OBSERVATIONS ON THE THEORY OF CROOKES' RADIOMEeter. Professors Dewar and Tait, in the prosecution of their researches into the movements of Crookes' radiometer, have devised methods of producing very perfect exhaustion of air from a receiver; the most perfect vacuum being obtained by taking advantage of the absorbent power of charcoal. The movement of the radiometer, whose disks are of rock-salt, are traced to the unequal heating of the movable parts of the apparatus; and the explanation of the phenomena observed by them is deduced from the kinetic theory of gaseous pressure. Proceedings Royal Society of Edinburgh, VIII.,

628.

THE MECHANICAL PRODUCTION OF COLD.

Mr. A. C. Kirk has offered a memoir on the mechanical production of cold to the Royal Institution of Civil Engineers, which attracted extended discussion, and will probably lead to material improvements in many of the mechanical arts. He states that his attention was first drawn to the subject by noticing the inconvenience experienced at certain paraffin-oil works, where it was customary to extract the solid paraffin in winter by exposing the material to a low temperature sufficient to cause the paraffin to crystallize. At those works chemical methods of producing low temperatures had been introduced in order to avoid the otherwise expensive loss of time. These methods were too objectionable to be continued long, and the author was requested, as engineer to the works, to examine the methods invented by Dr. Gorrie, who had constructed a machine that was said to have produced ice in Florida. Mr. Kirk's early experiments with machines similar to those of Dr. Gorrie having been unsatisfactory, attention was turned to an air-engine, the reversal of whose processes it was thought ought to make a good cooling-machine; and, in fact, after many modifications and reconstructions, a degree of cold was produced by it sufficient to freeze mercury. This machine may be in the main described as follows: A steam-engine moves a piston backward and forward, by which the confined air is in a state of alternate compression and expansion. While the air is compressed in one space, the heat generated thereby is

removed by a stream of water flowing through properly arranged pipes; and while the air is expanding and cooling in the other space, heat is abstracted from whatever has been placed in a second properly arranged receptacle. The only limit to the temperature attainable is the conducting power of the material of which the apparatus is composed. The air contained in the pistons may be at any pressure that the apparatus can withstand. The machines built for cooling large masses of water for the use of breweries, etc., are usually worked at from 100 to 120 pounds per square inch, the efficiency and capacity of the machine increasing with increased. pressure. To keep up this pressure and meet leakage, a small compressing pump draws air through two boxes containing chloride of calcium. This is necessary to dry the air, for if the air be damp the moisture is deposited as snow in certain parts of the apparatus, and causes mischievous obstructions.

A NEW ICE CALORIMETER.

The ice calorimeter in all its forms measures quantities of heat by determining how much ice that heat can melt. In Bunsen's apparatus advantage is taken of the change of volume which ice undergoes upon fusion. This change occurs at one end of a column of mercury sustained in a capillary tube: as the ice melts the mercury descends, and the amount of fall represents the quantity of heat. But the use of a capillary tube in this apparatus affects the delicacy of all meas urements made with it. So Schüller and V. Wartha propose a modification of Bunsen's calorimeter, in which graduated scales are done away with, the amount of mercury involved in any measurement made with the instrument being determined by direct weight. As the ice in the apparatus melts and the column of mercury falls additional mercury flows in through a peculiar suction tube, and the weight thus absorbed affords a measure of the heat. The details of the new calorimeter are not very complicated, but need the author's diagram for their explanation. The inventors tried to apply their instrument to determining the specific heat of some metallic titanium, and obtained a value apparently too high. Seeking the cause of their error, they found that the supposed metal contained nitrogen, being really a compound of the formula TiN2.-35 C, September, 1875.

EXPERIMENTS UPON NON-LUMINOUS FLAMES.

