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other. In order that the meteorologist may at any time
test the perfection of the vacuum within his tube, Mr. Hicks
has very ingeniously inserted two wires into the sides of
the bulb in such a way that a galvanic current applied to
the wires will, by the nature of the light that is spread
through the vacuum bulb, show with considerable accuracy
what proportion of gas, and especially of watery vapor, is
there present.

A pressure within the vacuum bulb exceed-
ing one tenth of an inch of mercury is not admissible if an
accnrate instrument is desired, and the vacuum can be easily
brought to within one fiftieth of an inch, in which condition
the radiation solar thermometers will prove strictly compa-
rable. Especially is it important that the bulb should be
filled with dry gas, and that not the slightest trace of moist-
ure should exist. Mr. Flicks said that, although he had made
hundreds of tubes with Torricellian vacua, he never knew
one to fail showing stratification and white light when the
tube was thoroughly clean and free from moisture.- Quar.
Jour. Metcor. Soc., II., April, 1874.

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THE TIIERMAL CONDUCTIVITY OF MERCURY.
Herwig has been continuing the inquiry previously insti-
tuted as to whether the thermal conductivity of mercury
varies with the temperature-a question of much moment in
connection with the reliability of the indications of the mer-
curial thermometer at different temperatures. He finds that
between 40° and 160° Centigrade the heat-conducting power
of pure mercury is perfectly constant. Ile is now occupied
in a series of experiments to show how far solid metals differ
in their behavior from mercury.-13 A, Feb. 27, 1875, 222.

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A NEW SOURCE OF ERROR WITH THE MERCURIAL THER

MOMETER.

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Mr. J. M. Morgan, in employing a mercurial thermometer in the operation of distillation, the instrument being iuserted into the apparatus to such a depth that the whole of the quicksilver thread was surrounded by heated vapors, observed after the operation had continued for several days that the temperature registered was too low by 3o. An examination showed that this error was due to the fact that a portion of the mercury had vaporized and condensed in

the upper part of the tube which was not exposed to the heat of the operation. When the portion condensed in the upper end of the tube was united, by cautious tapping, with the main column, the instrument again registered correctly. The observer of this phenomenon determined thereupon by experiment that a quantity of mercury corresponding to from 1° to 15o will be volatilized in the manner described if the mercury column of a thermometer is exposed for several days to a temperature of from 60° to 100° Centigrade. This observation is worthy of the special attention of experimenters, since the small quantity of mercury thus condensed in the upper end of the thermometer tube may be readily overlooked, and thus give rise to serious errors of observation.-Fresenius's Zeitsch. für Analyt. Chem., XIV., 81.

RELIABILITY OF SIEMENS'S PYROMETER. The Siemens pyrometer has been subject to a careful investigation by a committee of the British Association, in order to decide whether or not the resistance is altered after exposure to high temperatures. Four instruments were examined, three of which were found to be considerably altered after having been exposed to a high temperature; the fourth gave results showing it to be sufficient for industrial application, if not for strictly scientific observations.-12 Å, X., 373.

A NEW MERCURIAL MINIMUM AND MAXIMUM THERMOMETER.

Mi. Denton describes a maximum and minimum thermomcter combined in one, by which both registrations of temperature are obtained from one mercurial bulb, both indices are moved by the mercury pressing on their ends, and, independent of the self-registering feature, the actual temperature is shown, at any moment, by two separate columns of mercury. In the construction of the instrument the tube of the maximum thermometer is bent at the top and turned downward, and dips into an hermetically sealed chamber, which is itself more than half filled with mercury. An increase of temperature raises the index of the maximum thermometer and pushes down the mercury in the other leg of the tube. A diminution of temperature leaves the maximum index in its place, and allows the mercury in the other leg of the tube to rise, pushing up with it its index, which is, in its turn, left in its place where the maximum thermometer has passed. The graduations of the second or minimum stem of the thermometer are counted downward, and those of the maximum stem are counted upward. As constructed by Casella, it is said this instrument is extremely sensitive, convenient, and reliable.— Quar. Jour. Meteor. Soc. of London, 1875, II., 193.

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NEW SELF-RECORDING THERMOMETER. In constructing a thermometer in which the dilatation of the metal shall give the measure of the temperature of the air, Tremeschini states that he has endeavored to eliminate the inconvenience peculiar to the nature of glass by making use of a metallic band as an indicator of the temperature. In his thermometer, which he exhibited lately to the French Meteorological Society, he employs a band of copper slightly platinized in order to preserve it from oxidation. This band is nine centimeters long and seven millimeters broad, and has a thickness of one twentieth of a millimeter, and is therefore extremely sensitive to atmospheric temperature changes: it is coiled about a central axis, very much like the hairspring of a watch, and is contained within a case similar to that of an ordinary aneroid barometer. The temperature is read on the face of the thermometer by an index, which may even describe an entire circle in passing from – 40° to +100° Fahrenheit.-Nouv. Meteor., 1875, 14.

