Begin by reducing the size of the end of tube A till B will just slip
in quite easily. With B about one-quarter inch in diameter, a
clearance of about one-twentieth of an inch, or less, in all (i.e.
one-fortieth of an inch on each side) will be proper.
Heat B by itself at the proposed zone of junction, and blow out a very
narrow ring; then compress this slightly so that it forms an almost
closed ring of glass. The figure refers to the close of this
operation (Fig. 31, B). It does not matter much whether the ring
remains a mere flattened bulb, or whether it is a solid ring, but it
must be one or the other. Some judgment must be exercised in
preparing the ring. In general, the beginner will collect too much
glass in the ring, and consequently the joint, when made, will either
be thick and liable to crack easily, or it will be blown out into an
erratic shape in endeavours to reduce this thickness. Accordingly,
the operator will, if necessary, thin the tube B by drawing slightly,
if he considers it desirable, before the little enlargement is blown
out. In general, two heats must be used for this operation.
Fig. 32.
Get the approximating parts of both A and B up to a temperature just
below that at which they will adhere, and having closed the other end
of A, place B carefully within it up to the ring, and if it can be
arranged, have a mica wad in A, with a central hole through which the
end of B can project. This will very much facilitate the operation,
especially if B is long, but may be dispensed with by the exercise of
care and skill.
The operation is now simple. Fuse the junction and press the tubes
lightly together, being careful not to collect more glass than can be
helped; finally, blow out the joint and reduce the thickness by mild
drawing (Fig. 33). In order to make a really good joint, two points
must be particularly attended to--the rim must be thin and its plane
perfectly perpendicular to the axis of tube B; the end of tube A must
be cut off quite clean and perpendicular to its axis before B is
inserted. So important are these conditions--especially the latter
that the writer has even occasionally used the grindstone to get the
end of A into a proper condition, an admission which will probably
earn the contempt of the expert glass-worker.
Fig. 33.
Now for the second method, which is often practised in Germany, where
soda glass is chiefly used. With this glass the chief point is to get
a very even and not too thick ring at the junction, and consequently
the extra thickening produced by making a rim on B is rather a
drawback. The method consists in cutting off from B the length which
it is desired to insert, slipping this into A (which may be an
otherwise closed bulb, for instance), and then gradually melting up
the open end of A till the piece of B inside will no longer fall out.
By holding the joint downwards so that the inserted portion of B rests
on the edges of the opening, a joint may be made with the minimum
thickening.
The external part of B, previously heated, is then applied, and the
joint subjected to a "general" heat and blown out. Very nice joints
may be made by this method, and it is perhaps the better one where the
external part of B is to be less in diameter than the inserted part.
It was in this manner that the writer was taught to make glass
velocity pumps, one of which, of a good design, is figured as an
example.
In all cases good annealing should follow this operation. If the
inserted part of the inner tube (B) is anything like an inch in
diameter, and especially if it is of any length, as in some forms of
ozone apparatus, or in a large Bunsen's ice calorimeter, the
arrangements for supporting the inner part must be very good. A
convenient way of proceeding when the inner tube is well supported is
to make the mouth of A only very little larger than the diameter of B,
so that B will only just slip in. Then the mouth of A and the zone of
B may be heated together, and B blown out upon A. This, of course,
must be arranged for, if necessary, by temporarily stopping the inner
end of B.
The inner support of B should be removed as soon as practicable after
the joint is made, or, at all events, should not be perfectly rigid; a
tightly-fitting cork, for instance, is too rigid. The reason is, of
course, that in cooling there may be a tendency to set B a little to
one side or the other, and if it is not free to take such a set, the
joint most probably will give way. Good annealing both with flame and
asbestos is a sine qua non in all inserted work.
Fig. 34.
§ 35. Bending Tubes.
I have hitherto said nothing about bending tubes, for to bend a tube
of a quarter of an inch in diameter, and of ordinary thickness, is
about the first thing one learns in any laboratory, while to bend
large tubes nicely is as difficult an operation as the practice of
glass-blowing affords. However, even in bending a narrow tube it is
possible to proceed in the wrong way. The wrong way is to heat a
short length of the tube and then bend it rapidly, holding the plane
of the bend horizontal. The right way, per contra, is to use a
batswing burner to heat, say, two inches of the tube with constant
turning till it is very soft, and then, holding the glass so that the
bend will be in a vertical plane passing through one eye (the other
being shut), to make the bend rather slowly.
If an exact angle is required, it is as well to have it drawn out on a
sheet of asbestos board. In this case bend the glass as described
till it is approximately right, and finish by laying it on the
asbestos board and bringing it up to the marks. A suitable bit of
wood may be substituted for the asbestos on occasion.
N.B: The laboratory table is not a suitable piece of wood. A
right-angled bend is often wanted. In this case the corner of a table
will serve as a good guide to the eye, the glass being finished by
being held just above it. If great accuracy is wanted, make a wooden
template and suspend it by a screw from the side of the table, so that
the vertex of the gauge for the interior angle projects downwards,
then finish by bending the tube round it. The wood may be about half
an inch thick.
If a sharp bend is required, heat the tube in the blow-pipe, and bend
it rapidly, blowing out the glass meanwhile. The reason why a long
bend should be held in a vertical plane is that the hot part tends to
droop out of the plane of the bend if the latter be made in a
horizontal position. To bend a tube above half an inch in diameter is
a more or less difficult operation, and one which increases in
difficulty as the diameter of the tube increases.
A U-tube, for instance, may be made as follows: Use the four
blow-pipe arrangement so as to heat a fair length of tube, and get,
say, two inches of tube very hot--almost fluid, in fact--by means of
the carbon block supported from a stand. Remove the tube rapidly from
the flame and draw the hot part out to, say, three inches. Then,
holding the tube so as to make the bend in a vertical plane, bend it
and blow it out together to its proper size.
This operation seems to present no difficulties to experienced
glass-workers, even with tubes of about one inch in diameter, but to
the amateur it is very difficult. I always look on a large U-tube
with feelings of envy and admiration, which the complex trick work of
an elaborate vacuum tube does not excite in the least. It will be
noted that this method may, and often does, involve a preliminary
thickening of the glass.
With tubes over an inch in diameter I have no idea as to what is the
best mode of procedure--whether, for instance, a quantity of sand or
gas coke might not be used to stuff out the tube during bending, but
in this case there would be the difficulty of removing the fragments,
which would be sure to stick to the glass.
Of course, if the bend need not be short, the tube could be softened
in a tube furnace and bent in a kind of way. I must admit that with
tubes of even less than one inch in diameter I have generally managed
best by proceeding little by little. I heat as much of the glass as I
can by means of a gigantic blow-pipe, having a nozzle of about an inch
in diameter, and driven by a machine-blower.
When I find that, in spite of blowing, the tube begins to collapse, I
suspend operations, reheat the tube a little farther on, and so
proceed. If by any chance any reader knows a good laboratory method
of performing this operation, I hope he will communicate it to me.
After all, the difficulty chiefly arises from laboratory heating
appliances being as a rule too limited in scope for such work.
The bending of very thin tubes also is a difficulty. I have only
succeeded here by making very wide bends, but of course the blowing
method is quite applicable to this case, and the effect may be
obtained by welding in a rather thicker bit of tube, and drawing and
blowing it till it is of the necessary thinness. This is, however, a
mere evasion of the difficulty.
§ 36. Spiral Tubes.
These are easily made where good heating apparatus is available. As,
however, one constantly requires to bend tubes of about one-eighth
inch in diameter into spirals in order to make spring connections for
continuous glass apparatus, I will describe a method by which this is
easily done. Provide a bit of iron pipe about an inch and a quarter
in outside diameter. Cover this with a thick sheath of asbestos
cloth, and sew the edges with iron wire. Hammer the wire down so that
a good cylindrical surface is obtained. Make two wooden plugs for the
ends of the iron pipe. Bore one to fit a nail, which may be held in a
small retort clip, and fasten a stout wire crank handle into the other
one. Support the neck of the handle by means of a second clip. In
this way we easily get a sort of windlass quite strong enough for our
purpose.
Fig. 35.