The experiments by Knapp, and the subsequent ones of Blochmann, in which a luminous gas flame was rendered non-luminous by the introduction of gases indifferent to combustion, in place of air, have been modified by Wibel in such a way as to permit the heating of the mixed gases at the moment of combustion. The diluting gas was introduced through a glass tube, soldered into one of the draughtholes of the ordinary Bunsen burner, the others being closed, and the mixed gases were burned from a platinum tube, readily formed by rolling a piece of thin platinum foil spirally, slipped into the top of the burner. Two opposite horizontal Bunsen burners were arranged for heating this tube. Upon heating it when the flame was rendered non-luminous by air, as in the ordinary Bunsen burner, or by carbonic acid, nitrogen, or hydrogen, Wibel noticed the immediate formation of a luminous cone in the interior of the flame, which gradually disappeared when the heating burners were removed. The temperature required was not very high, especially if the gases were mixed in the proper proportions. The flame did not strike down, and in all respects resembled an ordinary luminous flame, affording soot, and also a continuous spectrum. Very slight decomposition took place in the hot tube with nitrogen and carbonic acid, as ascertained by analysis of gas taken from the flame by aspiration, as well as from the exceedingly slight deposit of carbon in the tube. A sheet-iron plate, perforated so as to fit the burner, prevented the products of combustion from the heating burners from coating the flame. It was also found that when the flames of two good Bunsen burners were brought in contact with the exterior mantle of flame, rendered non-luminous by carbonic acid, an interior luminous cone was formed, which disappeared immediately on their removal. He concludes from his experiments that the non-luminous character of the flame in Knapp's experiment is not due to dilution of the burning gas, according to the views of Frankland and of Blochmann, but rather to a cooling of the interior of the flame by the gas introduced a view that is supported by the peculiarities of the flame when oxygen is the diluting gas. The luminosity of the flame of a substance containing carbon, other things

being equal, depends upon the existence of a temperature in the interior which renders possible the processes of chemical decomposition and combination, from which the luminous. body proper results. In ordinary cases the more or less cold gases formed from the illuminating material rushing up from beneath are heated sufficiently by the temperature developed in the exterior zone of combustion to produce the change necessary to luminosity.-35 C, IV., 1875, 220.

INFLUENCE OF PRESSURE ON COMBUSTION.

Some interesting observations have been made by Cailletet in burning different substances under pressure. He finds that pressure slightly augments the temperature at which combustion occurs, and that the luminous and actinic rays. emitted by the burning body are considerably intensified. When a candle is made the subject of experiment, the base of the flame, ordinarily bluish and transparent, becomes white and very luminous. Soon, however, clouds of smoke are formed, due to incomplete combustion. Under similar circumstances the flame of phosphorus is not sensibly aug mented in brilliancy, but sulphur, potassium, alcohol, and carbon disulphide burn much more vividly than in free air. -Annales de Chimie et de Physique, November, 1875.

THE ABSORPTION OF SOLAR HEAT BY THE ATMOSPHERE. The discussions of the observations of the black-bulb-invacuum thermometer, made in England during the past five years, have led Mr. Stowe to the following conclusions in reference to the influence on solar radiation of the aqueous vapor in the atmosphere. First, when the tension of vapor is small, the radiation is greater than the average. Second, the north and northwest winds, which contain little vapor, show a greater intensity of solar radiation than the south and southeast winds. Third, the hourly changes, due to the va rying altitude of the sun above the horizon, are well marked, and allow the approximate determination of the solar radiation as unaffected by the absorption in the atmosphere; this latter varying from a minimum of ten per cent. to a maximum of twenty per cent. A change in the elevation of the station, from 470 up to 1800 feet, diminishes the absorption by five per cent.-7 C, II., 57.

THE MATHEMATICAL THEORY OF LIGHT.

An exhaustive memoir on the interference of light, by Professor Lommel, is contained in the Proceedings of the Physical Society of Erlangen, in which he brings the phenomena of thick and thin plates of glass and of curved surfaces into one point of view. Some of the diagrams given by him remind one strongly of those multiplex curves treated of by Professor Newton in the Connecticut Academy of Sciences for 1874.-Sitzb. Physikal.-Med. Gesells., Erlangen, 1875, 106.

MATHEMATICAL THEORY OF LIGHT.

The connection between the elliptic polarization of light reflected from mirrors, and the refraction and dispersion of light when passing through transparent media, is developed in a very elaborate manner by Ketteler, who deduces a complete theory from the consideration of the expression A+B√-1, which expression Fresnel met with in his investigation of the subject of total reflection. The occurrence of this expression, to which the term "complexe" is given by the French and German mathematicians, is shown by Ketteler to result from the fact that the wave of light reflected from any surface may be considered as a complex wave, consisting of a superposition of two partial waves differing from each other by one quarter of a wave-length. The interpretation which Fresnel himself seems to have given is submitted to a rigid demonstration. Ketteler states that we are led more and more to the conviction that in all dioptrical phenomena we have to do not so much with a special constitution of the optical ether as with the synchronous vibration of the atoms of ether, and those of material bodies; and that, in fact, the ether itself may have the same inertia and density within as without the transparent body. His memoir is especially devoted to the development of the idea that there is a vibration of the ponderable atoms corresponding to the vibrations of the ether.-Verhandl. Naturhist. Vereins, Bonn, XXXII., 1 to 224.

DEEP-SEA SOUNDING BY PHOTOGRAPHY.

Dr. Neumayer has presented to the Geographical Society of Berlin a remarkable photographic apparatus for determin