ON TUE EXPANSION OF INDIA RUBBER BY HEAT. According to the studies of Schmulewitsch, based in part on the studies of Puschl and Exner, as well as his own experiments, the somewhat anomalous behavior of caoutchouc under the influence of heat may be expressed by the following four propositions: First, caoutchouc is a body whose density is a minimum at a certain temperature. Second, this minimum temperature changes with the mechanical extension, being lower the more the body is extended by the application of some external force. Third, in the case of caoutchouc unexposed to any strain, the temperature of the minimum density is higher than ordinary temperatures, but approaches the latter by heating; its co-efficient of expansion is positive, but diminishes with increasing temperature. Fourth, in the case of strongly extended caoutchouc, the temperature at which its density is a minimum is lower than ordinary temperatures; its co-efficient of expansion is therefore negative at the latter temperature, and increases numerically with the temperature.—19 C, VIII., 146.

ON THE MOLECULAR HEATS OF SIMILAR COMPOUNDS. Professor F. W. Clarke states that as the result of an extensive comparison between the molecular heats of similar compounds, he finds that these have equal values, not at the same temperature, but at what are called corresponding temperatures, which are at equal or nearly equal distances from the respective melting points.Bull. Phil. Soc. Washington, June, 1874.

ON THE REPULSION DUE TO HEAT,

In his reply to the criticism of Professor Reynolds, Professor Crookes states that abundant observations which have been accumulated by himn during some years appear in every way to contradict the theory that the phenomena observed by him are due either to air-currents existing within vacuum tubes or to electrical phenomena. As to the theory of Professor Reynolds, that the effects are the results of evaporation and condensation, he satisfactorily shows that while this explanation might sometimes be admissible, yet in general it requires the adoption of assumptions that seem to be wholly at variance with the facts. He concludes by stating his belief that the repulsion observed by him as accompanying the radiation of heat and light is directly due to the impact of the waves upon the surface of the moving mass, and is not a secondary effect through the intervention of aircurrents, or electricity, condensation, etc. Whether the athereal waves actually strike the object moved, or whether at the boundary of the surface, solid or gaseous, there are intermediate layers of condensed gas which, taking up the blow, pass it on to the layer beneath, are problems the solution of which must be left to further research ; and, without insisting upon any theory of his own, he proposes it merely as a useful working hypothesis. Any theory will account for some facts, but only the true explanation will satisfy al

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the conditions of the problem, and this can not be said of
either of the theories which have thus far been discussed.
To quote the eloquent language of Sir Humphrey Davy,
“When I consider the variety of theories which may be form-
ed on the basis of one or two facts, I am convinced it is the
business of the true philosopher to avoid them altogether.
It is more laborious to accumulate facts than to reason con-
cerning them; but one good experiment is of more value
than the ingenuity of a brain like Newton's.”—7 A, XLVIII.,
94.

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THE SPECIFIC JIEAT AND CUBIC EXPANSION OF BODIES.

Mr. Walter Spring communicates to the Royal Academy of Belgium the following note with reference to the specific heat of bodies. He states that he sought to determine whether there were any relation between the specific heats and the co-efficients of cubic expansion by heat. He arrives at very beautiful results, both practically and theoretically: For instance, the computations which he makes of the specific heats of mercury and of graphite agree to the fourth decimal place with the observations of Regnault, Dulong, and Petit. He concludes that the product obtained by multiplying the specific heat of any body by its atomic weight can not be constant, since the specific heat is itself a function of a variable factor.- Bulletin of the Royal Academy of Belgium, 1874, 294.

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LIGHTNING-CONDUCTORS.
In a few remarks upon the action of lightning-conductors,
Secchi, the well-known astronomer, describes the storm of
November, 1872, in which the cathedral and palace of Alatri
were struck by lightning, these structures having been free
from such visitations for many years. The damage done on
this occasion was, as he shows, due in great measure to the
fact that the lightning-rods, instead of being directly con-
nected with the metallic gutters and other portions of the
roof, were isolated from them. The fluid, therefore, sought
to make its own way to such other good conductors as were
near. After quoting other instances, he expressed the opin-
ion that the conditions most favorable to safety consist in
joining the lightning-rod directly to all the metallic portions

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