2 
50 
NATURE 
[JANUARY 14, 1904 
is voiced in the language of one of the ablest descriptive 
writers on astronomical subjects of the present day. On 
p 127 of her late admirable work entitled ‘‘ Problems in 
Astrophysics,’’ Miss Clerke dismisses the subject with the 
words, *‘ The only genuine ‘ eclipse comet’ so far captured 
was that seen and photographed at Sohag 17th May, 1882.” 
This talented writer makes not the slightest reference to 
the fact that the 1893 phenomenon differed in one important 
particular from all those it is said to resemble—that it 
possessed the one observed element which even the genuine 
object does not lay claim to, namely, the photographs prove 
that this object moved, receded from the sun through an 
angular distance equal to two-thirds of the solar diameter 
in less than four hours. On the Mina Broncis photographs 
this object is plainly connected with the sun by a single, 
straight, isolated coronal stream.* J. M. ScuarBerte. 
Ann Arbor, December 19, 1903. 
Dynamical and Granular Media. 
I sHouLtp be very much obliged if any reader of Nature 
who has studied the matter could enlighten me on the 
following point. 
We may regard a dynamical system as commonly under- 
stood as being a system which, when left to itself, obeys 
the Hamiltonian equations 
ee OE _ — 
os or 
] 
| bromide, which is the most sensitive, emits more light than 
| half remains dark and emits no light when the plate is pul 
where x, 7, . . . are generalised coordinates, , 7. . gener- 
alised momenta, and 
U=4(82)2° + (Enint . . 2 $V + (2)i) 
ani V, (é), (En), &c., aré any functions whatever of x, y, . . - 
We may regard a granular medium as a particular kind 
of system coming under this heading for which U takes 
the form 
Uaz,ie tie FS 5 ss Fale, — 24)? — 90) *4H(e— 
2m, 
Ss)" Ht (2) 
where m, stands for any constant (being the mass of the 
rth particle, atom, corpuscle, grain, or whatever else you 
like to call it) and F,, is any function whatever, con- 
tinuous or discontinuous, determined by the law of force 
between different masses. 
What I want to know is this :— 
(1) Is every dynamical system which can exist in Euclidean 
three-dimensional ‘space transformable into a granular 
system according to the above definition by a proper choice 
of coordinates? 
(2) If not, what are the precise mathematical conditions 
under which a dynamical system can be so transformed ? 
We may put these questions in a somewhat different form. 
It is undoubtedly possible to conceive a universe the physical 
phenomena of which are represented by equations of any 
assumed form whatever, and therefore not necessarily by the 
equations of dynamics. Is it possible to conceive a universe 
the physical phenomena of which are represented by 
dynamical equations, but cannot be accounted for by means 
granular medium? I have read many treatises and 
‘s dealing with theories of the ether, in which it has 
tacitly assumed that the only possible answer to (1) must 
inevitably be ‘* yes,’’ and I cannot but feel that a discussion 
in vour columns might be of much use to physicists. 
G. H. Bryay. 
Phosphorescence of Photographic Plates. 
I osTaineD the following results, which are new to me, 
in the course of some experiments on the action of light 
on the salts of silver. 
I have not yet thoroughly examined the light or radiation 
emitted in these experiments, but its actinic power is low, 
and it appears to render the brush discharge from an in- 
duction coil more lur us. 
The sensitive silver salts, such as the bromide, iodide and 
chloride, if precipitated and kept in the dark, have the 
property, under certain conditions, of emitting light in 
degrees proportionate their sensitiveness. Thus the 
Also Mr. Wesley's article in 
to 
1 See Astronomical Journal, No. 
tory, No. 220, p. 349- 
= See rough sketch in “ Astronomy and Astrophysics,” 1804, p. 307. 
NO. 1785, VOL. 69] 
| action of light destroys their power of emitting it under the 
| 
the iodide and chloride. A convenient way of observing the 
phenomenon is to take a bromide photographic plate and 
place it at once (without having exposed it) in ordinary 
pyro soda developing solution and allow it to remain for 
ten minutes. Take out of the solution, wash, extinguish the 
“red lamp,’’ and in total darkness plunge it suddenly into 
a dish containing a saturated solution of aluminium 
sulphate. The plate immediately becomes phosphorescent, 
and the solution also is luminous, but not so bright as the 
plate is at first. The light gradually weakens, and in a 
minute or two dies away. On pouring the solution off the 
plate into a bottle, the whole body of the liquid becomes 
luminous, and has the appearance of ‘‘ bottled moonlight.” 
It remains so several minutes, and the light is increased 
by shaking the liquid. 
If half the plate be exposed to the action of white light for 
a second before treating with the pyro soda solution, that 
? 
into the aluminium sulphate. If the plate is given a short 
exposure in the camera, and developed and put into the 
aluminium sulphate solution, the image will appear dark on 
a phosphorescent background. 
On placing some precipitated bromide of silver (which 
had been kept a few days in a corked test-tube in the dark) 
in a porcelain dish and exposing it to a bright red light | 
whilst adding the pyro-soda solution, it appears black, but 
on pouring off the solution the precipitate gradually assumes — 
a bright green appearance under the red light, whilst in 
white light it appears dark grey or black. \ 
The remarkable part of these experiments appears to me 
to be the fact that the exposing of the silver salts to the 
treatment described, whilst the salt precipitated and treated | 
in total darkness emits light freely. 
T. A. VaucutTon. 
Ley Hill House, Sutton Coldfield. 
Format:on of Coal 
Some of your readers will no doubt be interested to see the 
photograph sent herewith, which represents a peculiar — 
growth of coal on a piece of timber. » ii 
The timber is part of a wooden trough into which, for 
a period of three years, water had been delivered by tanks 
lifting the water from a ccal-mine shaft. 
The formation of coal was found adhering to the vertical 
sides of the trough, forming a miniature coal seam about 
a quarter of an inch thick. This coal is hard and bright, 
and its texture and solidity differ in no respect from 
ordinary coal. f 
The explanation seems to be that the water contained 
small quantities of fine coal dust abraded from the seam 
i a a ce 
below, and these, either. through the motion of the water 
or by some other means, were filtered out and formed anew 
into solid coal. 
I believe this phenomenon has never previously — 
observed, and it appears to show that a coal seam may be 
broken up, washed away, and again built up in a new posi- — 
tion without the aid of either the passage of time, pressure 
or heat. Henry Hatt. i 
Rainhill, January 3. 
The Lamprey. a 
I sua feel obliged to any of your readers who will kindly — 
tell me where to procure specimens of the lamprey. They 
are unobtainable at the Marine Stations of Millport and 
Plymouth. J. PENTLAND-SMITH. 
St. Regulus, Park Place, Elie, N.B., January 5. 
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