Provide a large blow-pipe, such as the blow-pipe of a Fletcher
crucible furnace, Select a length of tubing and clean it. Lash one
end to the cylinder by means of a bit of wire, and hold the other end
out nearly horizontally. Then start the blow-pipe to play on the tube
just where it runs on to the asbestos cylinder, and at first right up
to the lashing. Get an attendant to assist in turning the handle of
the windlass, always keeping his eye on the tube, and never turning so
fast as to tilt the tube upwards. By means of the blow-pipe, which
may be moved round the tubing, heat the latter continuously as it is
drawn through the flame, and lay it on the cylinder in even spirals.
If the tubing is thin, a good deal of care will have to be exercised
in order to prevent a collapse. A better arrangement, which, however,
I have not yet tried, would, I think, be to replace the blow-pipe by
two bats-wing burners, permanently fastened to a stand, one of them
playing its flame downwards on to the top of the flame of the other.
The angle between the directions of the jets might be, say, 130°, or
whatever is found convenient. In this way the glass would not be so
likely to get overheated in spots, and better work would doubtless
result. However, I have made numbers of perfectly satisfactory
spirals as described. Three or four turns only make a sufficiently
springy connection for nearly all purposes.
§ 37. On Auxiliary Operations on Glass:-
Boring Holes through Glass:- This is much more easily done than is
generally supposed. The best mode of procedure depends on the
circumstances. The following three cases will be considered:-
1. Boring holes up to one-quarter inch diameter through thick glass
(say over one-eighth inch), or rather larger holes through thin glass.
2. Boring holes of any size through thick glass.
3. Boring round holes through ordinary window glass.
§ 38. Boring small Holes.
Take a three-cornered file of appropriate dimensions, and snip the
point off by means of a hammer; grind out most of the file marks to
get sharp corners. Dip the file in kerosene, and have plenty of
kerosene at hand in a small pot. Place the broken end of the file
against the glass, and with considerable pressure begin to rotate it
(the file) backwards and forwards with the fingers, very much as one
would operate a bradawl against a hard piece of wood. The surface of
the glass will shortly be ground away, and then the file bradawl will
make much quicker progress than might be expected. Two or three
minutes should suffice to bore a bit of sheet window-glass.
The following points require attention:
(1) Use any quantity of oil.
(2) After getting through the skin reduce the pressure on the file.
(3) Be sure to turn the file backwards and forwards through a complete
revolution at least.
(4) When the hole is nearly through reduce the pressure.
(5) When the hole is through the glass be exceedingly careful not to
force the file through too rapidly, otherwise it will simply act as a
wedge and cause a complete fracture.
(6) In many cases it is better to harden the file in mercury before
commencing operations; both files and glass differ so much in hardness
that this point can only be decided by a trial. If it is found
necessary to harden the file, use either a large blow-pipe and a coke
or charcoal bed, or else a small forge. A small blowpipe, such as is
generally found in laboratories, does more harm than good, either by
burning the end of the file or raising it to an insufficient
temperature.
(7) To sharpen the file, which is often necessary after passing
through the "skin" of the glass, put it in a vice so that the point
just protrudes clear of the jaws. Then, using a bit of waste iron as
an intermediary anvil or punch, knock off the least bit from the
point, so as to expose a fresh natural surface. The same result may
be brought about by the use of a pair of pliers.
If several holes have to be bored, it is convenient to mount the file
in the lathe and use a bit of flat hard wood to press up the glass by
means of the back rest. A drilling machine, if not too heavy, does
very well, and has the advantage of allowing the glass to remain
horizontal so that plenty of oil can be kept in the hole.
Use a very slow speed in either case--much slower than would be used
for drilling wrought iron. It is essential that the lubricant should
flow on to the end of the file very freely, either from a pipette or
from the regular oil-feed. If a little chipping where the file
pierces the back surface is inadmissible, it is better, on the whole,
to finish the bore by hand, using a very taper file. It is not
necessary to use a special file for the lathe, for a well-handled file
can be chucked very conveniently in a three-jaw chuck by means of the
handle.
Mr. Shenstone recommends a lubricant composed of camphor dissolved in
turpentine for general purposes. With the object of obtaining some
decisive information as to the use of this lubricant, and to settle
other points, I made the following experiments. Using an old
three-cornered French file, I chipped off the point and adjusted the
handle carefully. I also ground out the file marks near the point,
without hardening the file in mercury. Using kerosene and turpentine
and camphor, I began to bore holes in a hard bit of 3/32 inch window
glass.
Each hole was bored to about one-eighth inch in diameter in four
minutes with either lubricant. After hardening the file in mercury
and using kerosene, I also required four minutes per hole. After
mounting the file in a lathe which had been speeded to turn up brass
rods of about one-half-inch diameter, and therefore ran too fast, I
required one and a half minutes per hole, and bored them right
through, using kerosene. On the whole, I think kerosene does as well
as anything, and for filing is, I think, better than the camphor
solution. However, I ought to say that the camphor-turpentine
compound has probably a good deal to recommend it, for it has survived
from long ago. My assistant tells me he has seen his grandfather use
it when filing glass.
I beg to acknowledge my indebtedness to Mr. Pye, of the Cambridge
Scientific Instrument Company, for showing me in 1886 (by the courtesy
of the Company) the file method of glass-boring; it is also described
by Faraday in Chemical Manipulation, 1228.
It is not necessary, however, to use a file at all, for the twist
drills made by the Morse Drill Company are quite hard enough in their
natural state to bore glass. The circumferential speed of the drill
should not much exceed 10 feet per minute. In this way the author has
bored holes through glass an inch thick without any trouble except
that of keeping the lubricant sufficiently supplied. For boring very
small holes watchmaker's drills may be used perfectly well, especially
those tempered for boring hardened steel. The only difficulty is in
obtaining a sufficient supply of the lubricant, and to secure this the
drill must be frequently withdrawn.
My reason for describing the file method at such length is to be found
in the fact that a Morse drill requires to be sharpened after drilling
glass before it can be used in the ordinary way, and this is often a
difficulty.
I ought to say that I have never succeeded in boring the barrel of a
glass tap by either of these methods. [Footnote: I have been lately
informed that it is usual to employ a splinter of diamond set in a
steel wire holder both for tap boring and for drilling earthenware for
riveting. The diamond must, of course, be set so as to give
sufficient clearance for the wire holder.
For methods of using and setting diamond tools see § 55. It will
suffice to say here that a steel wire is softened and filed at one end
so as to form a fork; into this the diamond is set by squeezing with
pliers. The diamond is arranged so as to present a point in the axis
of the wire, and must not project on one side of the wire more than on
the other. It is not always easy to get a fragment satisfying these
conditions, and at the same time suitable for mounting. A drop of
solder occasionally assists the process of setting the diamond.
In drilling, the diamond must be held against the work by a constant
force, applied either by means of weight or a spring. I made many
trials by this method, using a watchmaker's lathe and pressing up the
work by a weight and string, which passed over a pulley. I used about
40 ounces, and drilled a hole 3/32 in diameter in flint glass at a
speed of 900 revolutions per minute to a depth of one-eighth of an
inch in eight minutes. I used soap and water as a lubricant, and the
work was satisfactory.
Since this was set up, I have been informed by Mr. Hicks of Hatton
Garden that it is necessary to anneal glass rod by heating it up to
the softening point and allowing it to cool very slowly under red-hot
sand or asbestos before boring. If this be done, no trouble will be
experienced. The annealing must be perfect.]
§ 39. For boring large holes through thick glass sheets, or, indeed,
through anything where it is necessary to make sure that
no accident can happen, or where great precision of position and form
of hole is required, I find a boring tube mounted as shown in the
picture (Fig. 36) is of great service. Brass or iron tube borers do
perfectly well, and the end of the spindle may be provided once for
all with a small tube chuck, or the tubes may be separately mounted as
shown. A fairly high speed is desirable, and may be obtained either
by foot, or, if power is available, is readily got by connecting to
the speed cone of a lathe, which is presumably permanently belted to
the motor.
Fig. 36.
After trying tubes armed with diamond dust, as will be presently
explained, I find that emery and thin oil or turpentine, if liberally
supplied below the glass, will do very nearly as well. The tube
should be allowed to rise from the work every few seconds, so as to
allow of fresh emery and oil being carried into the circular grooves.
