June 6, 1912] 
an inscription has already been set up on the Banne 
d’Ordenche, a viewpoint above the valley of the 
Dordogne; it explains briefly that the Banne itself 
is a volcanic neck, and indicates its relation to the | 
volcanic system of the Auvergne generally, most of 
the members of which are visible from it. The in- 
scription is stated greatly to interest those who ascend | 
the Banne. ‘Tables d’orientation”’ are rare in our 
own country; there is no organisation specially con- 
cerned to provide them, but if such as exist were 
equipped with explanations of the scenery on M. 
Glangeaud’s lines, they would probably become 
objects of pilgrimage not only for tourists, but for 
students and school classes. 
SOCIETIES AND ACADEMIES. 
Lonpon. 
Royal Society, May 23.—Sir Archibald Geilsie, 
K.C.B., president, in the chair.—H. S. Hele-Shaw:: 
Theory of a new form of the chamber crank chain. 
The paper commences by showing in what way the 
mechanism is derived from the ordinary type of crank 
mechanism, its various phases being indicated 
diagrammatically. One feature of the mechanism, 
which is of practical importance, is that the crank is 
fixed, and so a variable stroke can be obtained by 
very simple means. The new feature of 
properties, is the employment of what is called ‘“‘a 
floating guide ring.”” This device largely reduces the 
friction of the contrivance when working under high 
pressures.—Prof. R. A. Sampson: A new treatment of 
optical aberration. A method is developed by which 
Gauss’s method of relating original and emergent 
rays in a coaxial optical system 
v=Bx+b,) v’=B'x'+-b', 
Z=yx+c, | sire, |) 
by means of a transformation, 
b’=Gb+H8, 8’=Kb+L8, c'=Gc+Hy, 
y'=Kce+Ly, 
where GL—HK=z/yn'=N, may be applied so as to 
include the aberrations of the third order. The 
method is adequate for the numerical calculation of 
telescopic objectives, and offers a remarkable economy 
in the work hitherto necessary.—Sir W. de W. 
Abney: The extinction of light by an illuminated 
retina. In this communication the author describes 
an apparatus adapted for illuminating the retina with 
known amounts of light, coloured or white, and for 
extinguishing the sensation of the light in the 
different colours of the spectrum. Confining himself 
to the stimulation of the retina by white light only, 
he shows the movement in the spectrum of the rays 
requiring the maximum amount of diminution to 
extinguish their light according as the retina is more 
or less illuminated.—Dr. W. Wahl: Optical deter- 
minations at high pressures. Diagram of state of 
carbon tetrabromide.—The melting point of CBr, is 
raised 1° by a pressure of 16 kg. cm.? The transi- 
tion point from monoclinic to regular crystal form is 
raised 1° by 32 kg. cm.* The melting-point curve 
and the transition-point curve do not, therefore, inter- 
sect at high pressures to form a “‘triple point.’ In 
consequence, the monoclinic form of carbon tetra- 
bromide cannot be caused to melt at any temperature 
or pressure whatever. Diagram of .state of a-B- 
dibrompropionic acid.—Two modifications of the acid 
are known, a stable one melting at: 64° and an 
unstable melting at 51°.. The unstable modification 
is not spontaneously transformed into the stable one 
so readily as in most other cases of: ‘‘monotropy,” 
NO. 2223, VOL. 89] 
the — group. The addition of another solvent to these 
mechanism, which results in somewhat remarkable | solutions causes a slow disappearance of the lines 
NATURE Be 
and as only very small quantities are employed for 
these optical determinations, it has been possible to 
determine the melting-point curve of the unstable 
modification also. During isothermal melting of the 
unstable modification the pressure may be reduced 
as much as about r50 kg. cm.* below the true melt- 
ing-point pressure before melting takes place rapidly. 
This pressure difference corresponds to a superheat 
ing of 25°. The melting point of the stable modifi- 
cation is raised 1° by a pressure of 51-3 kg. cm.? 
_ The difference between the absolute melting points of 
the two polymorphic modifications is at any pressure 
similar to the difference between the absolute melting 
points at ordinary pressure.—T. R. Merton: The 
changes. in certain absorption spectra in different 
solvents. (1) The absorption spectra of uranous 
chloride in a number of organic solvents have been 
measured quantitatively, the results indicating that 
the differences cannot be considered as a shift of the 
bands, since the entire character and intensity of the 
absorption varies in different solvents. (2) The 
apparent gradual shifts observed when one acid 
radicle is replaced by another can be simply explained 
by the superposition of absorption curves, and 
evidence has been found in support of this explana- 
tion. (3) A’ marked change in the character of the 
absorption has been found in the presence of free 
acid, more especially in solvents containing a ketone 
| without shift, in accordance with the results of Jones 
and Strong. (4) The influence of pressures up to 
750 atmospheres on the absorption spectra of solu- 
_ tions has -been investigated with negative results.— 
| The viscosity of carbon dioxide is 
W. C. Ball: Changes in the absorption spectra of 
‘‘didymium ”’ salts. The absorption spectra given by 
aqueous solutions of ‘‘didymium”’ salts, such as the 
nitrate, chloride, &c., were observed to be consider- 
ably altered by sodium hypcsulphate, Na,S,O,, the 
lines and bands being altered in position, width, and 
intensity. These alterations were found to be in- 
dependent of any reducing action of the very strongly 
reducing hyposulphite, but to be connected with 
changes in the ionisation of the didymium ; for similar 
effects on the spectra of the didymium salt of strong 
acids were produced under conditions likely to 
diminish such ionisation.—Dr. PP. Phillips: The 
viscosity of carbon dioxide. In this experiment the 
method of determining the viscosity is that described 
before the society by A. O. Rankine in January, 1910. 
determined for 
temperatures of 20°, 30°, 32°, 35°, and 40° C., and 
for a range of pressures from 1 to 120 atmospheres. 
When the viscosity is plotted against the pressure, 
the form of the isothermals is very similar to the 
form of the density-pressure isothermals, but the 
former cross, whereas the latter do not. When the 
kinematic viscosity is plotted against ihe pressure, it 
is noticed that at the saturation pressure the kine- 
matic viscosity of the gas is the same as that of the 
liquid. The minimum value of the kinematic vis- 
cosity being approximately o-co069 at 30°, 32°, and at 
35° C., this is taken as the critical value of the kine- 
matic ‘viscosity, and therefore multiplying it by the 
critical density, 0-464, the critical value of the co- 
efficient, of viscosity is found to be 0-000320. When 
| the. viscosity is plotted against the square of the 
density it is found that, for a considerable range of 
density ‘near to the critical point, the viscosity is a 
linear function of the square of the density. This 
would seem to show that the viscosity is proportional 
to the molecular attraction between two adjacent 
layers of the fluid, that is, to the a/y? term in Van 
der Waals’s equation. 
