the higher parts of the 
volcano, and lower down it caused ordinary rain. 
The column of steam had by nine o’clock spread itself 
enormously in the sky nearly over Catania (a distance of 
30 km.), and caused a marked darkening. By 9.30 the 
column had disappeared. 
The eruption was accompanied by no perceptible move- 
ment of the earth, except a slight shock at the lower end 
of the Valle del Bove. The instruments at Catania only 
indicated a very slight oscillation. At the Etna Observ- 
atory two seismometers showed horizontal and vertical 
movements. The eruption was also accompanied by 
detonations, which were heard very slightly as far as 
Catania. 
On July 25 there occurred a similar eruption, but of 
less importance. 
PROFESSOR BUNSEN, 
N Wednesday morning, August 16, the illustrious 
Heidelberg chemist breathed his last, after a long 
life wholly devoted to the furtherance of science. In April 
1881 I communicated to the cclumns of this journal a 
sketch of the work of him whose death at the ripe age of 
eighty-eight all lovers of science now have to deplore. 
We can only now call attention to the magnitude and 
extent of that work, and lay on the grave of one of the truest 
and noblest of men the tribute of our admiration and 
respect. As expressing the position held by Bunsen 
amongst the standard-bearers of science, I may, perhaps, 
be forgiven for quoting the opening sentences of what 
I wrote eighteen years ago, as I cannot find more appro- 
NO. 1557, VOL. 60] 
priate words to indicate what all feel who know what his 
work was. 
“The value of a life devoted to original scientific work 
is measured by the new paths and new fields which such 
work opens out. In this respect the labours of Robert 
Wilhelm Bunsen stand second to those of no chemist of 
his time. Outwardly the existence of. such a man, 
attached, as Bunsen has been from the first, exclusively 
to his science, seems to glide silently on without causes 
for excitement or stirring incident. His inward life, how- 
ever, is on the contrary full of interests and of incidents 
of even a striking and exciting kind. The discovery of a 
fact which overthrows or remodels our ideas on a whole 
branch of science; the experimental proof of a general 
law hitherto unrecognised ; the employment of a new and 
happy combination of known facts to effect an invention 
of general applicability and utility ; these are the peaceful 
victories of the man of science which may well be thought 
to outweigh the high-sounding achievements of the more 
public professions.” 
In the columns which follow the above will be found a 
statement of the chief experimental researches which 
have not only raised Bunsen by the common consent of 
all who can understand the results of accurate and far- 
reaching methods to the highest point of scientific 
honour, but also of those more popular discoveries which 
have made his name a household word in circles far 
wider than those of purely scientific appreciation. Now, 
therefore, it is only necessary to recall the main facts 
of his life work ; to note, in the first place, that 
his desire to unravel the secrets of nature was un- 
alloyed by any attempt to make capital out of any 
application of his discoveries. “To one man,” he often 
said, ‘‘comes the duty of discovery, to another that of 
applying that discovery to practical uses.” Like our 
great countryman Faraday, Bunsen consistently refused 
to be drawn away from the paths of purely scientific 
investigation, and, though too clear-sighted a mind to 
belittle the importance of the application of scientific 
discovery to every-day life, rightly judged that to him 
belonged the undoubtedly higher and nobler work of 
enlarging the boundaries of knowledge. 
The next thing to be noted about Bunsen’s work is its 
originality and its accuracy. It matters not whether we 
look into his purely chemical investigations, at his 
chemico-geological researches, or at those—perhaps the 
most remarkable amongst the many questions he answered 
—which lie on the borderland of physics and chemistry, in 
every case we rise from the study not merely feeling that 
we have to do with a master’s mind and hand, but that 
each investigation is stamped by an original mode of 
treatment and by an accuracy of thought and of manipu- 
lative power which, it is not too much to say, has rarely 
if ever been equalled. 
In no instance was this rare combination of mental 
and manual dexterity more strikingly shown than in his 
investigation of the compounds of czesium, the rarest of 
the two alkali metals which he discovered by means 
of spectrum analysis. In order to prepare the pure 
salts of this metal, some scores of tons (I write this away 
from books, and therefore cannot give the exact figures) 
of Diirkheim mineral water had to be evaporated, and 
from this residue it was only possible to obtain some 
five or six grams of the pure chloride. Nevertheless, 
from this comparatively minute quantity Bunsen suc- 
ceeded not only in preparing and analysing all the im- 
portant salts of czesium, but in ascertaining by goniometric 
methods their exact crystalline form. So that he was 
able to supply all the information requisite to a complete 
understanding of the position of this new element and its 
compounds to those of its well-known relations potassium 
and sodium. 
Then look at his gasometric methods. He was the first 
to attempt anything like exactitude in the measurement of 
