PROPERTIES--_Continued_
Glass of new and most useful qualities . . . Metals plastic under
pressure . . . Non-conductors of heat . . . Norwegian cooking box
. . . Aladdin oven . . . Matter seems to remember . . . Feeble
influences become strong in time.
Jena Glass.
As in the case of the aluminium bronzes and nickel steels, alloys of the
utmost value have been formed by introducing new ingredients, often in
little more than traces, or by modifying but slightly the proportions in
which ingredients long familiar have been mingled together. An equal
gain has followed upon varying anew the composition of glass. For
centuries the only materials added to sand for its melting pot were
silicic acid, potash, soda, lead-oxide, and lime. As optical research
grew more exacting the question arose, Will new ingredients give us
lenses of better qualities? First of all came the demand for glasses
which combined in lenses would yield images in the telescope and
microscope free from color. In a simple lens, such as that of an
ordinary reading glass, we can readily observe the production of color
by a common beam of light. The rays of different colors, which make up
white light, are refrangible in different degrees, so that while the
violet rays come to a focus near the lens, the red rays have their focus
farther off; the images, therefore, instead of being sharply defined,
are surrounded by faint colored rings. In a telescope or microscope a
simple lens would be of no value from the indistinctness of its images.
To correct this dispersion of color a second lens of opposite action is
placed behind the first, that is, a crown-glass lens is added to a
flint-glass lens. (See cut, p. 255.) This remedy is not quite perfect
for the reason that the distribution of the spectrum from violet to red
varies with each kind of glass, and in such a way that through failure
of correspondence, color to color, in a compound lens, variegated
fringes of light, though faint, are perceptible, much to the annoyance
of the microscopist, the astronomer, and the photographer.
With a view to producing glasses which united in compound lenses should
be color free, Rev. Vernon Harcourt, an English clergyman, in 1834 began
experiments which he continued for twenty-five years. By using boron and
titanium in addition to ordinary ingredients of glass, he produced
lenses less troubled by color than any that had before been made. His
labors, only in part successful, were in 1881 followed by those of
Professor Ernst Abbe and Dr. Otto Schott at Jena. With resources
provided by the Government of Prussia, these investigators were able to
do more for the science and art of glass-making than all the workers who
stood between them and the first melters of sand and soda. They
immensely diversified the ingredients employed, carefully noting the
behavior of each new glass, how much light it absorbed, how it behaved
in damp air, what strength it had, how it retained its original
qualities during months of keeping, and in particular how variously
colored rays were distributed throughout its field of dispersion. As in
the blending of new alloys it was found that many of these novel
combinations were useless. Of the scores of new glasses produced some
were extremely brittle, others were easily tarnished by air, or so soft
as to refuse to be shaped as prisms or ground as lenses. A more
systematic plan of experiment was therefore adopted: for the production
of new glasses there were by degrees separately introduced in varied
quantities, carefully measured, boron, phosphorus, lithium, magnesium,
zinc, cadmium, barium, strontium, aluminium, berylium, iron, manganese,
cerium, didymium, erbium, silver, mercury, thallium, bismuth, antimony,
arsenic, molybdenum, niobium, tungsten, tin, titanium, fluorine,
uranium. An early and cardinal discovery was that the relation between
refraction and dispersion may be varied almost at will. For example,
boron lengthens the red end of the spectrum relatively to the blue;
while fluorine, potassium, and sodium have the opposite effect. With the
distribution of the diverse hues of the spectrum thus brought under
control, there were produced glasses which, when united as compound
lenses, were almost perfectly color-free, rendering images with a new
sharpness of definition. Yet more: in their unceasing round of
experiments Professor Abbe and Dr. Schott came upon glass so little
absorbent of light that combinations of much thickness intercepted only
a small fraction of a beam; they were indeed almost perfectly
transparent. This achievement is of great importance to the
photographer, whose planar combination of six lenses may be four inches
in thickness. At Jena the researchers are endeavoring to perfect another
gift for the camera: they seek to produce glasses each transmitting but
one color, for service in color-photography.
To microscopy they have recently given lenses which completely transmit
ultra-violet rays so as to photograph the diffraction discs of objects,
such as gold particles in colloidal solutions, otherwise invisible,
because below the resolving power of the most powerful microscope. It is
estimated that with this new aid an object but 1/250,000,000 of a
millimeter in length may indirectly be brought to view.
