Imagini ale paginilor
PDF
ePub

this problem appears to be due to Professor Müller, of Freiburg. According to him, we obtain a measure of the intensity of the glow by dividing the intensity of the galvanic current by the diameter of the wire; the current intensity being given by Ohm's law, we of course find that the effect will depend upon the number of elements in the galvanic battery, and the electro-motive force of each element; also upon the resistance of the wire and the battery. For the same battery acting on the same length of wire a maximum glow will be produced when the wire has a certain determinable diameter, and the intensity of the glow diminishes when the wire is either thicker or thinner than this. For instance, with six of Ruhmkorff's zinc and carbon elements acting on a platinum wire one decimeter long, the maximum glow is produced when the diameter of the wire is } of a millimeter; for a wire two decimeters long the thickness must be lo of a millimeter to produce the maximum effect. With a battery of two of Stohrer's elements a platinum wire, two decimeters long, can not be raised to a white-hot glow, but may be raised to a red heat when its diameter is 1} millimeters, or less. Again, in order to make red hot a platinum wire of 1 millimeter diameter and two meters long, a battery of 28 elements is necessary, while 40 such will not make this wire white hot.-Berichte d. Naturf. Gesell. Freiburg, VI., 2, 97.

THE MOLECULAR CONSTITUTION OF GASES AND LIQUIDS.

That the same substance at the same temperature and pressure can exist in two very different states, viz., as a liquid and as a gas, is a fact of the highest scientific importance, for it is by the careful study of the difference between these two states and the phenomena which occur at the surface which separates the liquid from its vapor that we may expect to obtain a dynamical theory of liquids. A dynamical theory of perfect gases is already in existence; that is to say, we can explain many properties of gaseous bodies by supposing their molecules to be in rapid motion, and that they act on one another only when they come very near or strike each other; but we can not extend this dynamic theory from the rater to the denser condition obtained by subjecting the gas to great pressure without at the same time obtaining some definite conception of the nature of the action .

[ocr errors][ocr errors][ocr errors]

that takes place between molecules when they are only for
an instant in close contact, which action, in fact, depends upon
the particular constitution of the encountering molecules.
The first contribution to a dynamic theory of liquids is made
by Maxwell in some comments on the labors of Vander-
waals, where he takes occasion to show that we have evi-
dence that the molecules of gases, besides encountering each
other in their motions, also attract each other at a certain
small distance, but when they are brought still nearer they
repel each other, a conclusion in accordance with Boscovich's
theory of atoms. On the other hand, the molecules of
liquids, or even these same gaseous molecules, when reduced
to the liquid condition, apparently repel each other at a cer-
tain small distance, which repulsive forces between contig.
uous molecules are overcome by the general attractions of
the mass of the body.-12 A, X., 479.

[merged small][ocr errors][ocr errors][merged small][ocr errors][ocr errors][merged small][merged small][ocr errors]

ON TIE REFLECTION OF SOUND FROM A LAYER OF FLAME

OR HEATED GAS,
Mr. Cottrell las observed the reflection of sound from a
coal-gas tlame in the following manner: Sonorous pulses sent
through an open tube agitate a sensitive flame placed at its
other end; but when a coal-gas flame is placed between
the end of the tube and the flame, the latter is no longer
affected by the sound sent through the former. He then
placed two tubes so that they were equally inclined to one
face of the gas flame, and, sending the sound into one of
these tubes, it was reflected from the fame, passed up the
other tube, and agitated a sensitive flame placed at its mouth,
In a similar manner he has shown that part of the sound is
reflected from the flame, and part is transmitted by the
flame, thus giving a complete analogy between the reflec-
tion of sound from a flame and the reflection of light from
a transparent plate. He obtained the same effects, as given
above, when he replaced the flame by the sheet of heated
gases rising from it.

[ocr errors]

TUE EVAPORATION OF METALS BY ELECTRICITY.
Mr. Hopkins describes an interesting experiment, which
consists in passing a charge of electricity through a very
fine thread of platinum, or other metallic foil, the thread

being kept in place between slides of microscope glass. The effect of the heat from the electric discharge is to vaporize the metal, which is instantly condensed in a transparent layer upon the cold glass, which can then be studied by the microscope, and can be used in various ways to determine the character of the metal and the peculiarities of the discharge.-12 A, X., 190.

STEAM FOG-WHISTLES. It has been found by General Duane, of the United States Engineers, in his experiments made to determine the best form of boilers for steam fog-signals, that as the steam used is at a high pressure, and is drawn off at intervals, there is a constant tendency to foam and throw out water with the steam. To counteract this, a horizontal tubular boiler, like those used in locomotives, is recommended by him. The steam-dome must be very large, and surmounted by a steampipe 12 inches in diameter. The steam should be drawn off at a point ten feet above the water level in the boiler. The diameter of the boiler whistle should be two thirds of its length, and the vertical distance of its lower edge above the coping, for a steam pressure of 50 pounds, should be from one third to one fourth of the diameter.- Elliot's Eu. ropean Light-house System, p. 25.