This is done by lifting the hinged upper bearing, the drill being
lifted by a spiral spring between the pulley and the lower bearing
shown at B. The glass may be conveniently supported on a few sheets
of paper if flat, or held firm in position by wooden clamps if of any
other shape. In any case it should be firmly held down and should be
well supported. Any desired pressure upon the drill is obtained by
weighting the hinged board A.
§ 40. The following method was shown to me by Mr. Wimshurst, but I
have not had occasion to employ it myself. It is suitable for boring
large holes through such glass as the plates of Mr. Wimshurst's
Influence machines are usually made of. A diamond is mounted as the
"pencil" of a compass, and with this a circle is drawn on the glass in
the desired position. The other leg of the compass of course rests on
a suitable washer.
To the best of my recollection the further procedure was as follows.
A piece of steel rod about one-eighth inch in diameter was ground off
flat and mounted in a vice vertically, so as to cause its plane end to
form a small horizontal anvil. The centre (approximately) of the
diamond-cut circle of the glass was laid on this anvil so as to rest
evenly upon it, and the upper surface (i.e. that containing the cut)
was then struck smartly with a hammer, completely pulverising the
glass above the anvil. The hole was gradually extended in a similar
manner right up to the diamond cut, from which, of course, the glass
broke away.
A similar method has long been known to glaziers, differing from the
preceding in that a series of diamond cuts are run across the circle
parallel to two mutually perpendicular diameters. A smart tap on the
back of the scored disc will generally cause the fragments to tumble
out. I have never tried this myself, but I have seen it done.
Large discs may easily be cut from sheet glass by drawing a circular
diamond cut, and gradually breaking away the outer parts by the aid of
additional cuts and a pair of pliers or "shanks" (see Fig. 44).
§ 41. Operations depending on Grinding: Ground-in Joints.
The process will be perfectly understood by reference to a simple
case. Suppose it is desired to grind the end of a tube into the neck
of a bottle. If a stoppered bottle is available, the stopper must be
taken out and measured as to its diameter at the top and bottom.
Select a bit of tube as nearly as possible of the same diameter as the
stopper at its thickest part. Draw down the glass in the blow-pipe
flame rather by allowing it to sink than by pulling it out. After a
few trials no difficulty will be experienced in making its taper
nearly equal to that of the stopper, though there will in all
probability be several ridges and inequalities. When this stage is
reached anneal the work carefully and see that the glass is not too
thin. Afterwards use emery and water, and grind the stopper into the
bottle.
There are six special directions to be note
(1 )Turn the stopper through at least one revolution in each
direction.
(2) Lift it out often so as to give the fresh emery a chance of
getting into the joint.
(3) Rotate the bottle as well as the stopper in case there is any
irregularity in the force brought to bear, which might cause one side
of the neck to be more ground than another, or would cause the tube to
set rather to one side or the other.
(4) Use emery passing a 50 sieve, i.e. a sieve with fifty threads to
the inch run (see § 144) to begin with, and when the stopper nearly
fits, wash this thoroughly away, and finish with flour emery,
previously washed to get rid of particles of excessive size; the
process of washing will be fully discussed in the chapter on
glass-grinding, which see.
(5) Any degree of fineness of surface may be obtained by using graded
emery, as will be explained, but, in general, it is unnecessary to
attempt a finer surface than can be got with washed flour emery. A
superficial and imperfect polish may be given by grinding for a short
time with powdered pumice stone.
(6) If the proper taper is not attained by blowing, or if ridges are
left on the tapered part, the process may be both hastened and
improved by giving the taper a preliminary filing with a
three-cornered file and kerosene, just as one would proceed with iron
or brass. A little filing will often save a good deal of grinding and
make a better job.
If a bottle without a tapered neck is to be employed, it is as well to
do the preliminary grinding by means of a cone turned up from a bit of
cast iron. This is put in the lathe and pushed into the mouth of the
bottle, the latter being supported by the hands. Use about the same
surface speed as would be employed for turning cast iron. In this
case the emery is better used with kerosene.
If a cylindrical bit of cast iron about an inch in diameter is turned
down conically nearly to a point, it will save a good deal of trouble
in making separate cones. If it gets ground into rings, and it
becomes necessary to turn it up, use a diamond tool until the skin is
thoroughly removed; the embedded emery merely grinds the edge off any
ordinary steel tool.
For diamond tools see § 55.
§ 42. Use of the Lathe in Glass-working.
If it is necessary to remove a good deal of glass, time may be saved
by actually turning the glass in a lathe. According to the direction
given above for grinding a tube into the neck of a bottle, very little
glass need be removed if the drawing down is well done, so that for
this purpose turning is often unnecessary.
If the taper of the stopper be small and it is permissible to use a
thick tube, or if a solid stopper only has to be provided, or an old
stopper quickly altered to a new form, turning is very useful. The
glass may be "chucked" in any suitable manner, and run at a speed not
exceeding 10 feet per minute. Prepare a three-cornered file by
mercury-hardening and by grinding the end flat so as to form a cutting
angle of about 80°, and use a moderate amount of kerosene lubrication,
i.e. enough to keep the glass damp, but even this is not essential.
Use the file as an ordinary brass turning tool, and press much more
lightly than for metal turning. The glass will be found to scrape off
quite pleasantly.
By chucking glass tubes on wooden mandrells the ends may be nicely
turned in this manner ready for accurate closing by glass plates.
The process of grinding also is made much more rapid--at all events
in the earlier stages--by chucking either the stopper or the bottle
and holding the other member in the fingers, or in a wooden vice held
in the hands. The finishing touches are best given by hand.
I ought to say that I think a good deal of glass-grinding, as
practised in laboratories, might be advantageously replaced by glass
turning or filing and certainly will be by any one who will give these
methods a trial.
If one tube is to be ground into another, as in grinding a retort into
a receiver, the latter must be drawn down from a larger piece, few
beginners being able to widen a tube by the method explained with
sufficient ease and certainty. The other operations are similar to
the operations above described.
§ 43. Funnels often require to be ground to an angle of 60°. For
this purpose it is well to keep a cast-iron cone, tapering from
nothing up to four inches in diameter. This may be mounted on a
lathe, and will be found of great use for grinding out the inside of
funnels. Care must be taken to work the funnel backwards and
forwards, or it will tend to grind so as to form rings, which
interfere with filtering. A rough polish may be given on the lines
explained in the next section.
§ 44. A rough polish may be easily given to a surface which has been
finished by washed flour emery, in the following manner. Turn up a
disc of soft wood on the lathe, and run it at the highest wood-turning
speed. Rub into the periphery a paste of sifted powdered pumice stone
and water.
Any fairly smooth ground glass surface may be more or less polished by
holding it for a moment against the revolving disc. Exact means of
polishing will be described later on. Meanwhile this simple method
will be found both quick and convenient, and is often quite sufficient
where transparency, rather than figure, is required. I daresay a fine
polish may be got on the same lines, using putty powder or washed
rouge (not jewellers' rouge, which is too soft, but glass-polishers'
rouge) to follow the pumice powder, but I have not required to try
this.
§ 45. It is sometimes required to give to ground glass surfaces a
temporary transparency. This is to be done by using a film of oil of
the same refractive index as the glass. Cornu has employed a varnish
consisting of a mixture of turpentine and oil of cloves, but the
yellow-brown colour of the latter is often a disadvantage. It will be
found that a mixture of nut oil and oil of bitter almonds, or of
bromo-napthalene and acetone, can be made of only a faint yellow
colour; and by exact adjustment of the proportions will have the same
refractive index for any ray as crown glass (ordinary window glass).
Procure a sample of the glass and smash it up to small fragments in an
iron mortar. Sift out the fine dust and the larger pieces; bits
about as large as small beads--say one-sixteenth inch every way--do
very well. Boil the sifted glass with strong commercial hydrochloric
acid to remove iron, wash with distilled water and a few drops of
alcohol, dry on blotting paper in the sun or otherwise. Put the dry
glass into a bottle or beaker, and begin by adding almond oil (or
bromo-napthalene), then add nut oil (or acetone) till the glass
practically disappears when examined by sodium light, or light of any
other wave-length, as may be required.
The adjustment of the mixture is a matter of great delicacy, one drop
too much of either constituent, in, say, 50 cubic centimetres, makes
all the difference. The final adjustment is best accomplished by
having two mixtures of the oils, one just too rich in almond, the
other in nut oil; by adding one or other of these, the required
mixture is soon obtained.
