zymotic and other germs whose presence is necessary to the development of epidemic diseases. In one experiment Frankland placed a solution of lithium chloride in a shallow basin, acidulated it with hydrochloric acid, and dropped in fragments of white marble. The effervescence carried off the lithium particles, and colored strongly the flame of a Bunsen burner held at the upper end of a paper tube five inches in diameter and five feet long, held vertically above the basin. A tin tube three inches wide and twelve feet long was placed above this, and the burner held over it, with the same result. The paper tube was then lengthened to nine and a half feet, and the amount of lithium present in the current seemed to be quite as great as before. The author concludes, 1st, that fresh sewage, through a properly constructed sewer, is not likely to be attended by the suspension of zymotic matters in the air of the sewer; 2d, that if the sewage be allowed to stagnate, the evolution of gas results, and the breaking of gas bubbles on the surface projects liquid particles into the air, and is a potent cause of the suspension of zymotic particles in the air of the sewer; and, 3d, that it is of the greatest importance that foul liquids should pass freely and quickly through sewers and drain-pipes.

Stoney has called attention to the erroneous conception ordinarily entertained of a vacuum. He assumes as probable that in a cubic millimeter of any gas at the ordinary temperature and pressure there is a "unit-eighteen" of molecules (1,000,000,000,000,000,000), and consequently asserts that in every cubic millimeter of the best vacuums of our air-pumps there remains a unit-fifteen " of molecules (1,000,000,000,000,000). Even in the so-called Sprengel vacuum, as indicated by one-tenth of a millimeter of mercury on the gauge, there is a "unit-fourteen" of molecules (100,000,000,000,000), one hundred million million, in every cubic millimeter.

Wagner has modified his form of apparatus for determining the densities of gases by their times of effusion through minute openings in metal plates. In place of a straight cylinder, closed at top by the plate and open at bottom, which was plunged into a cylinder of water or mercury, he now uses a long U tube, closed at one extremity by the perfo

rated plate, and below which is a three-way cock with a lateral tubulure, through which the gas is introduced. Two marks on the tube are the points between which the effusion time is noted, the gas being forced out through the plate by a water or mercury column in the open leg of the tube. The results obtained with the instrument were accurate for coalgas and oxygen, but varied widely for hydrogen.

Kraevitsch has proposed an improvement in the construction of the barometer which increases indefinitely the sensibility of this instrument. To the shorter leg of a siphon barometer is attached a long horizontal capillary tube terminating in an open cylinder, the space above the mercury and the capillary tube being filled with water free of air. Obviously, if the barometer rises or falls, a quantity of water is displaced by the mercury equal to the volume representing the change in height. If now a bubble of air be introduced into the capillary tube, it will be displaced by an amount equal to the change in the barometric height, multiplied by the ratio of the two sections-in Kraevitsch's instrument by 140; thus rendering it extraordinarily delicate. The bubble when observed by a microscope of low power is rarely in repose.

An extended posthumous paper upon the constants of aneroid barometers and upon those aneroids which have scales attached for measuring heights, by Professor Jelinek, of Vienna, has appeared. It contains a complete résumé of previous results obtained by various observers.

ACOUSTICS.

Lord Rayleigh has experimented to ascertain the maximum limit of the amplitude of sound-waves, using for this purpose a whistle mounted on a Wolfe's bottle, furnished with a manometer. It was found that the most suitable pressure was 9 centimeters of water, and that under these conditions the sound could be distinctly heard at 820 meters' distance. The amount of air passing through the whistle was found to be 196 cubic centimeters per second. From these data the required amplitude may be readily calculated. The result shows that the amplitude of vibration of the aerial particles was less than the ten-mill

ionth of a centimeter. Indeed, the author is inclined to think that, on a still night, a sound of this pitch (fiv), whose amplitude is only a hundred-millionth of a centimeter, would still be audible.

Mach has devised an apparatus for studying the sonorous waves produced by an explosion. The ball from a pistol perforates two disks of paper which close the ends of a long box, the walls of which are formed of glass smoked with lamp-black. The aerial waves produced by the two successive ruptures of the paper produce on the glass interference bands, by which the velocity of the ball may be calculated. The results are always lower than those given by the ballistic pendulum, and are brought into accord with these only by assigning 500 meters per second as the velocity of sound. It thus appears that the velocity with which sound travels increases with the suddenness of its production.