THE GAS GUN FOR FOG-SIGNALS. A very ingenious application has been made, by Mr. Wigham, of the explosive nature of a mixture of ordinary gas in air. He establishes at any point on the coast where a fog-signal is desired a gas gun. It is simply a tube of iron, connected with the gas-holder by the proper pipe; the holder, of course, may be at any convenient distance. The gas-holder is filled with a mixture of one fourth air, and the remainder coal-gas and oxygen, and this mixture is allowed to flow into the gas gun, when it may be fired off by touching a match to the proper orifice, taking care, of course, to close all communication with the holder. By using an electric spark, instead of the match, the service of the gun may be made still easier. The flash from this gun is said to illuminate the fog much better than that from a discharge of gunpowder.- Elliot's European Light-house System, p. 74.

[ocr errors][merged small]

n

0

[ocr errors]
[ocr errors][merged small][merged small]

Experiments made in England with gun-cotton in the open air are said to have demonstrated that a mass of ten ounces of compressed gun-cotton, fired by means of two ounces of dry gun-cotton, as a primer, the whole being detonated with fulminate of mercury, produced a discharge which could be heard very distinctly at a distance of ten niles in all directions. These results were so satisfactory that it has been determined to build a parabolic reflector of cast iron, by which the intensity of the sound of the explosion of a charge of compressed cotton placed in its focus will be greatly intensified in one direction. The trials of the adaptability of this device as a fog-signal will be made at the Royal Arsenal, Woolwich.— The Engineer.

[ocr errors][merged small][ocr errors][merged small][ocr errors][merged small][merged small][merged small]

NEW METHOD OF OBSERVING THE VIBRATIONS OF A TUNING

FORK.
A new method of determining the absolute number of vi-
brations corresponding to any musical note is described by
Poske, and has a high value in comparison with those that
have hitherto been employed, which may be classified as
graphic, acoustic, and optic: the first of these three is compar-
atively rough; the use of the siren is a good example of the
acoustic method, although its practicable employment is
found troublesome; and of the optical, that of Lissajons
is in high esteem. The new method proposed by Poske
consists, first, in replacing the clock or chronometer by the
electro-magnetic rotation apparatus of Helmholtz, whose ve-
locity of rotation is extremely constant, and can be deter-
mined accurately to its ten-thousandth part. The essential
portion of this apparatus consists in a centrifugal regu-
lator, which diminishes the strength of the electric current
by the diminution of the number of contacts, as soon as the
velocity of rotation exceeds a certain limit. The observer
examines, through a microscope, a minute bright point upon
the vibrating rod or cord, which point by its vibrations ap-
pears as a bright line; and between the eye and the vibrat-
ing point there also rotates a disk perforated with a known
number of slits. The combination of the revolving slits and
the vibrating point causes the latter to appear to move

[ocr errors][ocr errors][ocr errors][ocr errors]

slowly to and fro, in periods similar to the acoustic phenomena known as “beats.” It is evident that the number of vibrations of the point is determined by the velocity of rotation, the number of slits, and the duration of the beats, the accuracy of the method being very great.

In the application of his method of determining the time of vibration of a tuning-fork, Poske has also been able to show that the vibrations of the latter vary with the amplitude of the arc of vibration; that the durations diminish in a geometrical series as the amplitudes diminish ; and that, in general, the change in duration is proportional to the first power of the amplitude, and not, as in the pendulum, in proportion to the square of the amplitude.Poggendorff Annalen, CLII., 463.

THE ACTION OF ORGAN-PIPES. Mr. Hermann Smith states as the result of experimental studies that within an organ-pipe the “air reed” vibrates in arcs whose extent diminishes as we increase the speed of the reed, or that the times vary with the amplitude; and to this he adds the remarkable feature that the motion of vibration is an activity tempered by rests, and that the note of every open organ-pipe is not single, but a concord of two tones.—12 A, X., 162.

EFFECT ON SOUND AND LIGHT OF THE MOVEMENT OF THE

OBSERVER. The long-vexed question as to the effect, upon observations, of the movement of the obseryer, and the source of light or sound, has been elucidated by Baron Eotvos, of Pesth, who, in a recent communication, extends his former investigations, and offers a satisfactory refutation of several objections that have been raised. According to him, in case the source of sound or light be moving, the intensity must be defined as the living force that would fall, in a unit of time, upon a unit of surface, parallel to the wave surface, if all vibrations were like those which are imparted to the surface at that instant in which the intensity is to be determined. The formula for the intensity in question, as deduced by Eotvos, shows that the movement of the observer has a decided effect npon the result; and by applying this

« ÎnapoiContinuați »