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valley forms: of mountain plants there were :—Salix 
herbacea, L., Saxifraga stellaris, L., and Phyllodoce 
caerulea, L.; and of forms of general distribution 
which one does not usually associate with glaciers 
there were Alchemilla alpina, L., Trientalis europaea, 
L., Pirola minor, L., Pinguicula vulgaris, L., Pheg- 
opteris Dryopteris L., Lotus corniculatus, L., Sagina 
procumbens, L., and a species of Epilobium.—Dr. Otto 
Stapf: The Dragon Tree of Tenerife. The author 
showed various illustrations of the celebrated tree at 
Orotava, and especially a drawing by Don Augustin 
Monteverde, dating from the earlier months of 1819, 
before the tree was partially destroyed by a gale on 
July 21, in that year. This drawing is the property of 
Dr. Perez, of Orotava, who had sent it to Kew for 
comparison with other illustrations. Dr. Stapf dis- 
cussed the known history of the dragon tree of the 
Canaries and notices of it from early writers, referring 
inter alia to the resinous product known as “‘ Dragon’s 
Blood,”’ formerly used as a pigment and in medicine, 
but now almost restricted to colouring varnishes. 
Zoological Society, June 8.—Dr. S. F. Harmer, vice- 
president, in the chair.—G. Jennison; The ‘“‘nest”’ 
made by a chimpanzee in the Belle Vue Zoological 
Gardens, Manchester.—R. J. Pocock: The feet, scent- 
glands, and other external characters of the Paradox- 
urine Viverrids, belonging to the genera Paradoxurus, 
Arctogalidia, Arctictis, and Nandinia. It is shown 
how these may be distinguished collectively from the 
Viverrine genera (Genetta, Viverra, etc.), and also 
how they may be differentiated from each other in the 
characters discussed.—Dr. A. Smith Woodward: The 
skull of an extinct mammal related to A#luropus, 
obtained from a cave at the ruby mines, Mogok, 
Upper Burma. The skull is described as the type of 
a new genus and species.—Miss K. M. Parker: The 
early development of the heart and anterior vessels 
in marsupials, with special reference to Perameles.— 
Lieut. R. Broom: Certain Triassic Stegocephalians. 
Restorations are given of the skulls of Brachyops 
laticeps, Owen, and Bothriceps australis, Huxley, 
which are regarded as forming, with Batrachosuchus 
browni, Broom, a_ distinct family, Brachyopide. 
Bothriceps huxleyi, Lydekker, is shown to differ from 
Bothriceps australis in the structure of the occiput, and 
in having numerous small teeth on the parasphenoid, 
pterygoids, and prevomers, and thus to belong to a 
very distinct new genus. 
Geological Society, June 9.—Dr. A. Smith Woodward, 
president, in the chair.—R. H. Rastall and W. H. 
Wilcockson; The accessory minerals of the granitic 
rocks of the English Lake District. The rocks de- 
scribed are the granites of Skiddaw, Shap, and Esk- 
dale, the microgranite of Threlkeld, and the grano- 
phyre of Buttermere and Ennerdale. The material 
was pounded in a mortar, washed and panned, and 
the concentrate separated in bromoform after the 
removal of the magnetic portion. The results showed 
a variation of accessory minerals between the different 
intrusions, but a similarity between parts of the same 
intrusion, although the minerals of apophyses are not 
always the same as those of the main mass. One 
remarkable result obtained is the rarity of magnetite 
and the prevalence of pyrrhotite, which was present in 
every sample examined. Attention was directed to the 
characteristics of the zircon-crystals, which lent no 
support to the occurrence of definite types in granite 
and gneissose rocks respectively. In parts of both the 
Skiddaw granite and the Threlkeld microgranite, 
anatase and brookite were found in abundance. 
Epidote is the characteristic mineral of the Ennerdale 
granophyre, while garnet is abundant at Threlkeld 
and Eskdale. The Eskdale granite also contains much 
tourmaline. The Shap granite is characterised by 
NO. 2382, VOL. 95] 
NATURE 

[June 24, 1915 
apatite and sphene. Descriptions of accessory minerals 
founded only on examination of rock-slices are in- 
adequate and misleading.—F. P. Mennell: The rocks 
of the Lyd Valley, above Lydford (Dartmoor). <A 
small area on the north-east of Dartmoor is chiefly 
considered, though some of the conclusions are applic- 
able to nearly all that part of the moor which lies 
north of the portion mapped by the Geological Survey. 
In the neighbourhood of Lydford the alteration of the 
Carboniferous rocks within the metamorphic aureole 
surrounding the granite is described, and it is shown 
that they are consistently cordierite- and biotite-bear- 
ing. North of the altered limestone the type of altera- 
tion is different, and leads to the inference that the 
beds are distinct. The change is of more than local 
significance, as from this point all round the north of 
the moor there is no bed of any thickness containing 
cordierite, while chiastolite, white mica, and anda- 
lusite proper, are characteristic. Coarse andalusite-rock 
and altered shale, with remarkable skeleton-crystals 
of chiastolite, are described from the Nodden quarries, 
together with other types of hornfels. - The beds 
occupying the northern_part of the contact-zone belong 
to a definite series. There is evidence that the cover 
of the granite mass has a dome-like character, and 
that the same stratigraphical horizon is in contact 
with the granite all the way from Sourton to Drew- 
steignton. The granite of Brator is described. It is 
a biotite-bearing rock containing a little microcline, 
as well as orthoclase and oligoclase. It is rich in 
cordierite, recrystallised from sedimentary material 
absorbed into the magma. 
Physical Society, June 11.—Dr. A. Russell, vice-presi- 
dent, in the chair.—E. A. Griffiths and E. Griffiths : 
The coefficient of expansion of sodium. The thermal 
expansion and increase in volume on liquefaction of 
sodium were determined by a method based on the 
following principle :—The difference in expansion of 
a volume of sodium and an equal volume of glass 
(or quartz) was measured by differential weighing 
under oil at various temperatures. A volume of about 
250 c.c. of sodium was suspended from one arm of a 
short beam balance and a weighed glass bulb of equal 
displacement from the other arm. Sodium expands 
uniformly with the temperature up to its melting point. 
The value 0-000226 was deduced for the coefficient of 
expansion. In changing from the solid to the liquid 
state, an increase of 2-57 per cent. occurs in the 
volume.—T. Smith: Notes on the calculation of thin 
objectives. Lens systems which are symmetrical about 
an axis have in general six degrees of freedom for 
first-order aberrations. Thin systems have only three 
degrees of freedom, and in consequence of the limited 
range of glasses only two degrees of freedom are 
practically available. In achromatic combinations of 
two lenses these two degrees of freedom are controlled 
by the general shape as distinct from the total power 
of each lens. In general when two given conditions 
are satisfied the curvatures of the inner surfaces are 
not equal, so that a cemented combination of two 
lenses is not possible. Owing to the increased light . 
transmitting powers it is often necessary to have only 
two glass air surfaces, and thus more than two com- 
ponent lenses are necessary. The effect of bending any 
thin system as a whole by increasing the curvature of 
each surface by the same amount is investigated, and 
it is shown that with two given kinds of glass a 
triple cemented lens can be formed satisfying two 
arbitrary aberration conditions. Illustrations are given 
of astronomical objectives of both double uncemented 
and triple cemented forms, and the glasses are deter- 
mined for which a doublet can be cemented.—T. 
Smith: Tracing rays through an optical system. 
Trigonometrical formula have been used for tracing 
rays not lying entirely in one plane through optical 
