Nov. 24 1881 | 
probably the resulting extension which produces the effect, But 
when a tube is exposed to pressure from the interior there is 
dilatation of the walls, which aids the shear. Thus we see why 
a thin tube is so much more capable of resisting external than 
internal pressure. It is probable that, in the case of glass, the 
element which first gives way is not so much crushed as torn 
asunder, If so, the tube which is compressed from without is 
‘in a much more faycurable condition for resisting than that in 
which the pressure is applied internally. For, in the first, the 
-whole substance of the walls is compressed, and thus the linear 
‘extension produced by the shear is in part counteracted. In the 
second, the whole substance is expanded, and the linear exten- 
‘sion due to the shear is aided. As will be seen in Appendix A, 
the case of very thick tubes is considerably different. 
X. Description of the Apparatus for applying Pressure.—Sir 
Wyville Thomson handed over to me, with the thermometers, a 
press which was made for him before he started in the Challenger, 
and which he had carried all round the world ; but when we 
‘made some preliminary experiments with it, we found it to be 
objectionable in many ways. It was in the first place not safe 
at high pressures, although an attempt had been made to 
strengthen it by surrounding it with massive rings of Swedish 
iron. As the experiments had to be conducted in college, and 
to a great extent by students who volunteered their’ services, 
this was a fatal defect ; though I believe that the danger from 
the bursting of a hydrostatic press has been usually very much 
exaggerated. The bursting of the cylinder itself would probably 
be unattended with danger ; but some of the nuts and connecting 
pieces had occasionally been projected with great violence. 
A slight numerical calculation shows that a cubic foot of water 
at a pressure of one ton weight to the square inch is capable of 
doing only about 1210 foot lbs, of work in expanding, the reason 
being that although the pressure is intense, the amount of com- 
pression it produces is exceedingly small, But a cubic foot of 
air at a pressure of a ton weight to the square inch is capable of 
doing nearly 1300 times as much work in expanding. Hence 
the danger of having large quantities of air in the press before 
the compression is begun. 
Another defect of the apparatus was the comparatively small 
interior bore, which did not admit of the proper carrying out of 
my scheme for measuring pressures—the Bourdon gauge having 
-shown itself quite untrustworthy, Besides, two thermometers, 
at most, could be exposed to pressure simultaneously, even when 
no gauge was inserted along with them. 
The apparatus which Sir Wyville Thomson finally obtained 
from the Woolwich gun factories, through the intervention of 
the Admiralty, was in fact a Fraser gua with a few adaptations 
made to suit it to the purposes of the investigation, The gun 
was made of a cylinder of mild steel, round which were shrunk 
two succes-ive wrought-iron coils. The effective interior is 44 
inches in bore, and nearly 4 feet long, 
This cylinder was guaranteed to be safe under pressures up to 
18 or 20 tons weight per square inch, and we have for various 
purposes already worked up to pressures of 11 and 12 tons. 
‘Lhe rest of the apparatus, to fit it for our immediate purpose, 
consisted of a tightly-fitting steel plug which was forced into 
the upper end of the cylinder after the thermometers and other 
apparatus had been inserted, and the whole had been filled with 
water. The plug was forced down by the weight of an assistant 
standing on it, while a stop-cock at the bottom of the cylinder 
was kept open for the escape of water, until a massive steel key 
could be put in through a slot in the side of the cylinder to-lock 
the plug in its definite position, 
To the lower end of the steel cylinder were adapted a series of 
fittings by means of which it could be connected with a powerful 
force-pump, and simultaneously with a gauge whose construction 
will be afterwards described. The gauge enabled the experi- 
menters to know at every stage of the operation what amount of 
pressure had been reached in the interior of the cylinder, The 
pump was worked at first by hand. Of late a more powerful 
pump has been procured, and it can be fitted when necessary to 
the gas-engine of my laboratory, 
Only one real difficulty was met with in working this appa- 
ratus ; viz, the difficulty of making the plug fit perfectly tight, 
At first, when it came from Woolwich, the plug was finished by 
a piece of leather in the form of a cup; but this was found to 
leak seriously even at'very moderate pressures, so that even the 
comparatively small pressure of a ton weight per square inch was 
unattainable, 
But by taking off the leather from the plug and furnishing it 
with a ring of steel turned into cup form with an exceedingly 
NALTORE 
93 
thin and sharp edge, on thesame principle as that on which the 
piston of the pump was constructed, this difficulty was completely 
got over. The flexible steel edge was pressed against the interior 
of the tube more forcibly the greater the applied pressure, and it 
was found that the apparatus was then, except under the most 
unfavourable circumstances, perfectly tight, at least so far as the 
plug was concerned. Very great care was, however, requisite in 
cleaning the plug and the upper part of the bore of the cylinder 
before each experiment. The smallest fragment of cotton-waste, 
getting behind the edge of the cup, almost invariably produced 
serious leakage when high pressure was applied. The cup form 
was objectionable for one reason, that it always took down a 
considerable quantity of air, of which it was impossible to get 
rid. This difficulty was overcome by putting into the cup a 
quantity of tallow which completely filled it up and projected 
considerably below it, so that the apparatus, when pressure 
commenced, contained at the most a few small air bubbles only. 
