Imagini ale paginilor

required in order to separate these plates through a given distance was inversely proportional to the force that was applied. It increased nearly as the inverse square of the original distance between the plates, and as the fourth power of the radius of the plates; and in whatever liquid he immersed his plates, the times were proportional to those required by equal volumes of these liquids to flow through capillary tubes.— 19 C, VIII., 60.


In some remarks on the dissipation of energy, Lord Rayleigh states that the chemical bearings of this subject are very important. A chemical transformation is impossible if its occurrence involves the opposite of dissipation; but it is not true that a transformation which involves dissipation must necessarily take place; otherwise the existence of explosives, like gunpowder, would be impossible. The possi bility of chemical action must often depend upon the density of the reacting substances. Thus, in the case of a mixture of oxygen and hydrogen in proper proportions at a certain density, the mixture may be exploded by an electric spark, and energy be dissipated; but beyond a certain point of rarity the explosion can not be made, as it could not then involve any dissipation. It may probably be found that many mixtures which show no tendency to explode under ordinary conditions, will become explosive when sufficiently condensed.


Magneto-electric machines have not yet attained that point of perfection which permits them to be placed among the industrial apparatus; but they at least offer a method of producing electricity very economically. The principal machines are those of Siemens and Halske, of the French Society of L'Alliance, and of Gramme. The currents produced in the last-named machines are analogous to those furnished by the galvanic battery, thus permitting its ap plication to all works where voltaic electricity is useful. The Gramme, as adapted for electro-plating in the silverworks of Christoffle, of Paris, has a height of four feet and a

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

length of two and a half feet, and the resulting electric current deposits eight hundred grammes of silver per hour, requiring a motive force of one horse-power. The experiments made at London by this machine give it the first rank among apparatus for producing electric light. A curious experiment was performed with one of these machines at the Exposition of Vienna. The principal magneto-electric machine being driven by an air-engine on the Lenoir system, the electricity thus produced was carried to the electro magnets of two Gramme machines of more feeble dimensions, which, acting as an electric motor, kept a small centrifugal pump in action. Thus the mechanical effect of the electricity was by the connecting wires carried to a great distance from the motor. The principal advantage of the transmission of force by electricity is found in the possibility of overcoming vertical spaces which are inaccessible to the cables or belts in use in manufacturing works. Bulletin Hebdomadaire, XVI., 8.


It is often very important in chemistry or in crystallography to be able to seize exactly the delicate forms of crystals, as examined by the microscope; but the most conscientious draughtsman can not always reproduce the fineness of these crystallizations. According to Girard, many crystals can be photographed. As a preliminary, it is necessary generally to dilute the solution to different degrees, and to allow one specimen of each kind of crystal to be formed by itself. We thus acquire the faculty of choosing that degree of concentration where the crystallographic characters are most appropriate to the methods of photography, either by reason of the delicate grouping of the forms, or by their transparency to the penetration of light. The most simple. method of photographing the crystals consists in the use of an ordinary microscope, the body of which is placed in a horizontal position. In a dark room upon a table near a window the microscope is placed, and the object is illuminated by a beam of light coming through the window from a heliostat without. The luminous rays pass through the thin film of water containing the crystallized salt, traverse

[ocr errors]

the objective of the microscope, and form a magnified image
upon a distant screen. At the proper moment we substitute
for the screen a sensitized photographic plate, and obtain a
photograph by the ordinary processes. A number of pict-
ures should be taken of the same object, in order to choose
among them that which presents the nature of the crystal-
line system under the best conditions. This precaution is
especially necessary in certain products, where the solution
gives results distinct from each other, according to the de-
gree of saturation.-13 B, III., 171.


Dr. Phipson states that water strongly acidified with hydrochloric acid, and containing a small quantity of benzol, when placed over a spirit-lamp enters into a violent ebullition every sixty seconds. After a while the boiling ceases completely, and then recommences suddenly every thirty seconds, for a considerable period. The flask still being kept over the spirit-lamp, the periods between quiescence and violent ebullition dropped to twenty, ten, and finally to eight seconds, at which intervals the phenomena continued for a considerable time.

