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NATURE 
[APRIL 11, 1907 
SOCIETIES AND ACADEMIES. 
LONDON. 
Royal Society, December 13, 1906.—‘‘ Experiments on 
the Length of the Kathode Dark Space with Varying 
Current Densities and Pressures in Different Gases.’’ By 
F. W. Aston. Communicated by Prof. J. H. Poynting, 
BaRese 
This paper deals with experiments on the length of the 
“Crookes ’’ dark space under steady currents in air, 
oxygen, nitrogen, and hydrogen, and its relation to 
pressure, current density and potential in a cylindrical glass 
discharge tube, the electrodes of which are large aluminium 
discs closely fitting the tube. Under these conditions it is 
found that so long as the current is kept above a certain 
value, 7.e. sufficient to cover the kathode with glow and 
to cause the complete disappearance of the positive light 
on the anode, the distance between the electrodes has 
quite a negligible effect upon the dimensions of the dark 
space, the current, and the potential. The current ceases 
abruptly, however, when the length of the dark space 
becomes the same as that distance; also the negative glow 
terminates sharply (in the. case of oxygen amazingly so) 
over the greater part of its area at a plane exactly parallel 
to the kathode at a distance from it (D) accurately measur- 
able by means of a simple sighting arrangement. In order 
to eliminate edge effects and to get a more exact measure 
of current density, the kathode used was in the form of 
a disc and guard-ring, that current passing through the 
disc only being measured. If P=pressure, c=current 
density, V=potential between electrodes, then very approxi- 
mately 
A, B, F, and E being constants for a given gas, the last 
being very nearly the same as the accepted values of 
kathode fall with aluminium electrodes in the gas. 
These empirical relations, together with other observ- 
ations, led to the conclusion that the dark space may be 
regarded as a region of positive electrification travelling 
towards the kathode, in which the total positive charge 
exactly balances the negative charge on the kathode. The 
theoretical fall of potential across such a region in which 
the density of negative electrification is assumed negligible 
é Sire \Fo 8 = 4 
is shown to be (=) 3D", where A is the velocity of a 
positive ion in a unit field and c is the density of the 
current carried by the positive ions, so that if the latter 
bear a constant relation to the whole current density 
passing through the tube, we should expect cPD*V-* to 
be a constant for any gas. This is found to be the case 
for all values of the dark space between 0-5 cm. and 2-0 cm. 
in the gases under investigation. From the values so 
obtained the velocities of positive ions at the very low 
pressures (of the order of 0-2 mm. mercury) employed are 
calculated, and shown to be of the order expected from 
the values at atmospheric pressure determined by Zeleny. 
The stream of positive ions may be strikingly shown by 
a rotatory mica mill mounted inside the dark space, which 
rotates violently in the opposite direction to the familiar 
ones designed to show the motion of kathode rays away 
from the electrode. Suggestions are put forward to account 
for the almost incredible “‘ sharpness’ of the edge of the 
negative glow in oxygen, the most remarkable phenomenon 
of the investigation. 
Mineralogical Society, March 19.—Prof. H. A. Miers, 
B.R.S., president, in the chair.—The silver deposit 
or Sedgman lode in the Perran Mine, Cornwall: F. H. 
Butler. The lode runs through killas in an approximately 
north and south direction. The silver ore, consisting 
almost solely of cerargyrite, occurs in compact masses or 
finely disseminated in a gossany limonite. Splintery and 
ferruginous quartz, the ‘‘ cab-course,’’ is always a well- 
developed feature in the richest parts of the lode. The 
distribution of the cerargyrite, to the depth of 18 fathoms 
to which the mine has been worked, is roughly in accord- 
ance with the surface contour of the land, but segregations 
have also taken place along a series of lines running from 
NO. 1954, VOL. 75| 
above downwards. The source of the chlorine, the author 
suggests, might be sea water that has reached abyssal 
regions.—The minerals of the Silvermines District, co. 
