[5] “Glass working was practised by the ancient Egyptians at a very
early period of their national existence. Sir J. G. Wilkinson, in
his able work on the _Manners and Customs of the ancient Egyptians_,
has adduced three distinct proofs that the art of Glass working was
practised in Egypt before the Exodus of the children of Israel from
that land, three thousand five hundred years ago. At Beni Hassan
are two paintings representing Glass blowers at work, and from the
hieroglyphics accompanying them they are shown to have been executed
in the reign of the first Osirtasen at the early date above mentioned.
Such was the skill of the Egyptians in glass making, that they
successively counterfeited the Amethyst and other precious stones
worn as ornaments for the person. Winckelmann, a high authority, is
of opinion that glass was employed more frequently in ancient than in
modern times; it was used by the Egyptians even for coffins; (_within
the year 1847 a process was patented in England for making Coffins of
Glass_) they also employed it not only for drinking vessels but for
Mosaic work, the figures of deities, and sacred emblems, in which they
attained excellent workmanship, and surprising brilliancy of colour.
“It is certain that the glass houses of Alexandria were celebrated
among the ancients for the skill and ingenuity of their workmen; and
from thence the Romans, who did not acquire a knowledge of the art till
a later period, procured all their Glass ware.
[Illustration: Egyptian painting of glassblowers at a fire]
[Illustration: Egyptian painting of glassblowers blowing a large
vessel.]
“Most of the large cinerary vases in the British Museum, found in
Roman barrows which contained bones and bone-ashes, are, probably, the
production of extensive Egyptian or Roman works: they are large, and of
excellent form and workmanship: but the Glass is somewhat impure, of a
greenish tint, has numerous globules and striæ, and is not unlike the
modern common crown or sheet glass in quality.
“We have incidentally mentioned the discovery of Glass at Pompeii.
Glass vessels have also been found among the ruins of Herculaneum: and
it appears that Glass was used for admitting light to dwellings in
Pompeii.
“Mr. Auldjo, of Noel house, Kensington, who resided several years at
Naples, states, that he has seen glass in the window-frames of some of
the houses of Pompeii.
“Mr. Roach Smith has a specimen of ancient flat Glass such as he
believes to have been used by the Romans, or their predecessors for
windows.”—_Curiosities of Glass making by_ APSLEY PELLAT, London, 1849.
Mr. Layard in his interesting work on Nineveh, 1849, London, in Vol. I,
page 342, says: “I took the instrument, and, working cautiously myself,
was rewarded by the discovery of two small vases, one in alabaster,
the other in glass (both in the most perfect preservation) of elegant
shape, and admirable workmanship. Each bore the name and title of the
Khorsabad King, written in two different ways, as in the inscriptions
of Khorsabad.”
_No. 1. The bent Siphon._
[Illustration: Diagram of apparatus as described in text]
Let A B C, (fig. 1), be a bent siphon, or tube, of which the leg A B is
plunged into a vessel D E containing water. If the surface of the water
is in F G, the leg of the siphon, A B, will be filled with water as
high as the surface, that is, up to H, the portion H B C remaining full
of air. If, then, we draw off the air by suction through the aperture
C, the liquid also will follow from the impossibility, explained above,
of a continuous vacuum. And, if the aperture C be level with the
surface of the water, the siphon, though full, will not discharge the
water, but will remain full: so that, although it is contrary to nature
for water to rise, it has risen so as to fill the tube A B C; and the
water will remain in equilibrium, like the beams of a balance, the
portion H B being raised on high, and the portion B C suspended. But if
the outer mouth of the siphon be lower than the surface F G, as at K,
the water flows out; for the liquid in K B, being heavier, overpowers
and draws toward it the liquid in B H. The discharge, however,
continues only until the surface of the water is on a level with the
mouth K, when, for the same reason as before, the efflux ceases. But
if the outer mouth of the tube be lower than K, as at L, the discharge
continues until the surface of the water reaches the mouth A. If then
we wish all the water in the vessel to be drawn out, we must depress
the siphon so far that the mouth A may reach the bottom of the vessel,
leaving only a passage for the water.