When methyl alcohol was added to the above mixture of water, hydrochloric acid, and benzol, and the flask placed over a spirit-lamp, no ebullition at all occurred for a long period of time, and then it took place suddenly and continued. —1 A, April 23, 177.


From a number of experiments on the magnetism of iron and steel, Holtz concludes that galvano-plastic iron receives when made to glow by a galvanic current a greater specific gravity. The molecules are brought closer together, the intervals between them become smaller, and the permanent magnetic moment is diminished by one half. On the other hand, steel bars by being heated red-hot, and tempered, acquire a smaller specific gravity, the molecules are farther from each other than before, the intervals being greater, and the magnetic moment is notably increased. Again, Wiedeman has shown that permanent magnetism is diminished by

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

torsion, but temporary magnetism increases by detorsion. These effects can be explained by the same relation above found between density and the magnetic force of iron. The torsion increases the density of the iron, and the magnetism must therefore diminish. By detorsion, the molecules of the magnets are separated from each other, and the magnetism. itself increases. It can, then, be assumed as probable that the magnetic forces in general are functions of the spaces between the molecules, and dependent upon the dimensions of these spaces.—19 C, VIII., 151.


The study of the evolution of heat in chemical combinations is a new branch of science, belonging partly to physics and partly to chemistry, and the number of facts already observed is sufficiently numerous to indicate certain laws which are set forth by Berthelot. He premises that in the act of producing any chemical change, the molecules hit sharply against one another, and give off heat, just as when a hammer strikes a bar of iron. From the study of the relations between the amount of heat and the amount of work done, it is possible to establish some theorems of thermochemistry.-12 A, X., 473.


In a very suggestive article of Kohlrausch on thermoelectricity, he develops the hypothesis that currents of heat and of electricity are connected together in every conductor of heat, the heat being moved by an electric current whose heat-moving force is proportional to the electro-motive force of the heat current in the same body.-12 A, X., 278.


Professor Andrews, of Belfast, at the meeting of the British Association in 1874, made a further communication on the continuity of the liquid and gaseous states of matter. As the result of some of his more recent investigations, he stated that the compressibility of sulphurous liquids, unlike that of water, diminishes as the pressure increases. A mixture of three volumes of carbonic acid and four of nitrogen was sub

jected to a pressure of 300 atmospheres at various temperatures, from 20 to 48° C., with the very important result that, even at 20, the carbonic acid of such a mixture could not be liquefied under any pressure. Indeed, the "critical point" of carbonic acid proves to be lowered many degrees when that gas is mixed with a non-liquefiable gas, such as nitrogen.15 A, Aug. 29, 1874, 277.


Professor Osborne Reynolds, in a careful review of the remarkable observations recently made by Mr. Crookes, shows that, according to the kinetic theory of gases, whenever a molecule of liquid is evaporated, and becomes a molecule of gas, it must leave the liquid surface with a velocity equal to that with which the other particles of the gas are rebounding among themselves; that is to say, instead of being first detached, it must be shot off with a velocity greater than that of a cannon-ball, and there must be an equal reaction on the surface of the remaining liquid; the contrary effect takes place in the case of condensation. Applying the necessary mathematical formulæ, he finds that, at a temperature of 60°, the evaporation of one pound of water from a surface is sufficient to maintain a force of 65 foot-pounds for one second, the force being proportional to the square root of the absolute temperature. In the case of mercury the force is only 6 foot pounds instead of 65. And again, whenever heat is communicated from a hot solid surface to a gas, an effect similar to that of evaporation is produced, while for every English unit of heat communicated to steam, at a temperature of 60°, the reaction on the surface is equivalent to 0.38 of a pound acting for one second; but is, for air, 0.55 of a pound.-12 A, X., 175.


Since the galvanic effect upon metal wires, by means of which they are made to glow, has acquired a practical importance in galvano-caustics, it appears desirable to present, in the clearest manner, the connection between the power of the battery, the dimensions of the wire, and the resulting phenomenon; and the first attempt at a partial solution of

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][ocr errors][merged small][ocr errors]
« ÎnapoiContinuați »