It is to be noted
(1) That adjustment is only perfect for light of one wave-length.
(2) That adjustment is only perfect at one temperature.
On examining a bottle of rather larger fragments of glass immersed in
an adjusted mixture by ordinary daylight, a peculiarly beautiful play
of colours is seen.
Of course, if it is only desired to make ground glass fairly
transparent, these precautions are unnecessary, but it seemed better
to dispose of the matter once for all in this connection.
M. Cornu's object was to make a varnish which would prevent reflection
from the back of a photographic plate on to the film. I have had
occasion to require to do the same when using a scale made by cutting
lines through a film of black varnish on a slip of glass. This
succeeded perfectly by making the varnish out of Canada balsam stained
with a black aniline dye.
Mr. Russell, Government Astronomer of New South Wales, finds that the
"halation" of star photographs can be prevented by pouring over the
back of the plate a film of collodion suitably stained.
§ 46. Making Ground Glass.
This is easily done by rubbing the surface of polished glass with a
bit of cast iron and washed "flour of emery." Of course, if the
fineness of grain of the surface is of importance, appropriate sizes
of emery must be employed. The iron may be replaced by a bit of glass
cut with transverse grooves to allow the emery to distribute itself,
or even by a bit of glass without such grooves, provided it does not
measure more than one or two inches each way. If great speed is an
object rather than the fineness of the surface, use a bit of lead and
coarse emery, say any that will pass a sieve with fifty threads to the
inch.
It may perhaps be mentioned here that it is a pity to throw away emery
which has been used between glass and glass. In the chapter dealing
with fine optical work the use of emery of various grades of fineness
will be treated, and the finer grades can only be obtained (to my
knowledge) from emery which has been crushed in the process of glass
or metal grinding, especially the former. A large jam-pot covered
with a cardboard lid does well as a receptacle of washings.
§ 47. Glass-cutting.
This is an art about which more can be learned in five minutes by
watching it well practised than by pages of written description. My
advice to any one about to commence the practice of the art would be
to make friends with a glazier and see it done. What follows is
therefore on the supposition that this advice has been followed.
After some experience of cutters made of especially hardened steel, I
believe better work can generally be got out of a diamond, provided
the cost is not an objection. It is economy to pay a good price for a
good diamond. As is well known, the natural angle of the crystal
makes the best point, and a person buying a diamond should examine the
stone by the help of a lens, so as to see that this condition is
fulfilled. The natural angle is generally, if not always, bounded by
curved edges, which have a totally different appearance from the sharp
edges of a "splinter."
When a purchase is to be made, it is as well for the student to take a
bit of glass and a foot-rule with him, and to test the diamond before
it is taken away. When a good diamond has been procured, begin by
taking cuts on bits of clean window glass until the proper angle at
which to hold the tool is ascertained. Never try to cut over a
scratch, if you value your diamond, and never press hard on the glass;
a good cut is accompanied by an unmistakable ringing sound quite
different from the sound made when the diamond is only scratching.
Perhaps the most important advice that can be given is, Never lend the
diamond to anybody--under any circumstances.
The free use of a diamond is an art which the physicist will do well
to acquire, for quite a variety of apparatus may be made out of glass
strips, and the accuracy with which the glass breaks along a good cut
reduces such an operation as glass-box-making to a question of
accurate drawing.
§ 48. Cementing.
One of the matters which is generally confused by too great a
profusion of treatment is the art of cementing glass to other
substances.
The following methods will be found to work, subject to two
conditions:
(1) The glass must be clean;
(2) it must be hot enough to melt the cement.
For ordinary mending purposes when the glass does not require to be
placed in water (especially if hot) nothing is better than that kind
of glue which is generally called "diamond cement." This may be
easily made by dissolving the best procurable isinglass in a mixture
of 20 per cent water and 80 per cent glacial acetic acid--the exact
proportions are not of consequence.
First, the isinglass is to be tightly packed into a bottle with a wide
neck, then add the water, and let the isinglass soak it up.
Afterwards pour in the acetic acid, and keep the mixture near 100°C.
for an hour or two on the water bath--or rather in it. The total
volume of acetic acid and water should not be more than about half of
the volume of isinglass when the latter is pressed into the bottle as
tightly as possible.
The proper consistency of the cement may be ascertained by lifting a
drop out of the bottle and allowing it to cool on a sheet of glass.
In ten minutes it ought not to be more than slightly sticky, and the
mass in the bottle, after standing a few hours cold, should not be
sticky at all, and should yield, jelly-like, to the pressure of the
finger to only a slight degree. If the glue is too weak, more
isinglass may be added (without any preliminary soaking).
A person making the mixture for the first time almost always gets it
too weak. It is difficult to give exact proportions by weight, as
isinglass and gelatine (which may replace it) differ greatly in
quality. This cement is applied like glue, and will cement nearly
anything as well as glass. Of course, as much cement as possible must
be squeezed out of any joint where it is employed. The addition of
gums, as recommended in some books, is unnecessary.
Ordinary glue will serve perfectly for cementing glass to wood.
"Chipped glass" ware is, I understand, made by painting clean glass
with glue. As the glue dries and breaks by contraction, it chips off
the surface of the glass. I have never seen this done. In nearly all
cases where alcohol is not to be employed very strong joints may be
made by shellac. Orange shellac is stronger than the "bleached"
variety.
A sine qua non is that the glass be hot enough to melt the shellac.
The best way is to heat the glass surfaces and rub on the shellac from
a bit of flake; the glass should not be so hot as to discolour the
shellac appreciably, or its valuable properties will be partly
destroyed. Both glass surfaces being thus prepared, and the shellac
being quite fluid on both, they may be brought together and clamped
tightly together till cool. Shellac that has been overheated, or
dissolved in alcohol, or bleached, is of little use as compared with
the pale orange flaky product. Dark flakes have probably been
overheated during the preliminary refining.
For many purposes a cement is required capable of resisting carbon
bisulphide. This is easily made by adding a little treacle (say 20
per cent) to ordinary glue. Since the mixture of glue and treacle
does not keep, i.e. it cannot be satisfactorily melted up again after
once it has set, no more should be made up than will be wanted at the
time. If the glue be thick, glass boxes for carbon disulphide may be
easily put together, even though the edges of the glass strips are not
quite smooth, for, unlike most cements, this mixture remains tough,
and is fairly strong in itself.
I have found by experiment that most fixed and, to a less degree,
essential oils have little or no solvent action on shellac, and I
suspect that the same remark applies to the treacle-glue mixture, but
I have not tried. Turpenes act on shellac slightly, but mineral oils
apparently not at all. The tests on which these statements are based
were continued for about two years, during which time kerosene and
mineral oils had no observable effect on shellac--fastened
galvanometer mirrors.
§ 49. Fusing Electrodes into Glass.
This art has greatly improved since the introduction of the
incandescent lamp; however, up to the present, platinum seems to
remain the only substance capable of giving a certainly air-tight
result. I have not tried the aluminium-alumina method.
Many years ago it was the fashion to surround the platinum wire with a
drop of white enamel glass in order to cause better adhesion between
it and the ordinary glass. [Footnote: Hittorf and Geissler (Pogg.
Ann. 1864, § 35; English translation, Phys. Soc. London, p. 138)
found that it was impossible to make air-tight joints between platinum
and hard potash glass, but that soft lead glass could be used with
success as a cement.] However, in the case of flint glass, if one may
judge from incandescent lamps, this is not essential--a fact which
entirely coincides with my own experience.
On the other hand, when sealing electrodes into German glass I have
often used a drop of enamel with perfect results, though this is not
always done in Germany. In all cases, however, in which electrodes
have to be sealed in--especially when they are liable to heat--I
recommend flint glass, and in this have the support of Mr. Rain (The
Incandescent Lamp and its Manufacture, p. 131). The exact details
for the preparation of eudiometer tubes are given by Faraday (Chemical
Manipulation, § 1200).
In view of what has preceded, however, I will content myself with the
following notes. Make the hole through which the wire is to protrude
only slightly larger than the wire itself, and be sure that the latter
is clean. Allow the glass to cool sufficiently not to stick to the
wire when the latter is pushed in. Be sure that, on heating, the
glass does not get reduced, and that it flows up to the wire all
round; pull and push the wire a little with a pair of pincers, to
ensure this.