Now some writers have given the above explanation of the action of the
siphon, saying that the longer leg, holding more, attracts the shorter.
But that such an explanation is incorrect, and that he who believes so
would be greatly mistaken if he were to attempt to raise water from a
lower level, we may prove as follows. Let there be a siphon with its
inner leg longer and narrow, and the outer much less in length but
broader so as to contain more water than the longer leg. Then, having
first filled the siphon with water, plunge the longer leg into a vessel
of water or a well. Now, if we allow the water to flow, the outer leg,
containing more than the inner, should draw the water out of the longer
leg, which will at the same time draw up the water in the well; and
the discharge having begun will exhaust all the water or continue for
ever, since the liquid without is more than that within. But this is
not found to be the case; and therefore the alleged cause is not the
true one. Let us then examine into the natural cause. The surface of
every liquid body, when at rest, is spherical and concentric with that
of the earth; and, if the liquid be not at rest, it moves until it
attains such a surface. If then we take two vessels and pour water into
each, and, after filling the siphon and closing its extremities with
the fingers, insert one leg into one vessel plunging it beneath the
water, and the other into the other, all the water will be continuous,
for each of the liquids in the vessels communicates with that in the
siphon. If, then, the surfaces of the liquids in the vessels were at
the same level before, they will both remain at rest when the siphon is
plunged in. But if they were not, as soon as the water is continuous
it must inevitably flow into the lower vessel through the channel of
communication, until either all the water in both vessels stands at the
same height, or one of the vessels is emptied. Suppose that the liquids
stand at the same height; they will of course be at rest, so that the
liquid in the siphon will also be at rest. If, then, the siphon be
conceived to be intersected by a plane in the surface of the liquids in
the vessels, even now the liquid in the siphon will be at rest, and, if
raised without being inclined to either side, it will again be at rest,
and that, whether the siphon is of equal breadth throughout or one leg
is much larger than the other. For the reason why the liquid remained
at rest did not lie in this, but in the fact that the apertures of
the siphon were at the same level. The question now arises why, when
the siphon is raised, the water is not borne down by its own weight,
having beneath it air which is lighter than itself. The answer is that
a continuous void cannot exist; so that, if the water is to descend,
we must first fill the upper part of the siphon, into which no air
can possibly force its way. But if we pierce a hole in the upper part
of the siphon, the water will immediately be rent in sunder the air
having found a passage. Before the hole is bored, the liquid in the
siphon, resting on the air beneath, tends to drive it away, but the air
having no means of escape does not allow the water to pass out: when
however the air has obtained a passage through the hole, being unable
to sustain the pressure of the water, it escapes. It is from the same
cause that, by means of a siphon, we can suck wine upwards, though this
is contrary to the nature of a liquid; for, when we have received into
the body the air which was in the siphon, we become fuller than before,
and a pressure is exerted on the air contiguous to us, and this in turn
presses on the atmosphere at large, until a void has been produced at
the surface of the wine, and then the wine undergoing pressure itself
will pass into the exhausted space of the siphon; for there is no other
place into which it can escape from the pressure. It is from this cause
that its unnatural upward movement arises.
[Illustration: Two overlaping circles, A, B, C are on the left circle.
F, B, D are on the right, a is a center point, F is on a line from a
to A. There is a line from a to B, the intersection point of the two
circles.]
That the water in the siphon will rest when its surface is spherical
and concentric with that of the earth may be shewn otherwise. It is
required to prove that a liquid is stationary when its surface is
spherical and concentric with that of the earth. If possible let it
not be stationary; it will of course become so after being moved; let
it then have become stationary. Its surface will now be spherical
and concentric with that of the earth, and it will cut the former
surface; for, when the same liquid has taken two positions, there must
be a line of intersection common to both. Let both surfaces be cut by
a plane passing through the centre of the earth; the intersections
will be the circumferences of circles concentric with the earth. Let
these circumferences be A B C and F B D, (fig. 1 a.) Join B G; B G is
equal to each of the lines G F, G A, which is absurd. The liquid will
therefore be in equilibrium.