One ancient art, that of annealing glass, Professor Abbe and Dr. Schott
greatly improved, eliminating from their products the stresses which
distort an image. By means of an automatic heat-regulator, the
temperature of a batch of glass could be kept steadily for any desired
period at any point between 350° and 477° C.; or allowed to fall
uniformly at any prescribed rate. The glass was usually contained in a
very thick cylindrical copper vessel, on which played a large gas flame.
The highest temperature found necessary to banish stress, that is, to
cause softening to begin, was 465° C. The lowest temperature required to
ensure complete hardening was about 370° C. Thus the temperatures of
solidification all lie between 370° and 465°. This fall of 95° was
spread over an interval of four weeks, instead of a few days as
formerly, with the result that stress was banished utterly.
[Illustration: Photograph by Bräunlich & Tesch.
THE LATE PROFESSOR ERNEST ABBE, OF JENA.]
A practical example of the benefits gained in the properties of Jena
glass is exhibited by its use in measuring heat. A thermometer of common
glass when first manufactured may tell the truth, and in a month or two
may vary from truth so much as to be worthless. The reason is that the
dimensions of the glass slowly change day by day, as in a less degree
do those of many alloys. It was one of the aims of the Jena laboratory
to produce a glass which should remain constant in its dimensions while
exposed to varying temperatures, so that, made into thermometers, it
would be thoroughly trustworthy. Here, too, success was attained, so
that thermometers of Jena glass are found to be reliable as are no
instruments of ordinary glass. This product is available for
astronomical lenses, otherwise liable to serious changes of form as
exposed successively to warmth and cold.
Heat was to be staunchly withstood not only in moderate variations, but
in extreme degrees. From time immemorial heat suddenly applied to glass
has riven it in pieces. Could art dismiss this ancient fault? To-day a
beaker from Jena may be filled with ice and placed with safety on a gas
flame. In its many varieties this glass furnishes the chemist with
clean, transparent and untarnishing vessels for the delicate tasks of
the laboratory, all of singular indifference to heat and cold. Yet
again. Special kinds of this glass in chemical uses are attacked by cold
or hot corrosive liquids only one-twelfth to one-fourth as much as good
Bohemian glass, the next best material.
Not only to heat but to light Jena glass renders a service. Glass of
ordinary kinds when used for the tubes of a Hewitt mercury-vapor lamp,
absorbs a considerable part of the ultra-violet rays upon which
photography chiefly depends. A Jena glass free from this fault is formed
into Uviol lamps of great value in taking photographs, photo-copying,
and photo-engraving. These lamps are also employed in ascertaining the
comparative stability of inks and artificial dyes; so intense is their
action that brief periods suffice for the tests. Uviol rays severely
irritate the eyes and skin; they may prove useful in treating skin
diseases. They moreover quickly destroy germs. In all these activities
reminding us of radium.
Thus by a bold departure from traditional methods in glass-making, the
eye receives aid from lenses more powerful and more nearly true than
ever before swept the canopy of heaven, or peered into the structure of
minutest life. Meanwhile instruments of measurement take on a new
accuracy and retain it as long as they last. All this while a material
invaluable for its transparency is redeemed from brittleness and
corrodibility, and given a strength all but metallic; at the same time
transmitting light with none of the usual subtraction from its beams.
[Illustration: Bliss forming die. A, bed plate. B, blank-holder. C,
drawing punch. D, push-out plate. O, P, annular pressure surfaces.]
[Illustration: Bliss process of shell making.]
Power Presses in Metal Working.
From glass let us now turn to metals. It is their tenacity that chiefly
gives them value; this tenacity is usually accompanied by a hardness
which disposes us to regard nickel, for example, as of a solidity quite
unyielding. But the coins in our pockets prove that under the pressure
of minting machinery they are as impressible as wax. In molds and dies,
each the counterpart of the other, brass, bronze, iron, steel, and
tin-plate take desired forms as readily as if paste. Solid though these
metals appear they yield under severe stress with a semi-fluid quality.
We have long had stamped kitchen ware, baking pans, and the like; the
principle of their manufacture has of late years been extended to ware
of more importance. Bliss power presses are to-day turning out hundreds
of articles which until recently were either slowly hammered or spun
into form, pieced with solder, or shaped by the gear cutter or the
milling machine. These presses furnish the United States Navy with
sharp-pointed projectiles, some of them so large as to demand a million
pounds pressure for their production; they make strong seamless drawn
bottles, cylindrical tanks for compressed air and other gases, and cream
separators able to withstand the bursting tendency of extremely swift
rotation.