Later, when I found it was impossible to obtain certain neces- 
sary data, on account of the slowness with which pressure was 
got up in so large an apparatus, I procured a very much smaller 
apparatus of similar character, in which the cylinder was only 
an inch in bore, and rather less than a foot in effective interior 
length. With this apparatus two or three strokes, only, of the 
pump were required to get up the desired pressure, and there 
was the great additional advantage that temperatures could be 
independently measured by means of thermo-electric junctions. 
[This could not be done in the large cylinder without seriously 
affecting its strength, and rendering it at the same time almost 
unmanageable. ] 
(To be continued.) 
TRANSFORMATION OF OLD COINS IN A 
LAKE 
M. 
DAUBREE read an interesting paper on this subject at 
the Paris Academy of Sciences on October 17. 
In the commune of Flines-les-Roches, canton of Douai, there 
is a small lake of very limpid water, known as the Mer-de- 
Flines. It is of circular shape, and about 300 m. in diameter ; 
its surface remains stationary in position, and neither springs 
supplying it, nor any outlet, are apparent. The depth at the 
centre has not been determined. There are fishes in the lake, 
and the water attracts numerous bathers. Among other shells 
on the banks, one observes many specimens of Uni», The 
water rests on Tertiary strata of the Landenian formation, con- 
sisting of sand and gravel, with dark ashes and pyrites. 
According to the researches of MM. Termink and Loustan, 
this lake appears to have been, in ancient times, a place of 
sacred resort, and various precious objects were thrown into it, 
as an offering doubtless to some divinity. Numerous coins, 
more especially, have been recovered, along with statuettes of 
bronze and much pottery a@ééris. Of the coins, some are Gaulish, 
but the majority are Roman, of the time of the early Empire. 
They are mostly bronze, but some are of gold. 
Some remarkable changes have occurred in many of these 
coins, and have been brought to the notice of the French Academy 
by M. Daubrée. The coins have been attacked and are com- 
pletely enveloped with a crystalline substance formed at their 
expense. Their general aspect recalls that of the coins incrusted 
with metallic minerals, found in various thermal springs, notably 
at Bagnéres-de-Bigorre, at Bourbonne-les-Bains, and at Baracct 
in Corsica. 
An extremely fine external pellicle, of the brass-yellow colour 
which characterises chalcopyrite, first attracts notice. But the 
incrustation is mostly formed, to a depth of 2 mm., of a dark 
crystalline substance, with metallic lustre, consisting of sulphide 
of copper, and it is to it that the crystalline forms belong, which 
one might be apt, at first sight, to attribute to yellow sulphur. 
The central part consists of a laminated substance. Here and 
there may be observed small dark hexagonal metallic crystals, in 
which one can see the characteristic striz of chalcocite. The 
same substance forms small brilliant leaves, alternating with the 
dark leaves, on which appear some deposits of bright green. 
Heated in a tube, the crystalline substance gives a very slight 
sublimate of sulphur, which apparently results from the mixture 
of a little pyrites. The sulphide contains neither tin nor zinc. 
The form of the exterior substance is that of hexagonal plates 
bearing truncated pyramidal faces inclined about 127° to the 
base. The cleavage is basic, This latter character and the 