Tipperary: A. Russell. The mines extend along an east 
and west line of fault in which Silurian, Old Red Sand- 
stone, and Carboniferous rocks are brought into juxta- 
position. Along its course in certain places mineralisation 
has taken place, resulting in contact lodes and metasomatic 
deposits. In the Ballygowan South mine is an interest- 
ing occurrence of hemimorphite, the only one of the kind 
known in the United Kingdom. The mineral is found in 
brilliant crystals lining cavities in limonite. The gossan 
also contains irregular masses of argentiferous galena, 
partially altered to cerrusite. At the Ballynoe mine, copper 
pyrites, galena, and barytes form a lode between walls 
of Silurian and Carboniferous limestone. At the 
Gortnadyne mine argentiferous tetrahedrite is found with 
copper pyrites and cerrusite (‘‘ cat-tooth ore’’). An ex- 
tensive series of old open workings of galena can be seen 
at the Shallee East mine.—Baddeleyite from Ceylon: G, S. 
Blake and G. F. Herbert Smith. Three brilliant crystals 
of the mineral were picked out from a number of speci- 
mens of the heavy minerals from the gem district of 
Ceylon which were sent to the Imperial Institute in 1905. 
Of the three crystals, one possessing only the prism zone 
was used for analysis, and found to contain nearly 99 per 
cent. of zirconia. On the other two crystals, one of which 
was a twin, were observed eleven forms, including one 
new one (210).—Zinciferous tennantite from the Binnen- 
thal: R. H. Solly*and G. T. Prior. Crystals of tennan- 
tite, one of which was a large cube, with faces deeply 
striated parallel to small tetrahedral faces, were found on 
analysis to contain nearly 8 per cent. of zinc.—Striiverite, 
a new mineral: F. Zambonini and G. T. Prior. This 
new mineral was found in detrital masses of pegmatite 
near Craveggia, in N. Piedmont. Crystallographically 
it is very similar to rutile and tapiolite, with axial ratio 
a:c=0-6456. Some of the crystals are elongated along 
the pyramid edge, and are probably twins similar to those 
of ilmenorutile. The mineral is black and opaque, and 
has a specific gravity of 5-59. It contains titanic acid, 
zirconia, oxide of iron, and niobic and tantalic acid. The 
result of analyses suggests the formula 
3FeO.(TaNb),0,.4ZrO,.9TiO,, 
which may be written as a mixture of the three molecules 
Fe(TaNb),O,, FeZr,O,, and TiTi,O, in the proportion 
of 1:2:3. Chemically it is very similar to ilmenorutile, 
but contains about 28 per cent. of ZrO, replacing part of 
the TiO,. 
Zoological Society, March 19.—Dr. Henry Woodward, 
F.R.S., vice-president, in the chair.—Recently discovered 
subfossil Prosimiz from Madagascar, and their affinities 
with extant lemurs and with the higher Primates: H. F. 
Standing. The remains were obtained in the muddy 
bed of a swamp formed by the blocking up of the river 
Mazy by a lava flow, at from a few inches to 3 feet or 
4 feet below the surface. They consisted of a large 
number of skulls and limb-bones of lemurs and lemur-like 
animals. This great amount of material enabled the 
author to corroborate the view, previously put forward 
by Dr. Forsyth Major, that the extinct lemurs of Mada- 
gascar were, in many respects, intermediate between exist- 
ing lemurs and monkeys, and to express his belief that the 
New World monkeys and the Lemuridz, as well as the 
Malagasy Indrisinaz, had a common origin. He also stated 
his opinion that it was not possible to separate the Primates, 
as hitherto, into the two suborders Lemuroidea and 
Anthropoidea.—Animal parasites: Dr.. L. W. Sambon. 
Three new species were described :—Wellcomia mitchelli, 
gen. et sp. nov., habitat, small intestine of Pedetes caffer; 
Sparganum baxterit, sp.n.? habitat, connective tissue of 
man; Schistosomum mansoni, sp.n., habitat, blood-vessels 
of man. Dr. Sambon also described five new Hamo- 
gregarines discovered by Dr. C. G. Seligmann and himself 
in snakes.—A collection of mammals, the seventh of the 
series, made by Mr. C. H. B. Grant at Coguno, Inham- 
bane, and presented to the National Museum by Mr. C. D. 
Rudd: Oldfield Thomas and R. C. Wroughton. The 
collection consisted of 212 specimens belonging to thirty- 
nine species, of which six were described as new. 