[Illustration: Mandolin pressed in aluminium.]
[Illustration: Pressed Seamless pitcher.]
[Illustration: Barrel of pressed steel.]
Presses less powerful produce scores of parts for sewing machines,
typewriters, cash registers, bicycles, and so on; or, at a blow, strike
out a gong from a disc of bronze. Presses of another kind stamp out cans
in great variety, and even a mandolin frame in all its irregular curves.
Tubs are quickly pressed from sheets of metal; a pair of such tubs,
tightly joined at their rims by a double seam, form a barrel impervious
to oil or other liquid, and hence preferable to a wooden barrel. A press
operated by a double crank may be arranged to supersede the forging of
hammers, axes, and mattocks. Another press at a blow cuts out the front
for a steel range. Still another press invades the foundry, producing
excellent gear wheels for trolley cars, not weakened by being cut from a
casting across the grain of the metal. Sometimes the article
manufactured requires a series of operations, as in the case of a kettle
cover with its knob. At the Lalance & Grosjean factory, Woodhaven, New
York, a Bliss press makes such covers in a single continuous round.
Another press treats soft alloys, so that a disc one inch in diameter
when hit by a plunger is forced into the shape of a tube suitable to
hold paint or oil.
In large manufactures as in small the hydraulic forge has wrought a
quiet revolution. If a steel freight car were produced by planing,
turning, slotting and similar machines, it would be much heavier and
dearer than as turned out to-day from ingeniously fashioned dies under
severe pressure. Its girders are molded of the same strength throughout
with no waste of material, and without rivets; corner pieces are
avoided; stiffeners are built up from the plates themselves through the
introduction of ridges and depressions: and in a structure having the
fewest possible parts, uniform strength is attained because dimensions
everywhere may freely depart from uniformity.
[Illustration: Range front pressed from sheet steel.]
[Illustration: Pressed paint tube and cover.]
Non-Conductors of Heat.
In a vast manufactory of steel cars, of steel structural forms, steam
has to be conveyed long distances from the boilers. Here, as in similar
huge establishments, or in the heating of towns and cities from central
stations, it is desirable to lose as little heat as possible by the way,
for undue waste means enormous inroads upon profits. There are other
reasons for wishing to keep heat within a steam pipe; much damage may be
done to fruit, flour and other merchandise unduly warmed. Furthermore
there is a risk of setting fire to woodwork, paper, cotton and the like;
it has been observed that after a month’s exposure to heat from
steampipes, wood takes fire at a temperature which at first would not
have led to ignition, because then the wood contained a little moisture.
To guard against loss and danger it has long been the practice to cover
steampipes with jackets of non-conducting material, such as
mineral-wool,--furnace-slag blown into short glassy fibres by a sharp
blast of air. Felt, loosely folded, also serves well. Many advertised
claims for asbestos are not well founded; this mineral is incombustible
and is therefore useful in thick curtains to separate a stage from the
auditorium of a theatre. But it is a fairly good conductor, and for
steampipes should be used as a direct covering of the metal simply to
keep an outer and much thicker coat of felt from being charred. Whatever
the material chiefly employed, one point is clearly brought out by
experiment, namely, that the air detained by the fibres of a covering
greatly aids in obstructing the passage of heat. Hence it is well to
keep the materials from becoming compacted together, as do ashes when
moistened. Asbestos fibres, which are smooth and glassy, do not take
hold of air as do cork and wool.
Professor J. M. Ordway, of the Massachusetts Institute of Technology,
Boston, tells us that non-conductors should be of materials that are
abundant and cheap; clean and inodorous; light and easy to apply; not
liable to become compacted by jarring or to change by long keeping; not
attractive to insects or mice; not likely to scorch, char or ignite at
the long-continued highest temperature to which they may be exposed; not
liable to spontaneous combustion when partly soaked in oil; not prone to
attract moisture from the air; not capable of exerting chemical action
on the surfaces they touch. No material combines all these desirable
qualities, but a considerable range of substances fulfil most of the
requirements.
Tests of steam-pipe coverings at Sibley College, Cornell University, and
at Michigan University, have resulted as follows:--
Relative
Amount
Kind of Covering of Heat
Transmitted
Naked pipe 100.
