170 
NATURE 
[FEBRUARY 3, 1923 

the same plate, which is stored away during the 
interval and developed after the second exposure. 
It was arranged that the images fell near each other 
(within 1 mm.—they were, however, too small for 
the Ross effect to come into action) and only the 
small differences separating the images were measured, 
any film displacement which would affect both images 
equally, consequently disappearing. This method 
has now been given up in favour of single plates, but 
a number of the Kapteyn pairs have since been 
measured treating each photograph as a separate 
plate. From a discussion of the residuals of some 
300 plates the film error is +0-:0003 mm. 
The Faraday Society, January 15.—Sir Robert 
Robertson, president, in the chair.—E. W. 
Mardles: Study of the reversible sol to get transition 
in non-liqueous systems. Pt. 1: The change of 
viscosity with time during gelation. The viscosity 
value of a sol during its gelation is dependent on the 
method and conditions of its determination, and since 
the system is heterogeneous, it loses its real signifi- 
cance. The change of apparent viscosity with time 
during the gelation of a sol of cellulose acetate in 
benzyl alcohol can be expressed by an empirical 
formula. The temperature, when the rate of gelation 
becomes nil, is regarded as the maximum gelation 
temperature, since above it the sol is relatively stable 
with time and below it a part or whole of the dispersed 
particles aggregate to form a gel structure. The 
relation between the maximum gelation temperature 
and concentration resembles that between tempera- 
ture and the saturation concentration for crystalloids. 
Pt. 2: Viscosity changes associated with the gel to 
30l transition. These have been measured at various 
temperatures and with different concentration systems 
of cellulose acetate in benzyl alcohol. The viscosity 
at first rapidly diminishes, the rate of change becoming 
smaller until a constant value is obtained. The 
minimum temperature at which there is a complete 
return to the original viscosity of the sol without 
mechanical treatment is termed the minimum 
solation temperature. Mechanical treatment hastens 
solation in the same way that it retards gelation. 
The time taken for a system to attain constant 
viscosity or mobility depends on the previous treat- 
ment of the gel. The hysteresis effect observed during 
the sol—gel transition can be measured by the 
difference in the temperature of minimum solation 
from that of maximum gelation, and the cause of it 
has been ascribed to the different conditions of the 
particles in the gel and sol state.—E. W. J. Mardles : 
Changes of volume and refractive index associated 
with (a) the formation of organosols and gels; (bd) 
the reversible sol to gel transition. In general, the 
volume changes are largest (a) at the lower concentra- 
tions, (b) with the best solvents and optimum solvent 
mixtures, and (c) at higher temperatures. They 
are much smaller than those observed by other 
workers for hydrosols and gels. The reversible sol 
to gel transition is associated with a small volume 
change which varies with time as in the case of the 
Tyndall number changes. There are also indications 
of a change of refractive index corresponding to the 
volume and Tyndall number changes during the 
reversible sol to gel transition.—E. W. J. Mardles: 
The scattering of light by organo-sols and gels of* 
cellulose acetate. Measurements of the change with 
temperature of the Tyndall number of’sols and gels 
of cellulose acetate in benzyl alcohol during the 
reversible sol to gel transition show that with fall in 
temperature of the sol the rate of change is small 
until a certain critical temperature, after which it 
increases with acceleration. Eventually there may 
be a point of inflexion on the curve, the position of 
NO. 2779, VOL. 111] 
which depends on the rate of cooling, and is deter- 
mined by the formation of a firm jelly structure which 
inhibits the development of opalescence. The Tyn-_ 
dall number of a gel is a function of the mechanical 
treatment as well as rate of gelation and it varies 
with time, the rate of change gapidly diminishing in 
absence of mechanical treatment. A gel tends to 
increase its Tyndall number, and mechanical treat- 
ment may induce opalescence. The Tyndall number- 
concentration curve contains a maximum which 
tends to disappear at higher temperatures, thus the 
size of the particles in a gel structure is a function of 
the concentration and the temperature at which it 
was formed. The curve resembles Tammann’s curve . 
relating the number of crystallisation nuclei, or rate 
of crystallisation, with the degree of supersaturation., — 
—J.R. Partington and W. J. Shilling: The variation 
of the specific heat of air with temperature. The 
velocity of sound in the gas contained in a large silica 
tube arranged as an electrically heated furnace was 
measured at intervals of approximately 100° C., from — 
room temperature up to r1000° C. The values 
obtained up to 700° C. lie practically on the line given 4 
by C,=4:849+0-000358T grm. cal. Above 700° . 
C, appears to increase more rapidly with temperature, 
but at present the values above 800° are uncertain. ; 
} 
: 
d 
Royal Anthropological Institute, January 16.—Dr. 
F. C. Shrubsall, treasurer, in the chair.—Mr. M. 
Addison: Human heads carved in steatite from 
Sierra Leone. The Mende tribes, in whose territory 
the heads were obtained, know nothing of their 
origin, but although the heads exhibit certain char- 
acteristics, such as nose- and ear-rings and long 
drooping moustaches, which do not occur among the 
inhabitants of the district at the present day, it is 
not probable that they are of a very high antiquity, 
their age possibly being two or three hundred years. 
Among the Mende the heads are used for magical 
purposes, and, placed on mounds in the fields, are © 
thought to increase the fertility of the crops.—F. W. 
H. Migeod: The Bedde group of tribes of Northern 
Nigeria. Though extending from Lake Chad as far 
as the City-State of Hadeija the Bedde are not a 
well-known people. They have the legend that they © 
originated in Yemen in Arabia and that they were the — 
first people driven out of Arabia in consequence of their 
refusal to accept Mohammedanism. The Western-— 
most branch of the Bedde, the Awuyoka, have a_ 
list of kings going back to the 12th century. The 
language shows no traces of an Arabian origin. The — 
Bedde live in round huts grouped in compounds, and 
formerly all the towns were surrounded by a rect- 
angular mud embankment, but most of the defensive 
works are now in ruins. Swords, spears, and bows 
and arrows are the offensive weapons. Children are 
named by the father, but there are fixed names for 
twins, male and female. Marriage may take place 
into any family. Corpses are buried on the right side 
with head to the south and so facing east, the hands 
being drawn down and placed together. The Bedde 
seem to be very superstitious and to believe in 
omens. They are divided up into animal tribes such 
as Leopard, Hippopotamus, etc., which form a bond 
outside family life. No worship is offered to the 
tutelary animal, but it must not be killed or eaten by 
those who bear its name. The people now nearly all 
wear gowns of cotton like the Hausas or Bornuese. 
They slash or cut their faces with a main design and 
subsidiary marks. The men shave their heads but 
the women indulge in fancy head-dressing. They 
are not a short race. They usually have broad faces, 
some being very broad across the cheek-bones, and 
usually have heads low above the ears and recedin 
foreheads ; but there is great variety. 




