It is not a bad plan to get the glass exceedingly fluid round the
wire--even if the lump has to be blown out a little afterwards--as it
cools. The seal should finally be well annealed in asbestos, but
first by gradually moving it into the hot air in front of the flame.
It was observed by Professor J. J. Thomson and the author some years
ago (Proc. Roy. Soc. 40. 331. 1886) that when very violent
discharges are taken through lightly sealed-in electrodes in
lead-glass tubes--say from a large battery of Leyden jars--gas
appears to be carried into the tube over and above that naturally
given off by the platinum, and this without there being any apparent
want of perfection in the seal. This observation has since been
confirmed by others. Consequently in experiments on violent
discharges in vacuo where certainty is required as to the exclusion of
air, the seals should be protected by a guard tube or cap containing
mercury; this must, of course, be put in hot and clean, on hot and
clean glass, and in special cases should be boiled in situ.
A well-known German physicist (Warburg, I think) recommends putting
the seals under water, but I cannot think that this is a good plan,
for if air can get in, why not water? which has its surface tension
in its favour. The same reasoning prevents my recommending a layer of
sulphuric acid above the mercury-a method used for securing
air-tightness in "mercury joints" by Mr. Gimingham, Proc. R. S.
1874.
Further protection may be attained for many purposes by coating the
platinum wire with a sheath of glass, say half an inch long, fused to
the platinum wire to a depth of one-twentieth of an inch all round.
In some cases the electrodes must be expected to get very hot, for
instance, when it is desired to platinise mirrors by the device of
Professor Wright of Yale. In this and similar cases I have met with
great success by using "barometer" tubes of about one-twelfth of an
inch bore, and with walls, say, one-tenth of an inch thick.
[Footnote: "Barometer" tube is merely very thick-walled glass tubing,
and makes particularly bad barometers, which are sold as weather
glasses.]
This tube is drawn down to a long point--say an inch long by
one-eighth of an inch external diameter, and the wire is fused in for
a length, say, of three-quarters of an inch, but only in the narrow
drawn--down part of the tube. At different times I have tried four
such seals, and though the electrodes were red hot for hours, I have
never had an accident--of course they were well annealed.
Fig. 37.
For directions as to the making of high vacuum tubes, see the section
dealing with that matter.
§ 50. As economy of platinum is often of importance, the following
little art will save money and trouble. Platinum is easily caused to
join most firmly to copper--with which, I presume, it alloys--by the
following method. Hold the platinum wire against the copper wire, end
to end, at the tip of the reducing flame of a typical blowpipe--or
anywhere--preferably in the "reducing" part of the oxygas flame; in
a moment the metals will fuse together at the point of contact, when
they may be withdrawn.
Such a joint is very strong and wholly satisfactory, much better than
a soldered joint. If the work is not carried out successfully so that
a considerable drop of copper-platinum alloy accumulates, cut it off
and start again. The essence of success is speed, so that the copper
does not get "burned." If any considerable quantity of alloy is formed
it dissolves the copper, and weakens it, so that we have first the
platinum wire, then a bead of alloy, and then a copper wire fused into
the bead, but so thin just outside the latter that the joint has no
mechanical strength.
§ 51. The Art of making Air-light Joints.
Lamp-manufacturers and others have long since learned that when glass
is in question not only are fused joints made as easily as others, but
that they afford the only reliable form of joint. An experimenter who
uses flint glass, has a little experience, an oxygas blow-pipe and a
blowing apparatus, will prefer to make his joints in this way, simply
from the ease with which it may be done. When it comes to making a
tight joint between glass and other substances the problem is by no
means so easy. Thus Mr. Griffiths (Phil. Trans. 1893, p. 380)
failed to make air-tight joints by cementing glass into steel tubes,
using hard shellac, and the tubes fitting closely. These joints were
satisfactory at first, but did not last; the length of the joint is
not stated. The difficulty was finally got over by soldering very
narrow platinum tubes into the steel, and fusing the former into the
glass.
Mr. Griffiths has since used an alloy with success as a cement, but I
cannot discover what it is made from. Many years ago Professor Hittorf
prepared good high vacuum tubes by plugging the ends of glass tubes
with sealing wax merely, though in all cases the spaces to be filled
with wax were long and narrow (Hittorf, Pogg. Ann. 1869, § 5,
English translation, Phys. Soc. p. 113). Again, Regnault
habitually used brass ferules, and cemented glass into them by means
of his mastic, which can still be procured at a low rate from his
instrument-makers (Golan, Paris). Lenard also, in his investigations
on Cathode Rays (Wied. Ann, vol. li. p. 224), made use of sealing
wax covered with marine glue.
Surely in face of these facts we must admit that cement joints can be
made with fair success. I do not know the composition of M.
Regnault's mastic, but Faraday (Manipulations, § 1123) gives the
following receipt for a cement for joining ferules to retorts, etc:
Resin 5 parts.
Beeswax 1 part.
Red ochre or Venetian red,
finely powdered and sifted 1 part.
I believe this to be substantially the same as Regnault's mastic,
though I have never analysed the latter.
For chemical work the possibility of evolution of gas from such a
cement must be taken into account, and I should certainly not trust it
for this reason in vacuum tube work, where the purity of the confined
gas could come in question. Otherwise it is an excellent cement, and
does not in my experience tend to crack away from glass to the same
extent as paraffin or pure shellac.
This cracking away from glass, by the way, is probably an effect of
difference in rate of expansion between the glass and cement which
probably always exists, and, if the cement be not sufficiently
viscous, must, beyond certain temperature limits, either produce
cracks or cause separation. Professor Wright of Yale has used a hard
mineral pitch as a cement in vacuum work with success.
My attention has been directed to a fusible metal cement containing
mercury, and made according to the following receipt, given by Mr. S.
G. Rawson, Journal of the Society of Chemical Industry, vol. ix.
(1890), P. 150:-
Bismuth 40 per cent
Lead 25 per cent
Tin 10 per cent
Cadmium 10 per cent
Mercury 15 per cent
This is practically one form of Rose's fusible metal with 15 per cent
mercury added. It takes nearly an hour to set completely, and the
apparatus must be clean and warm before it is applied.
As the result of several trials by myself and friends, I am afraid I
must dissent from the claim of the author that such a cement will make
a really air-tight joint between glass tubes. Indeed, the appearance
of the surface as viewed through the glass is not such as to give any
confidence, no matter what care may have been exercised in performing
all the operations and cleaning the glass; besides which the cement
is rigid when cold, and the expansion difficulty comes in.
On the other hand, if extreme air-tightness is not an object, the
cement is strong and easily applied, and has many uses. I have an
idea that if the joints were covered with a layer of soft wax, the
result would be satisfactory in so far as air-tightness is concerned.
This anticipation has since been verified.
In many cases one can resort to the device already mentioned of
enclosing a rubber or tape-wrapped joint between two tubes in a bath
of mercury, but in this case the glass must be clean and hot and the
mercury also warm, dry, and pure when the joint is put together,
otherwise an appreciable air film is left against the glass, and this
may creep into the joint.
Perhaps the easiest way of making such a joint is to use an outer tube
of thin clean glass, and bore a narrow hole into it from one side to
admit the mercury; if the mercury is to be heated in vacuo, it is
better to seal on a side joint. It is always better, if possible, to
boil the mercury in situ, which involves making the wrapping of
asbestos, but, after all, we come back to the position I began by
taking up, viz. that the easiest and most reliable method is by
fusion of the glass--all the rest are unsuitable for work of real
precision.
I should be ungrateful, however, were I not to devote a few lines to
the great convenience and merit of so-called "centering cement." This
substance has two or three very valuable properties. It is very tough
and strong in itself, and it remains plastic on cooling for some time
before it really sets. If for any reason a small tube has to be
cemented into a larger one, which is a good deal larger, so that an
appreciable mass of cement is necessary, and particularly if the joint
requires to have great mechanical strength, this cement is invaluable.
I have even used a plug of it instead of a cork for making the joint
between a gas delivery tube and a calcium chloride tower. (Why are
these affairs made with such abominable tubulures?)
