At a recent meeting of the Engineers' Club of Philadelphia, Mr. James
Christie presented a paper upon "The Adaptation of Steel to Structural
Work." The price of steel has now fallen so low, as compared with iron,
that its increased use will be actively stimulated as the building
industries revive. The grades and properties of the steels are so
distinct and various that opinions differ much as to the adaptability of
each grade for a special purpose. Hitherto, engineers have favored open
hearth steel on account of uniformity, but recent results obtained from
Bessemer steel tend to place either make on equality. The seeming
tendency is to specify what the physical properties shall be, and not how
the steel shall be made.
For boiler and ship plates, the mildest and most ductile steel is
favored. For ships' frames and beams, a harder steel, up to 75,000 pounds
tenacity, is frequently used. For tension members of bridges, steel of
65,000 to 75,000 pounds tenacity is usually specified; and for
compression members, 80,000 to 90,000 pounds. In the Forth Bridge,
compression steel is limited to 75,000 to 82,000 pounds. Such a marked
advantage occurs from the use of high tension steel in compression
members, and the danger of sudden failure of a properly made strut is so
little, that future practice will favor the use of hard steel in
compression, unless the material should prove untrustworthy. In columns,
even as long as forty diameters, steel of 90,000 pounds tenacity will
exceed the mildest steel 35 per cent., or iron 50 per cent., in
compressive resistance.
The present uncertainty consists largely as to how high-tension steel
will endure the manipulation usual with iron without injury. A few
experiments were recently made by the writer on riveted struts of both
mild and hard steel, which had been punched, straightened, and riveted,
as usual with iron, but no indication of deterioration was found.
Steel castings are now made entirely trustworthy for tensile working
stresses of 10,000 to 15,000 pounds per square inch. In some portable
machinery, an intermittent tensile stress is applied of 15,000 pounds,
sometimes rising to 20,000 pounds per square inch of section, without any
evidence of weakness.
* * * * *
Equal volumes of amyl alcohol (rectified fusel oil) and pure concentrated
hydrochloric acid, shaken together in a test tube, unite to form a single
colorless liquid; if one volume of benzine (from petroleum) be added to
this, and the tube well shaken, the contents will soon separate into
_three_ distinct colorless fluids, the planes of demarkation being
clearly discernible by transmitted light. Drop into the tube a particle
of "acid magenta;" after again shaking the liquids together, the lower
two zones will present different shades of red, while the supernatant
hydrocarbon will remain without color.
* * * * *
A METHOD OF MEASURING THE ABSOLUTE SENSITIVENESS OF PHOTOGRAPHIC DRY
PLATES.
[Footnote: From the Proceedings of the Academy of Arts and
Sciences.--_Amer. Jour._]
By WILLIAM H. PICKERING.
Within the last few years the subject of dry plate photography has
Increased very rapidly, not only in general popularity, but also in
importance in regard to its applications to other departments of science.
Numerous plate manufacturers have sprung up in this country as well as
abroad, and each naturally claims all the good qualities for his own
plates. It therefore seemed desirable that some tests should be made
which would determine definitely the validity of these claims, and that
they should be made in such a manner that other persons using instruments
similarly constructed would be able to obtain the same results.
Perhaps the most important tests needed are in regard to the
sensitiveness of the plates. Most plate makers use the wet plates as
their standard, giving the sensitiveness of the dry plates at from two to
sixty times greater; but as wet plates vary quite as much as dry ones,
depending on the collodion, condition of the bath, etc., this system is
very unsatisfactory. Another method, employed largely in England, depends
on the use of the Warnerke sensitometer. In this instrument the light
from a tablet coated with luminous paint just after being exposed to a
magnesium light is permitted to shine through a colored transparent film
of graduated density upon the plate to be tested. Each degree on the film
has a number, and, after a given exposure, the last number photographed
on the plate represents the sensitiveness on an empirical scale. There
are two or three objections to this instrument. In the first place, the
light-giving power of the luminous tablet is liable to variations, and,
if left in a warm, moist place, it rapidly deteriorates. Again, it has
been shown by Captain Abney that plates sensitized by iodides, bromides,
and chlorides, which may be equally sensitive to white light, are not
equally affected by the light emitted by the paint; the bromides being
the most rapidly darkened, the chlorides next, and the iodides least of
all. The instrument is therefore applicable only to testing plates
sensitized with the same salts.
