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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 pictures should be taken of the same object, in order to choose among them that which presents the nature of the crystalline 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 degree of saturation.-13 B, III., 171.

EBULLITION PHENOMENA.

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.

ON THE RELATION BETWEEN SPECIFIC GRAVITY AND MAG

NETISM OF IRON.

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

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.

RELATIONS BETWEEN CHEMISTRY AND THERMOTICS.

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.

RELATIONS OF HEAT AND ELECTRICITY.

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.

CONTINUITY OF THE LIQUID AND GASEOUS STATES OF

MATTER.

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 2° 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.

THE FORCES DEVELOPED BY EVAPORATION AND CONDENSATION.

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.

THE HEAT PRODUCED BY GALVANIC CURRENTS.

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

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 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 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 rarer 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.

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 Vanderwaals, where he takes occasion to show that we have evidence 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 certain small distance, which repulsive forces between contiguous molecules are overcome by the general attractions of the mass of the body.-12 4, X., 479.

ON THE REFLECTION OF SOUND FROM A LAYER OF FLAME OR HEATED GAS.

Mr. Cottrell has observed the reflection of sound from a coal-gas flame 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 flame, 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 reflection 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.

THE 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

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