The joint in question has never allowed the tube to sag though it
projects horizontally to a distance of 6 inches, and has had to
withstand nearly two years of Sydney temperature. The cement consists
of a mixture of shellac and 10 per cent of oil of cassia.
The shellac is first melted in an iron ladle, and the oil of cassia
quickly added and stirred in, to an extent of about 10 per cent, but
the exact proportions are not of importance. Great care must be taken
not to overheat the shellac.
APPENDIX TO CHAPTER I
ON THE PREPARATION OF VACUUM TUBES FOR THE PRODUCTION OF PROFESSOR
ROENTGEN'S RADIATION
[Footnote: Written in May 1896.]
WHEN Professor Roentgen's discovery was first announced at the end of
1895 much difficulty was experienced in obtaining radiation of the
requisite intensity for the repetition of his experiments. The
following notes on the production of vacuum tubes of the required
quality may therefore be of use to those who desire to prepare their
own apparatus. It appears that flint glass is much more opaque to
Roentgen's radiation than soda glass, and consequently the vacuum
tubes require to be prepared from the latter material.
Fig. 39.
A form of vacuum tube which has proved very successful in the
author's hands is sketched in Fig. 38. It is most easily constructed
as follows. A bit of tubing about 2 centimetres diameter, 15
centimetres long, and 1.5 millimetre wall thickness, is drawn down to
a point. The larger bulb, about 5 centimetres in diameter, is blown
at one end of this tube. The thinner the bulb the better, provided
that it does not collapse under atmospheric pressure. A very good
idea of a proper thickness may be obtained from the statement that
about 4 centimetres length of the tubing should be blown out to form
the bulb. This would give a bulb of about the thickness of an
ordinary fractionating bulb. Before going any further it is as well
to test the bulb by tapping on the table and by exhausting it by means
of an ordinary water-velocity pump.
The side tube is next prepared out of narrower tubing, and is provided
with a smaller bulb, a blowing-out tube, and a terminal, to be made as
will be described. This side tube is next fused on to the main tube,
special care being taken about the annealing, and the cathode terminal
is then sealed into the main tube. After using clean glass it is in
general only necessary to rinse the tube out with clean alcohol, after
which it may be dried and exhausted.
The success of the operation will depend primarily on the attention
given to the preparation and sealing-in of the electrode facing the
large bulb.
Preparation of Terminals. Some platinum wire of about No. 26
B.W.G--the exact size is unimportant--must be provided, also some
sheet aluminium about 1 millimetre thick, some white enamel cement
glass, and a "cane" of flint-glass tube of a few millimetres bore.
The electrodes are prepared by cutting discs of aluminium of from 1 to
1.5 centimetres diameter. The discs of aluminium are bored in the
centre, so as to admit the "stems" which are made of aluminium wire
of about 1 millimetre diameter. The stems are then riveted into the
discs. The "stems" are about I centimetre long, and are drilled to a
depth of about 3 millimetres, the drill used being about double the
diameter of the platinum wire to be used for making the connections.
The faces of the electrodes--i.e. the free surfaces of the aluminium
discs--are then hammered flat and brought to a burnished surface by
being placed on a bit of highly polished steel and struck by a "set"
provided with a hole to allow of the "stem" escaping damage. The
operation will be obvious after a reference to Figs. 39 and 40; it
is referred to again on page 96.
The platinum wires may be most conveniently attached by melting one
end of the piece of platinum wire in the oxygas blow-pipe till it
forms a bead just large enough to pass into the hole drilled up the
stem of the electrode. The junction between the stein and the
platinum wire is then made permanent by squeezing the aluminium down
upon the platinum wire with the help of a pair of pliers. It is also
possible to fuse the aluminium round the platinum, but as I have had
several breakages of such joints, I prefer the mechanical connection
described.
Fig. 39. Sets for striking aluminium electrodes
Fig. 40.
i. Aluminium electrode.
ii. Aluminium electrode connected to platinum wire.
iii. Aluminium electrode connected to platinum wire and protected by
glass.
iv. Detail of fastening platinum wire.
The stem and platinum wire may now be protected by covering them with
a little flint glass. For this purpose the flint-glass tube is pulled
down till it will just slip over the stem and wire, and is cut off so
as to leave about half a centimetre of platinum wire projecting. The
flint-glass tube is then fused down upon the platinum wire, care being
taken to avoid the presence of air bubbles. At the close of the
operation a single drop of white enamel glass is fused round the
platinum wire at a high temperature, so as to make a good joint with
the protecting flint-glass tube.
The negative electrode being nearly as large as the main tube, it must
be introduced before the latter is drawn down for sealing. After
drawing down the main tube in the usual manner, taking care not to
make it less than a millimetre in wall-thickness, it is cut off so as
to leave a hole not quite big enough for the enamel drop to pass
through. By heating and opening, the aperture is got just large
enough to allow the enamel drop to pass into it, and when this is the
case the joint is sealed, pulled, and blown out until the electrode
occupies the right position--viz. in the centre of the tube and with
its face normal to the axis of the tube.
The glass walls near the negative electrode must not be less than a
millimetre thick, and may be rather more with advantage, the glass
must be even, and the joint between the flint glass and the soda
glass, or between the wire and the soda glass, must be wholly through
the enamel. The "seal" must be well annealed. It will be found that
the sealing-in process is much easier when the stem of the electrode
is short and when the glass coating is not too heavy. Half a
millimetre of glass thickness round the stein is quite sufficient.
The diagram, of the tube shows that the main tube has been expanded
round the edges of the cathode. This is to reduce the heating
consequent on the projection of cathode rays from the edges of the
disc against the glass tube.
The anode is inserted into its bulb in a quite similar manner. If
desired it may be made considerably smaller, and does not need the
careful adjustment requisite in sealing-in the cathode, nor does the
glass near the entry wire require to be so thick.
More intense effects are often got by making the cathode slightly
concave, but in this case the risk of melting the thin glass is
considerably increased. No doubt, Bohemian glass might be used
throughout instead of soft soda glass, and this would not melt so
easily; the difficulties of manipulating the glass are, however, more
pronounced.
It will be shown directly that the best Roentgen effects are got with
a high vacuum, and it is for this reason that the glass near the
cathode seal requires to be strong. The potential right up to the
cathode is strongly positive inside the tube, and this causes the
glass to be exposed to a strong electric stress in the neighbourhood
of the seal.
Although the glass-blowing involved in the making of a so-called focus
tube is rather more difficult than in the case just described, there
is no reason why such a difficulty should not be overcome; I will
therefore explain how a focus tube may be made.
Fig. 41.
A bulb about 3 inches in diameter is blown from a bit of tube of a
little more than 1 inch diameter. Unless the walls of the tube are
about one-eighth of an inch in thickness, this will involve a
preliminary thickening up of the glass. This is not difficult if care
be taken to avoid making the glass too hot. The larger gas jet
described in connection with the soda-glass-blowing table must be
employed. In blowing a bulb of this size it must not be forgotten
that draughts exercise a very injurious influence by causing the glass
to cool unequally; this leads to bulbs of irregular shape.
In the method of construction shown in Fig. 41, the anode is put in
first. This anode simply consists of a square bit of platinum or
platinum-iridium foil, measuring about 0.75 inch by 1 inch, and
riveted on to a bent aluminium wire stem.
As soon as the anode is fused in, and while the glass is still hot,
the side tube is put on. The whole of the anode end is then carefully
annealed. When the annealing is finished the side tube is bent as
shown to serve as a handle when the time comes to mount the cathode.
Before placing the cathode in position, and while the main tube is
still wide open, the anode is adjusted by means of a tool thrust in
through this open end. This is necessary in view of the fact that the
platinum foil is occasionally bent during the operation of forcing the
anode into the bulb.
The cathode is a portion of a spherical surface of polished aluminium,
a mode of preparing which will be given directly. The cathode having
been placed inside the bulb, the wide glass tube is carefully drawn
down and cut off at such a point that when the cathode is in position
its centre of curvature will lie slightly in front of the anode plate.
For instance, if the radius of curvature of the cathode be 1.5 inches,
the centre of curvature may lie something like an eighth of an inch or
less in front of the anode.
