May 21, 1885] 
decided red colour, the rest being white, as usual. Taking up 
some that fell in the gig, Mr. Mullan found that the colour was 
not merely superficial, but pervaded the substance of the hail- 
stone, and, on melting, they stained the fingers. He did not 
think, or had not the means, of preserving any of the water 
resulting. Has the like been observed before? 
Spectral Images 
Mr. BIDWELL’S notice of spectral images (NATURE, vol. 
xxxiil. p. 30) calls to mind certain phenomena I witnessed 
while riding in a railway train in Kentucky last October. The 
fence of the railway consisted of posts of about 6 inches in 
diameter, and twenty paces apart, connected by wires. The 
posts had newly been painted green. I was seated on the right 
side of the carriage, face forwards; the speed fully twenty miles 
an hour, with the sun behind my right shoulder, when looking 
at the posts on the left side, brightly illuminated by the sun, I 
observed that each post had the appearance of a twin post imme- 
diately in advance of it—touching it—of a red colour. To make 
myself sure that I was not deceived by some abnormal affection, 
IT called the attention of a niece of mine to the phenomenon, 
and she saw it quite as well as I did. Another niece, however, 
failed to make it out. Iam under the belief that the red post 
was the complementary colour of the green one, appearing the 
instant after the latter had been seen, and though apparently 
in advance in space of the green pust, really was seen later in 
time. The fact of both being apparently seen simultaneously, 
is accounted for by the well-known law of retinal images 
lingering on vision. HENRY MUIRHEAD 
Cambuslang 
THE NEW OUTBURST OF LAVA FROM 
VESUVIUS 
Ry PESTERDAY, May 2, up to two o’clock, Vesuvius 
appeared to be in its natural state of activity, such 
as persisted with slight variations for some considerable 
time. At that hour the lava, which was at some height 
within the cone of eruption, forced a way out at its base, 
traversing the plain of old lava filling the crater of 1872, 
and producing a rent about one quarter the way down 
the great Vesuvian cone. This rent represents the exten- 
sion outwards of a volcanic dyke that has been in process 
of formation for over two years. A visitor during that 
period who walked around the southern rim of the 1872 
crater, might have noticed a fissure varying from a few 
inches up to 2 feet wide, and extending inwards across 
the crater plain, until lost beneath the efectamenta of the 
cone of eruption. From this fissure issued a powerful 
current of hot air, and in part of its course an abundance 
of HCl. This latter was indicated by the continual de- 
composition of the scoria and ash in its immediate 
neighbourhood, so that a large patch of yellow dust filled 
with the unattached pyroxene crystals was a point of 
bright colour in the black scoria-covered lava-plain. 
The lava at first actually issued, or, more properly, 
welled up from this fissure, but its point of exit was 
soon lowered by the cutting down of the outer slope. 
The lava soon commenced to flow down the cone with 
considerable rapidity, forming two distinct parallel streams 
averaging fifty metres apart, so that in the evening the 
landscape was lit up by these two brilliant streaks of fire. 
This morning I started early, and ascended on foot to 
the eastern side of the two streams, though often incon- 
venienced by the hot wind and exhalations blown off the 
lava. The streams take origin close together, and no 
doubt conjoin, but are covered by scoria—a vast quantity 
of /apil/o and ash that has been slipped downwards and 
forward, forming a rough annular space which would 
require a drawing to explain. At the upper end of this 
we have part of the great cone slipped down, showing in 
section the dyke, which I may call hollow; we have a 
fissure which was filled by lava, and which consolidated 
and adhered to its sides, forming sadbam_, but before the 
central part solidified, the general level was lowered, and 
NATURE 
ay) 
it drained away, leaving the dyke divided in two by an 
empty space. At 2 p.m. to-day the streams of lava had 
the following dimensions at their exit :— 
Eastern Western 
Breadth about 14 metres About 24 metres 
Depth estimated at 1 metre : at 2 metres 
Rate of flow on both, about 1 metre per second. 
The output therefore equals for the eastern stream 
about 90 cubic metres per hour, or 2160 cubic metres in 
24 hours, whilst that of the western stream represents 
300 cubic metres per hour, or 7200 in 24 hours. The two 
streams, therefore, represent an otttput of 9360 cubic 
metres during the 24 hours, from May 2 to 3, at 2 p.m. 
This quantity would equal a deposit of rock of about 
1 km. long, 9 m. broad, and 1 m. thick, which is rather 
an under-estimation of what now lies on the side of the 
mountain, for the two streams had at the hour of obser- 
vation traversed more than two-thirds of the fedzmenture. 
The amount of lava represents far more than what 
occupied the chimney above the level of the lateral open- 
ing, and the mechanism of the increased quantity ex- 
truded I have gone into fully ina paper read last week 
before the Geological Society. The cone of eruption 
only now gives forth vapour, its stone-throwing propen- 
sities being stopped by the lowering of the magma level. 
In consequence of the want of support of its inner walls 
by disappearance of the fluid column, these are rapidly 
crumbling in, and the craterial inner cavity much in- 
creased in size. In the same way a breach has been made 
in the line of the dyke by falling in of that part of loose 
materials immediately above it. 
This change in Vesuvius will no doubt be put down in 
history as an eruption, and possibly a relationship sought 
between contemporaneous earthquakes, or some other 
phenomena. It is nothing more nor less than the final 
giving way of part of the cone before a dyke that has 
been working its way out for years. 
I send you these few notes after a long day’s climb, 
exposed to great changes of temperature and mephitic 
vapours. I ask, therefore, that this will be taken as an 
excuse for these rough and ready notes, which I thought 
your readers would be interested to have quickly. 
Naples, May 3 H. J. JOHNSTON-LAVIS 
EXPERIMENTS WITH COAL-DUST AT 
NEUNKIRCHEN, IN GERMANY 
eS a former article on this subject which appeared in 
NATURE of Nov. 6 last (p. 12), I described the appa- 
ratus employed by the Prussian Firedamp Commission 
in making their experiments, and at the same time I gave 
an account of four experiments that were seen by Mr. 
Wm. Thomas Lewis and myself. 
No official account of these experiments had been pub- 
lished at that time, but quite recently Herr Hilt and Herr 
Margraf have made a joint report in the name of the 
Commission. As this report is intended to be only a 
preliminary one, it does not give the whole of the details 
of each experiment, but it shows as far as it goes that 
everything has been conceived and carried out in a spirit 
of liberality and thoroughness. 
At the outset Herr Hilt states that the uncertainty 
which seemed to surround this important question, and 
in particular the peculiar views that had been enunciated 
by MM. Mallard and Le Chatelier, who reported upon it 
to the French Commission du Grisou,! had induced him 
to address a letter on the subject, dated December 15, 
1883, to the Prussian Wetter-Commission, urging them 
as a matter of duty to take it up and investigate it by a 
series of large-scale experiments. The French Commis- 
sioners, referred to, stated at the end of their report 
that “they considered it established that coal-dust in the 
absence of fire-damp does not constitute an element of 
* Annales des Mines, Janvier—Février, 1882. 
