80 
NATURE 
[Marcu 18, 1915 

pieces were prepared for the determination of the 
refractive properties of the glasses. 
The three open and three complete rings were 
examined in plane polarised light for strain. _ The 
examination showed that in both rings of the French 
glass, and in the open ring from the German lamp 
chimney, the amount of strain was negligible. In 
the unbroken ring from the German chimney there 
was very decided strain. Strain was also present in 
both rings of the ‘“‘Durax”’ glass, the unbroken ring 
of this material showing much more pronounced 
strain than any of the other rings. The appearance 
presented by the rings when examined in the dark 
field is a very strong and sharp black circular line in 
the middle of the glass with a decided black cross 
upon it, and the rest of the ring either white or milky. 
The character of the strain present in the unbroken 
ring from the German chimney was determined by 
distorting the ring into an elliptical form by com- 
pression between two points at opposite ends of a 
diameter of the ring inclined at 45° to the plane of 
polarisation. By this means the milkiness could be 
made to disappear entirely from the regions about the 
diametrical plane perpendicular to the line of com- 
pression, but the strain in the neighbourhood of the 
points of compression was increased. This shows 
that the strain is relieved by an increase in the 
curvature, and augmented by a reduction in the curva- 
ture; in other words, the inner layers of the ring are 
in a state of tension, and the outer layers in a state 
of compression. The fact that in the open ring the 
strain throughout is practically entirely relieved sug- 
gests that these chimneys are made in one operation, 
and not by the combination of layers of material at 
different temperatures. It is evident that with the 
distribution of strain in the cold state indicated above, 
the chimney will tend to be relieved of strain when 
there is a radial temperature gradient throughout its 
substance with the outer surface cooler than the inner 
surface. 
With the ‘‘Durax”’ tubing there is in neither ring 
freedom from strain. In the open ring the strain 
becomes worse on altering the curvature in either 
direction. Perhaps this indicates that the tube is 
built up of layers of the same or different materials, 
but brought together with one layer decidedly cooler 
than another. The presence of a thin white band in 
the substance of the glass may also indicate that the 
tube has been built up in the way suggested. An 
examination of all the rings in ordinary light showed 
the presence of striz likely to prevent the formation 
of really sharp spectrum lines in the determination 
of their refractive properties. This expectation was 
fully borne out by the appearance of the lines in the 
refractometer. No indication was obtained that any 
tube consisted of more than one type of glass, but the 
want of sharpness in the lines was _ sufficient to 
obscure the difference between glasses of very nearly 
identical optical properties. The results obtained in 
these measurements are as follows :— 
Wp Nco— NE v 
German chimney 1-4795 0:00729 65-8 
French chimney 1-5748 0:01313 43:8 
“Durax”’ combustion 
tubing 15156 — — 
The optical glasses which resemble the above in 
refractive properties are, for the German chimney 
one of the new ‘Fluor Crowns,” for the French 
chimney a light flint, and for the ‘“‘Durax” tubing a 
hard crown. 
Experiments were made to determine possible varia- 
tions in the strained condition of lamp chimneys due 
to internal heating and resulting temperature gradient 
across the glass. 
NO. 2368, VOL. 95] 




| of workmen or managers. 
Coils of wire were wound round a cylindrical metal 
core, the whole enclosed in asbestos paper and fitted 
closely (without mechanical strain) into the ring of 
the chimney examined. A thermo-couple was intro- 
duced between the lagging and the glass to enable 
the temperature of the latter to be determined on the 
passage of a current through the heating coil. 
French Chimney.—This was initially when cool 
without strain. On heating up strain was found to 
develop progressively as the temperature rose, and 
there is no evidence that at any temperature the strain 
declines to a minimum again. 
German Chimney.—When cool and at uniform 
temperature this showed very decided strain in the 
shape of a central ring and cross. On heating up the 
ring the strain was relieved, but the glass is never 
entirely freed from the strain as evidenced by a com- 
plete absence of the stauroscopic figure. The tendency 
is rather for the central dark ring to move outwards 
and to be replaced eventually by another ring moving 
out from the inner surface of the glass ring. There 
is nevertheless a decided minimum effect, i.e. mini- 
mum strain, obtained when the temperature of the 
inner surface of the glass ring is about 150° C. (420°). 
The temperature gradient across the glass is such 
that in the steady state the temperature of the outer 
surface of the lamp, corresponding with the above in- 
ternal temperature, is roughly 70°, within the same 
limits of accuracy. Excessive heating above these 
limits results merely in producing a more pronounced 
strain. 
A further chemical analysis of the inner layers con- 
firmed the view that the German miner’s lamp 
chimney is of a single material. 
The incandescent lamp chimneys are very thin and 
optical examination is not possible, but it appears 
fairly certain that in their case also the extra strength 
is attained by some process whereby the outside is 
chilled before the interior cools down. It seems pos- 
sible also that the extra heat-resisting qualities of 
certain beakers, flasks, etc., of German glass may 
be due to the converse process, the interior being 
chilled previously to the exterior. All these glasses 
appear to be of the borosilicate type, such as is used 
for the well-known thermometer glass 59™!, but with 
somewhat more boric anhydride; the chemical glass 
contains zinc oxide, which is absent from the ther- 
mometer glass. 
As already stated, the object of these notes is to put 
such information as is available before English manu- 
facturers in the hopes of encouraging some of them 
to take up the manufacture of some of these glasses. 

UNIVERSITY AND EDUCATIONAL 
INTELLIGENCE. 
Dr. R. Travers Situ has been appointed to the 
chair of materia medica, therapeutics, and pharmaco- 
logy in the school of surgery of the Royal College of 
Surgeons in Ireland. 
THE will of the late Mr. W. E. Allen, chairman of 
Messrs. Edgar Allen and Co., Ltd., of the Imperial 
Steel Works, Sheffield, who died in Jnuary last, pro- 
vides that two-fifths of the residue of his property 
after the payment of numerous bequests to charities 
and employees, shall be given to the University of 
Sheffield, to be applied as to 5o0o0l. to the Applied 
Science Department, and as to the balance, in the 
discretion of the University Council, in founding’ 
Edgar Allen scholarships or exhibitions for students: 
of that University, of whom one-half shall be sons of 
workmen earning daily or weekly wages or foremen 
The gross value of the 