Two layers asbestos pipe, 1 inch hair felt, canvas cover 15.2
The same, wrapped with manila paper 15.
Two layers asbestos paper, 1 inch hair felt 17.
Hair felt sectional covering, asbestos lined 18.6
One thickness asbestos board 59.4
Four thicknesses asbestos paper 50.3
Two layers asbestos paper 77.7
Wool felt, asbestos lined 23.1
Wool felt with air spaces, asbestos lined 19.7
Wool felt, plaster paris lined 25.9
Asbestos molded, mixed with plaster paris 31.8
Asbestos felted, pure long fibre 20.1
Asbestos and sponge 18.8
Asbestos and wool felt 20.8
Magnesia, molded, applied in plastic condition 22.4
Magnesia, sectional 18.8
Mineral wool, sectional 19.3
Rock wool, fibrous 20.3
Rock wool, felted 20.9
Fossil meal, molded, 3/4 inch thick 29.7
In general the thickness of the coverings tested was one inch. Some
tests were made with coverings of different thicknesses, from which it
would appear that the gain in insulating power obtained by increasing
the thickness is very slight compared with the increase in cost.[19]
[19] Rolla C. Carpenter, “Heating and Ventilating Buildings,” p.
229. New York, John Wiley & Sons, 1905.
Some properties of matter seem to have family ties. Tenacity and
conductivity for heat, as an example, go together; all the tenacious
metals as a group are conducting as well. Conversely, the
non-conductors,--felt, gypsum, and the rest, are structurally weak. If
the inventor could lay hands on a material able to withstand high
pressure and, at the same time, carry off wastefully but little heat, he
would build with it cylinders for steam engines much more economical
than those of to-day He would also give cooking apparatus of all kinds a
covering which would conduce to the health and comfort of the cook,
while, at the same time, heat would be economized to the utmost. One of
the advantages of electric heat is that it can be readily introduced
into kettles and chafing dishes surrounded by excellent non-conductors;
the result is an efficiency of about ninety-five per cent., quite
unapproached in the operations of a common stove or range.
Norwegian Cooking Box.
The costliness of electric heat forbids the housekeeper from using much
of it. Her main source of heat must long continue to be the common
fuels. These, however, thanks to cheap non-conductors, may be used with
much more economy and comfort than of old. Take, for example, the
Norwegian cooking box, steadily gaining favor in Europe and well worthy
of popularity in America. It consists of a box, preferably cubical, made
of closely fitted thick boards, with a lid which fits tightly. Box and
lid are thickly lined with felt or woolen cloth, and filled with hay
except where pots are placed. These pots, filled with the materials for
a soup, a stew, a ragout, are brought to a boil on a fire and then
placed within the box, its lid being then fastened down. For two hours
or so the cooking process goes on with no further application of heat.
To be sure the temperature has fallen a little, but it is still high
enough to complete the preparation of a wholesome and palatable dish,
with economy of fuel and labor, without unduly heating the kitchen.
[Illustration: Norwegian cooker.]
Aladdin oven.
On the same principle is the Aladdin oven, invented by the late Edward
Atkinson of Boston, and manufactured by the Aladdin Oven Company,
Brookline, Mass. It is built of iron, surrounded with air cell asbestos
board, so as to maintain a cooking temperature of 400° Fahr. with little
fuel or attention. Its drop door when open forms a shelf, when closed it
is fastened by a brass eccentric catch, ensuring tightness; its wooden
stand has an iron top to hold the oven firmly in place. This apparatus
cooks a wide range of dishes admirably, retaining the natural flavors of
meats, fish, vegetables and fruits as ordinary excessive temperatures
never do. Mr. Atkinson wrote “The Science of Nutrition,” which sets
forth the construction and uses of this oven.[20]
[20] Published by Damrell & Upham, Boston. $1.00.
[Illustration: Aladdin Oven.]
Matter Impressed by Its History.