The cathode as shown in Fig. 41 is rather smaller than is
advantageous. To make it much larger than is shown, however, the
opening into the bulb would require to be considerably widened, and
though this is not really a difficult operation, still it requires
more practice than my readers are likely to have had. The difficulty
is not so much in widening out the entry as in closing it down again
neatly.
Now as to making the anode. A disc of aluminium is cut from a sheet
which must not be too thick--one twenty-fifth of an inch is quite
thick enough. This disc is bored at the centre to allow of the stem
being riveted in position. The disc is then annealed in the Bunsen
flame and the stem riveted on.
The curvature is best got by striking between steel dies (see Figs.
39 and 40). Two bits of tool steel are softened and turned on the
lathe, one convex and the other concave. The concave die has a small
hole drilled up the centre to admit the stem. The desired radius of
curvature is easily attained by cutting out templates from sheet zinc
and using them to gauge the turning. The two dies are slightly ground
together on the lathe with emery and oil and are then polished, or
rather the convex die is polished--the other one does not matter.
The polishing is most easily done by using graded emery and oil and
polishing with a rag. The method of grading emery will be described
in the chapter on glass-grinding.
The aluminium disc is now struck between the dies by means of a
hammer. If the radius of curvature is anything more than one inch and
the disc not more than one inch in diameter the cathode can be struck
at once from the flat as described. For very deep curves no doubt it
will be better to make an intermediate pair of dies and to re-anneal
the aluminium after the first striking.
When the tube is successfully prepared so far as the glassblowing goes
it may be rinsed with strong pure alcohol both inside and out, and
dried. The straight part of the side tube is then constricted ready
for fusing off and the whole affair is placed on the vacuum pump.
In spite of the great improvements made during recent years in the
construction of so-called Geissler vacuum pumps--i.e. pumps in which
a Torricellian vacuum is continually reproduced--I am of opinion that
Sprengel pumps are, on the whole, more convenient for exhausting
Crooke's tubes. A full discussion of the subject of vacuum pumps will
be found in a work by Mr. G. S. Ram (The Incandescent Lamp and its
Manufacture), published by the Electrician Publishing Company, and it
is not my intention to deal with the matter here; the simplest kind of
Sprengel pump will be found quite adequate for our purpose, provided
that it is well made.
Fig. 42 is intended to represent a modification of a pump based on
the model manufactured by Hicks of Hatton Garden, and arranged to suit
the amateur glass-blower. The only point of importance is the
construction of the head of the fall tube, of which a separate and
enlarged diagram is given. The fall tubes may have an internal
diameter up to 2 mm. (two millimetres) and an effective length of 120
cm.
Free use is made of rubber tube connections in the part of the pump
exposed to the passage of mercury. The rubber employed should be
black and of the highest quality, having the walls strengthened by a
layer of canvas. If such tube cannot be easily obtained, a very good
substitute may be made by placing a bit of ordinary black tube inside
another and rather larger bit and binding the outer tube with tape or
ribbon. In any case the tubing which comes in contact with the
mercury should be boiled in strong caustic potash or soda solution for
at least ten minutes to get rid of free sulphur, which fouls the
mercury directly it comes in contact with it. The tubing is well
washed, rinsed with alcohol, and carefully dried.
Fig. 42.
The diagram represents what is practically a system of three Sprengel
pumps, though they are all fed from the same mercury reservoir and run
down into the same mercury receiver. It is much easier to make three
pumps, each with separate pinch cocks to regulate the mercury supply,
than it is to make three jets, each delivering exactly the proper
stream of mercury to three fall tubes.
Sprengel pumps only work at their highest efficiency when the mercury
supply is carefully regulated to suit the peculiarities of each fall
tube, and this is quite easily done in the model figured. Since on
starting the pump the rubber connections have to stand a considerable
pressure, the ends of the tubes must be somewhat corrugated to enable
the rubber to be firmly wired on to them. The best binding wire is the
purest Swedish iron wire, previously annealed in a Bunsen gas flame.
The wire must never be twisted down on the bare rubber, but must
always be separated from it by a tape binding. By taking this
precaution the wire maybe twisted very much more tightly than is
otherwise possible without cutting the rubber.
The only difficulty in making such a pump as is described lies in the
bending of the heads of the fall tubes. This bending must be done
with perfect regularity and neatness, otherwise the drops of mercury
will not break regularly, or will break just inside the top of the
fall tube, and so obstruct its entrance that at high vacua no air can
get into the tube at all.
The connections at the head of the fall tubes must also be well put on
and the joints blown out so that the mercury in dropping over the head
is not interfered with by the upper surface of the tube. However, a
glance at the enlarged diagram will show what is to be aimed at better
than any amount of description. In preparing the fall tubes it is
generally necessary to join at least two "canes" together. The joint
must be arranged to occur either in the tube leading the mercury to
the head of the fall, or in that part of the fall tube which remains
full of mercury when the highest vacuum is attained. On no account
must the joint be made at the fall itself (at least not by an
amateur), nor in that part of the fall tube where the mercury falls
freely, particularly at its lower end, where the drops fall on the
head of the column of mercury.
When a high vacuum is attained the efficacy of the pump depends
chiefly on the way in which the drops fall on the head of the column.
If the fall is too long the drops are apt to break up and allow the
small bubble of air to escape up the tube, also any irregularity or
dirt in the tube at this point makes it more easy for the bubbles of
air to escape to the surface of the mercury.
Any pump in which the supply of mercury to the fall tube can be
regulated nicely will pump well until the lowest available pressures
are being attained; a good pump will then continue to hold the air
bubbles, while a bad one will allow them to slip back [Footnote: For
special methods of avoiding this difficulty see Mr. Ram's book.] ...
Though three fall tubes are recommended, it must not be supposed that
the pump will produce a Crooke's vacuum three times more rapidly than
one fall tube. Until the mercury commences to hammer in the pump the
three tubes will pump approximately three times faster than one tube,
but as soon as the major portion of the air collected begins to come
from the layer condensed on the glass surface of the tube to be
exhausted and from the electrodes, the rate at which exhaustion will
go on no longer depends entirely on the pump.
In order that bubbles of air may not slip back up the fall tube it is
generally desirable to allow the mercury to fall pretty briskly, and
in this case the capacity of the pump to take air is generally far in
excess of the air supply. One advantage of having more than one fall
tube is that it often happens that a fall tube gets soiled during the
process of exhaustion and no longer works up to its best performance.
Out of three fall tubes, however, one is pretty sure to be working
well, and as soon as the mercury begins to hammer in the tubes the
supply may be shut off from the two falls which are working least
satisfactorily.
Thus we are enabled to pump rapidly till a high degree of exhaustion
is attained, having practically three pumps instead of one, whereas
when the final stages are reached, and three pumps are only a drawback
in that they increase the mercury flow, the apparatus is capable of
instant modification to meet the new conditions.
The thistle funnels at the head of the fall tubes are made simply by
blowing bulbs and then blowing the heads of the bulbs into wider ones,
and finally blowing the heads of the wider bulbs off by vigorous
blowing. The stoppers are ground in on the lathe before the tubes are
attached to the fall tubes. The stoppers require to be at least half
an inch long where they fit the necks, and must be really well ground
in. The stoppers must first be turned up nicely and the necks ground
out by a copper or iron cone and emery. The stoppers are rotated on a
lathe at quite a slow speed, say 30 or 40 feet per minute, and the
necks are held against them, as described in the section dealing with
this art. The stoppers must in this case be finished with "two
seconds" emery, and lastly with pumice dust and water (see chapter on
glass-grinding).
Unless the stoppers fit exceedingly well trouble will arise from the
mercury (which is poured into the thistle heads to form a seal) being
forced downwards into the pump by atmospheric pressure.
The joints between the three fall tubes and the single exhaust main
are easily made when the tubes are finally mounted, the hooked nozzle
of the oxygas blow-pipe being expressly made for such work.
It is, on the whole, advisable to make the pump of flint glass, or at
all events the air-trap tube and the fall tubes. A brush flame from
the larger gas tube of the single blowpipe table is most suitable for
the work of bending the tubes. The jointing of the long, narrow bore
fall tubes is best accomplished by the oxygas flame, for in this way
the minimum of irregularity is produced; the blowing tubes will of
course be required for the job, and the narrow tubes must be well
cleaned to begin with.