In this investigation it was first shown that the plates most sensitive
for one colored light were not necessarily the most so for light of
another color. Therefore it was evident that the sun must be used as the
ultimate source of light, and it was concluded to employ the light
reflected from the sky near the zenith as the direct source. But as this
would vary in brilliancy from day to day, it was necessary to use some
method which would avoid the employment of an absolute standard of light.
It is evident that we may escape the use of this troublesome standard, if
we can obtain some material which has a perfectly uniform sensitiveness;
for we may then state the sensitiveness of our plates in terms of this
substance, regardless of the brilliancy of our source. The first material
tried was white filter paper, salted and sensitized in a standard
solution of silver nitrate. This was afterward replaced by powdered
silver chloride, chemically pure, which was found to be much more
sensitive than that made from the commercial chemicals. This powder is
spread out in a thin layer, in a long paper cell, on a strip of glass.
The cell measures one centimeter broad by ten in length. Over this is
laid a sheet of tissue paper, and above that a narrow strip of black
paper, so arranged so as to cover the chloride for its full length and
half its breadth. These two pieces of paper are pasted on to the under
side of a narrow strip of glass which is placed on top of the paper cell.
The apparatus in which the exposures are made consists of a box a little
over a meter in length, closed at the top by a board, in which is a
circular aperture 15'8 cm. in diameter. Over this board may be placed a
cover, in the center of which is a hole 0.05 cm. in diameter, which
therefore lets through 0.00001 as much light as the full aperture. The
silver chloride is placed a distance of just one meter from the larger
aperture, and over it is placed the photographic scale, which might be
made of tinted gelatines, or, as in the present case, constructed of long
strips of tissue paper, of varying widths, and arranged like a flight of
steps; so that the light passing through one side of the scale traverses
nine strips of paper, while that through the other side traverses only
one strip. Each strip cuts off about one-sixth of the light passing
through it, so that, taking the middle strip as unity, the strips on
either side taken in order will transmit approximately--
1 2 3 4 5 6 7 8 9
2.0 1.65 1.4 1.2 1.0 0.85 0.7 0.6 0.5
The instrument is now pointed toward the zenith for about eight minutes,
on a day when there is a bright blue sky. On taking the apparatus into
the dark room and viewing the impression by gaslight, it will be found
that the markings, which are quite clear at one end, have entirely faded
out by the time the middle division is reached. The last division clearly
marked is noted. Five strips cut from sensitized glass plates, ten
centimeters long and two and a half in width, are now placed side by side
under the scale, in the place of the chloride. By this means we can test,
if we wish, five different kinds of plates at once. The cover of the
sensitometer containing the 0.05cm. hole is put on, and the plates
exposed to sky light for a time varying anywhere between twenty seconds
and three minutes, depending on the sensitiveness of the plates. The
instrument is then removed to the dark room, and the plates developed by
immersing them all at once in a solution consisting of four parts
potassium oxalate and one part ferrous sulphate. After ten minutes they
are removed, fixed, and dried. Their readings are then noted, and
compared with those obtained with the silver chloride. The chloride
experiment is again performed as soon as the plates have been removed,
and the first result confirmed. With some plates it is necessary to make
two or three trials before the right exposure can be found; but if the
image disappears anywhere between the second and eighth divisions, a
satisfactory result may be obtained.
The plates were also tested using gaslight instead of daylight. In this
case an Argand burner was employed burning five cubic feet of gas per
hour. A diaphragm 1 cm. in diameter was placed close to the glass
chimney, and the chloride was placed at 10 cm. distance, and exposed to
the light coming from the brightest part of the flame, for ten hours.
This produced an impression as far as the third division of the scale.
The plates were exposed in the sensitometer as usual, except that it was
found convenient in several cases to use a larger stop, measuring 0.316
cm. in diameter.
The following table gives the absolute sensitiveness of several of the
best known kinds of American and foreign plates, when developed with
oxalate, in terms of pure silver chloride taken as a standard. As the
numbers would be very large, however, if the chloride were taken as a
unit, it was thought better to give them in even hundred thousands.