Every property of matter seems universal. The best non-conductor of heat
transmits a little heat; the best conductor is by no means perfect: the
two classes of substances are joined by materials which gradually
approach one end of the scale or the other. Nothing is so hard but that
it may be indented or engraved, and where neither a blow nor severe
pressure is employed, we may have, as in the photographic plate, an
impression which is chemical instead of mechanical, displaying itself to
the eye only when treated with a suitable developer. A bar of steel
hammered on an anvil is changed in properties; as it becomes closer in
texture its tenacity is increased. When that bar takes its place in a
structure, the work it has to do, the shocks it bears, equally tell upon
its fibres. Stresses and strains leave their effects upon the stoutest
machines, engines, bridges; they are never the same afterward as
before, and usually their experience does them harm. Says an eminent
engineer, Mr. W. Anderson: “The constant recurrence of stresses, even
those within the elastic limit, causes changes in the arrangement of the
particles which slowly alter their properties. In this way pieces of
machinery, which theoretically were abundantly strong for the work they
had to do, have after a time failed. The effect is intensified if the
stress is suddenly applied, as in the case of armor plate, or in the
wheels of a locomotive. . . . When considerable masses of metal have
been forged, or severely pressed while heated, the subsequent cooling of
the mass imposes restrictions on the free movement of some if not all
the particles, hence internal stresses are developed which slowly assert
themselves and often cause unexpected failures. In the manufacture of
dies for coinage, of chilled rollers, of shot and shell hardened in an
unequal manner, spontaneous fractures take place without apparent cause,
through constrained molecular motion of the inner particles gradually
extending the motion of the outer ones until a break occurs.”
Sir Benjamin Baker says:--“Many engineers ignore the fact that a bar of
iron may be broken in two ways--by a single application of a heavy
stress, or by the repeated application of a comparatively light stress.
An athlete’s muscles have often been likened to a bar of iron, but if
‘fatigue’ be in question, the simile is very wide of the truth.
Intermittent action, the alternative pull and thrust of the rower, or of
the laborer turning a winch, is what the muscle likes and the bar
abhors. A long time ago Braithwaite correctly attributed the failure of
girders, carrying a large brewery vat, to the vessel being sometimes
full and sometimes empty, the repeated deflection, although
imperceptibly slow and free from vibration, deteriorating the metal,
until in the course of years it broke. These girders were of cast iron,
but it was equally well known that wrought iron was similarly affected,
for Nasmyth afterward called attention to the fact that the alternate
strain in axles rendered them weak and brittle, and suggested annealing
as a remedy, having found that an axle which would snap with one blow
when worn, would bear eighteen blows when new or just after annealing.
We know that the toughest wire can be broken if bent backward and
forward at a sharp angle; perhaps only to locomotive and marine
engineers does it appear that the same result will follow in time even
when the bending is so slight as to be unseen by the eye. A locomotive
crank-axle bends but 1/34 inch, and a straight driving axle but 1/64,
under the heaviest bending stresses to which they are exposed, and yet
their life is limited. Experience proves that a very moderate stress
alternating from tension to compression, if repeated about a hundred
million times, will cause fracture as surely as bending to a sharp angle
repeated a few hundred times.”
Hence an axle, or other structure, should be tested by just such
stresses as it is to withstand in practice. A steel bar may
satisfactorily pass a tensile test applied in one direction, only to
break down disastrously under alternating stresses each less severe.
Magnetization.
That matter virtually remembers its impressions is plain when we study
magnetism. Steel when magnetized for the first time does not behave as
when magnetized afterward. It is as if magnetism at its first onset
threw aside barriers which never again stood in its way. If the steel is
to be brought to its original state it must be melted and recast, or
raised to a white heat for a long time. In quite other fields of
channeled motion we remark that violins take on a richer sonority with
age; their fibres, under the player’s hand, seem to fall into such lines
as better lend themselves to musical expression.
In 1878 the late Professor Alfred M. Mayer of the Stevens Institute of
Technology, Hoboken, New Jersey, published a series of remarkable
experiments in the “American Journal of Science.” He there told and
pictured how he had magnetized several small steel needles, thrust
through bits of cork set afloat in water, the south pole of each needle
being upward. As the needles repelled each other, or had their repulsion
somewhat overcome by a large magnet held above them with its north pole
downward, the needles disposed themselves symmetrically in outlines of
great interest, which varied, of course, with the number of needles
afloat at any one time. Three needles formed an equilateral triangle,
four made up a square, five disposed themselves either as a pentagon or
as a square with one magnet at its centre, and so on in a series of
regular combinations, all suggesting that magnetic forces may underlie
the structure of crystals.
[Illustration: Mayer’s floating magnets.]