The air trap is an important though simple part of the pump. Its
shoulder or fall should stand rather higher than the shoulders of the
fall tubes, so that the mercury may run in a thin stream through a
good Torricellian vacuum before it passes down to the fall tubes.
This is easily attained by regulating the main mercury supply at the
pinch cock situated between the tube from the upper reservoir and the
air-trap tube, the other cocks being almost wide open.
It might be thought that the mercury would tend to pick up air in
passing through the rubber connections to the fall tubes, but I have
not found this to be the case in practice. There is, of course, no
difficulty in eliminating the rubber connections between the fall
tubes and the mercury supply from the air trap, but it impresses a
greater rigidity on the structure and, as I say, is not in general
necessary. It must not be forgotten that the mercury always exercises
considerable pressure on the rubber joints, and so there is little
tendency for gas to come out of the rubber.
The thistle funnels at the head of the fall tubes provide a simple and
excellent means of cleaning the fall tubes. For this purpose some
"pure" sulphuric acid which has been boiled with pure ammonium
sulphate is placed in each thistle funnel, and when the fall tube is
dirty the connection to the mercury supply is cut off at the pinch
cock so as to leave the tube between this entry and the head of the
fall tube quite full of mercury, and the sulphuric acid is allowed to
run down the fall tube by raising the stopper. The fall tube should
be allowed to stand full of acid for an hour or so, after which it
will be found to be fairly clean.
Of course the mercury reservoir thus obtains a layer of acid above the
mercury, and as it is better not to run the risk of any acid getting
into the pump except in the fall tubes, the reservoir is best emptied
from the bottom, by a syphon, if a suitable vessel cannot be procured,
so that clean mercury only is withdrawn.
The phosphorus pentoxide tube is best made as shown simply from a bit
of wide tube, with two side connections fused to the rest of the pump.
It is no more trouble to cut the tube and fuse it up again when the
drying material is renewed than to adjust the drying tube to two fixed
stoppers, which is the alternative. The practice here recommended is
rendered possible only by the oxygas blow-pipe with hooked nozzle.
The connection between the pump and tube to be exhausted is made
simply by a short bit of rubber tube immersed in mercury.
The phosphorus pentoxide should be pure, or rather free from
phosphorus and lower oxides; unless this be the case, the vapour
arising from it is apt to soil the mercury in the pump. The
phosphorus pentoxide is purified by distilling with oxygen over
red-hot platinum black; if this cannot be done, the pentoxide should
at least be strongly heated in a tube, in a current of dry air or
oxygen, before it is placed in the drying tube.
The mercury used for the pump must be scrupulously clean. It does
not, however, require to have been distilled in vacuo. It is
sufficient to purify it by allowing it to fall in a fine spray into a
large or rather tall jar of 25 per cent nitric acid and 75 per cent
water. The mercury is then to be washed and dried by heating to, say,
110° C. in a porcelain dish.
Exhausting a Roentgen Tube.
With a pump such as has been described there is seldom any advantage
in fusing an extra connection to the vacuum tube so as to allow of a
preliminary exhaustion by means of a water pump. About half an hour's
pumping may possibly be saved by making use of a water pump.
The tube to be exhausted is washed and dried by careful heating over a
Bunsen burner and by the passage of a current of air. The exhausting
tube is then drawn down preparatory to sealing off, and the apparatus
placed upon the pump. It is best held in position by a wooden clamp
supported by a long retort stand.
Exhaustion may proceed till the mercury in the fall tubes commences to
hammer. At this point the tube must be carefully heated by a Bunsen
flame, the temperature being brought up to, say, 400° C. The heating
may be continued intermittently till little or no effect due to the
heating is discernible at the pump. When this stage is reached, or
even before, the electrodes may be connected up to the coil and a
discharge sent through the tube.
Care must be taken to stop the discharge as soon as a purple glow
begins to appear, because when this happens, the resistance of the
tube is very low, the electrodes get very hot, and may easily get
damaged by a powerful discharge, and the platinum of the anode (if a
focus tube is in question) begins to be distilled on to the glass.
The heating and sparking are to be continued till the resistance of
the tube sharply increases. This is tested by always having a spark
gap, conveniently formed by the coil terminals, in parallel with the
tube. If the terminals are points, it is convenient to set them at
about one quarter of an inch distance apart.
As soon as sparks begin to pass between the terminals of the spark gap
it becomes necessary to watch the process of exhaustion very
carefully. In the first place, stop the pump, but let the coil run,
and note whether the sparks continue to flow over the terminals. If
the glass and electrodes are getting gas free, the discharge will
continue to pass by the spark gap, but if gas is still being freely
given off, then in perhaps three minutes the discharge will return to
the tube, and pumping must be recommenced. The Roentgen effect only
begins to appear when the tube has got to so high a state of
exhaustion that the resistance increases rapidly.
By pumping and sparking, the resistance of the tube may be gradually
raised till the spark would rather jump over 2 inches of air than go
through the tube. When this state is attained the Roentgen effect as
tested by a screen of calcium tungstate should be very brilliant. No
conclusion as to the equivalent resistance of the tube can be arrived
at so long as the discharge is kept going continually. When the spark
would rather go over an inch of air in the spark gap than through the
tube the pumping and sparking may be interrupted and the tube allowed
to rest for, say, five minutes. It will generally be found that the
equivalent resistance of the tube will be largely increased by this
period of quiescence. It may even be found that the spark will now
prefer to pass an air gap 3 inches long.
In any case the sparking should now be continued, the pump being at
rest, and the variations of tube resistance watched by adjusting the
spark gap. If the resistance falls below an equivalent of 2 inches of
air in the gap the pump must be brought into action again and
continued until the resistance as thus estimated remains fairly
constant for, say, ten minutes. When this occurs the narrow neck of
the exhaust tube may be strongly heated till the blow-pipe flame
begins to show traces of sodium light. The flame must then be
withdrawn and the discharge again tested. This is necessary because
it occasionally happens that gas is given off during the heating of
the neck to the neighbourhood of its fusion temperature.
If all is right the neck may now be fused entirely off and the tube is
finished. Tubes of the focus pattern with large platinum anodes are
in general (in my experience) much more difficult to exhaust than
tubes of the kind first described. This is possibly to be attributed
mainly to the gas given off by the platinum, but is also, no doubt,
due to the tubes being much larger and exposing a larger glass
surface. The type of tube described first generally takes about two
hours to exhaust by a pump made as explained, while a "focus" tube has
taken as long as nine hours, eight of which have been consumed after
the tube was exhausted to the hammering point.
The pressure at which the maximum heating of the anode by the cathode
rays occurs is a good deal higher than that at which the maximum
Roentgen effect is produced. There is little doubt that the Roentgen
radiation changes in nature to some extent as the vacuum improves
either as a primary or secondary effect. It is therefore of some
importance to test the tube for the purpose for which it is to be used
during the actual exhaustion. It has been stated, for instance, that
the relative penetrability of bone and flesh to Roentgen radiation
attains a maximum difference at a certain pressure; this is very
likely the case. Whether this effect is a direct function of the
density of the gas in the tube, or whether it is dependent on the
voltage or time integral of the current during the discharge, are
questions which still await a solution.
The preparation of calcium tungstate for fluorescent screens is very
simple.
Commercial sodium tungstate is fused with dried calcium chloride in
the proportion of three parts of the former to two parts of the
latter, both constituents being in fine powder and well mixed
together. The fusion is conducted in a Fletcher's crucible furnace in
a clay crucible. The temperature is raised as rapidly as possible to
the highest point which the furnace will attain--i.e. a pure white
heat. At this temperature the mixture of salts becomes partly fluid,
or at least pasty, and the temperature may be kept at its highest
point for, say, a quarter of an hour. At the end of this time the
mass is poured and scraped on to a brick, and when cold is broken up
and boiled with a large excess of water to dissolve out all soluble
matter. The insoluble part, which consists of a gray shining powder,
is washed several times with hot water, and is finally dried on
filter paper in a water oven.
In order to prepare a screen the powder is ground slightly with very
dilute shellac varnish, and is then floated over a glass plate so as
to get an even covering. Unless the covering be very even the screen
is useless, and no pains should be spared to secure evenness. It is
not exactly easy to get a regular coat of the fluorescent material,
but it may be done with a little care.