The Crystal Foreshadows the Plant.
One of the remarkable attributes of a crystal is its ability to grow and
act as a unit, as if it had a life of its own, despite the evident
variety and great number of its parts. Take a crystal of alum, break off
a corner and then immerse the broken mass in its mother liquor; at once
the crystal will repair itself, new molecules building themselves into
its structure as if they knew where to go. This unity of effect may be
observed during a northern winter on a scale much more striking. In cold
weather on a large sheet of plate glass exposed as a window, a frost
pattern will extend itself as if a tree, beautiful branches spreading
themselves from a main stem which may be seven feet in height. It is
altogether probable that polar forces, such as we observe in the magnet,
are here at work. Their harmony of effect, in spaces comparatively vast,
is astonishing. Forces of allied character rise to a plane yet higher in
vegetation, culminating in the magnificent sequoia of California, whose
life, measured by thousands of years, goes back almost to the dawn of
human civilization. The union of tools, levers, wheels, as an organized
machine; the co-ordination in research of the parts to be played by
observers, recorders, depicters, generalizers; the regimentation of
soldiers, so that all march, advance and fire as one man under the
control of a single will, is prefigured in the forces which make a unit
of every crystal of saltpetre in a soldier’s cartridge-box. Of all the
characteristics of matter none is more pervasive and more marvelous than
its ability to form a unit which moves and acts as if no part were
separable from any other, while manifesting a highly complicated
structure, with functions at once intricate and co-ordinate.
[Illustration: A
Alum crystal.
B
After a part has been broken off.
C
Restored by immersion in alum solution.
From photographs by Herr Hugo Schmidt, Hackley School, Tarrytown, N. Y.]
During Long Periods Minute Influences Become Telling.
Qualities of matter, much more simple, may now engage our attention.
First, then, let us note how minute influences, acting for long
stretches of time, may change the qualities of metals and rocks. Forces,
too slight for measurement as yet, are known in the course of a year or
two to affect steel at times favorably, at other times unfavorably. The
highest grades of tool-steel are improved by being kept in stock for a
considerable time, the longer the better. It seems that bayonets,
swords, and guns are liable to changes which may account for failure
under sudden thrust or strain. Gauges of tool steel, which are required
to be hard in the extreme, are finished to their standard sizes a year
or two after the hardening process. Slow molecular changes register
themselves in altered dimensions. In the Bureau of Standards at
Washington are a yard in steel and a yard in brass, at first identical
in length; after twenty years they were found to vary by the 1/5000 of
an inch. Take another case, familiar enough to the railroad engineer: in
a mine, or a tunnel, the roof or wall may tumble down a month or more
after a blasting. The stone which fell immediately upon the explosion
was far from representing all the work done by the dynamite. A stress
was set up in large areas of rock and this at last, beginning in slight
cracks, overcame the cohesion of masses of huge extent.
[Illustration: Iron tube enclosing marble before and after deformation.]
[Illustration: Marble before deformation and after.]
Properties undergo change during the simple flight of time: a parallel
diversity is worthy of remark. A substance exhibits quite diverse
qualities according to whether the action upon it is slow or speedy. A
paraffine candle protruding horizontally half way out of a box, during a
New York summer will at last point directly downward, for all its
brittleness. If shoemaker’s wax is struck a sudden blow, it breaks into
bits as might a pane of window glass. But place leaden balls on the
surface of this same wax and in the course of ten or twelve weeks you
will find them sunk to the bottom of the mass. When sharply smitten, the
wax is rigid and brittle; to a long continued, moderate pressure the
wax proves plastic, semi-fluid almost. All this is repeated when stone
is subjected to severe pressure for as long a period as two months. At
McGill University, Montreal, a small cylinder of marble thus treated by
Professor Frank D. Adams became of bulging form, without fracture, but
with a reduction in tensile strength of one-half. When the pressure was
applied during but ninety minutes the tensile strength of the resulting
mass was but one-third that presented by the original marble; when the
experiment occupied but ten minutes the tenacity fell to somewhat less
than one-fourth its first degree. These researches shed light on the
stratifications of rocks often folded under extreme pressure as if
rubber or paste.
Take another and quite different example of how variations in time bring
about wide contrasts of result: a rubber ball thrown in play at a wall
rebounds; send it forth from a cannon, with a hundred-fold this
velocity, and it pierces the wall as might a shot of steel.