Mercadier has further studied the laws of the vibrations of tuning-forks, considering especially their isochronism with varying amplitudes. Three methods were used: in the first, the amplitude was maintained constant during each experiment, but was lessened from one experiment to another, the vibrations being recorded on a rotating cylinder; in the second, a large amplitude of vibration was given and then suffered to die out, the vibrations per second at various times being noted; in the third, a Lissajous curve was inspected as the amplitude of the fork lessened. The author concludes, 1st, that the duration of the vibration period of forks varies with the amplitude and in the same direction; 2d, that this variation, even for amplitudes as great as one centimeter, is small, affecting only the second decimal place; and, 3d, that if a certain limit, say four millimeters, be not surpassed, the duration of the period may be regarded as

constant.

The same physicist has published a description of a new form of apparatus for showing optically the resultants of the combination of two rectangular vibrations, by means of which any desired difference of period and of phase can be obtained and maintained. The apparatus consists of a heavy fork vibrating by means of an electro-magnet, and having heavy sliders so as to vary the rate an entire oc

tave. A small movable weight upon one leg, which can be adjusted while the fork is vibrating, and a mirror on the other, complete the apparatus. A second fork, without adjustment, but having a mirror and an electro-magnet, acts conjointly with the first one to produce the curves.

Decharme has investigated the pitch which bars of various metals and alloys of exactly the same size yield when vibrated transversely. The rods were twenty centimeters long and one centimeter in diameter, and they were supported at their nodal point, i. e., four centimeters from the ends, upon prisms of cork, and struck with a wooden hammer covered with India-rubber. The pitch varied widely: lead gave only 690 single vibrations per second; while gold gave 970; silver, 1034.6; tin, 1161.3; zinc, 1422; copper, 1642.3; cast iron, 1843.6; wrought iron, 2192.2; steel, 2322.6; and aluminum, 2762. There is thus an interval of two octaves from lead to aluminum. From the data thus given the author calculates the coefficients of elasticity of these metals, which agree very well with those obtained by Wertheim.

Lootens has studied the phenomenon of air-motion in organ-pipes. By means of little pith propellers he has shown the existence in the pipe of cyclonic currents rising on one side of the pipe and falling on the other, the air producing them being that portion of the current which enters the pipe. If the pipe does not speak, this portion mixes with the other portion of the air by which the pipe is blown; but if it does speak, this cyclonic current does not mix with the other one, but takes a direction on issuing notably more inclined. These results being directly connected with the vibration of the walls of the pipe, the author concludes that this intermittent current, whose vibrations are determined by the walls of the pipe, acts the part of the perforated plate of a siren.

Ridout has described a simple burner for obtaining a very sensitive flame at feeble gas pressures. A tube five inches long and five-eighths inch wide is closed at one end by a perforated cork, through which slides a piece of tube oneeighth inch wide and six inches long, having the inner end drawn to a jet one-sixteenth inch wide. The inner tube is pushed up, the gas issuing from it lighted, and the tube slowly drawn down. A long steady flame is obtained which

is quite sensitive. By arranging two such jets, and connecting the gas-tubes with a horizontal tube in which is a drop of water, any difference of pressure is readily shown; lighting one jet causes motion towards the other side, as also does shortening the flame by noise.

Barrett has given in Nature a description of a flame extremely sensitive to entirely inaudible sounds. The flame came from an ordinary steatite burner, having an aperture of 0.04 inch in diameter, the gas being under a pressure of ten inches of water. This flame, which was two feet high, fell fully sixteen inches at every inaudible puff of a Galton whistle, and this even at the distance of fifty feet from the instrument.

A beautiful acoustic experiment by Tylor has been described in Nature, in which atmospheric vibrations are received on a soap film instead of a membrane. The end of a lamp-chimney is dipped into the ordinary bubble solution, and a film is formed over the opening. On singing near the open end, the series of forms belonging to the various notes. become plainly visible in the film, and on reflecting the calcium light to a screen by the film, the figures come out on the screen with great beauty. If the solution be thin, the film is almost devoid of color; but if thick, a gorgeous scenic effect is produced by the masses of prismatic color whirled hither and thither by the musical vibrations.

Jeannel has observed that the radiometer is influenced by sound vibrations. In a dim light, when three radiometers were placed on the sounding-board of a parlor organ, all moved, two in the direction produced by light, the other in the opposite. He explains the result by the transmission of the vibrations mechanically to the vanes.

HEAT.

1. Thermometry.

Hervé Mangon has contrived a new registering thermometer of extreme sensibility and delicacy. A capillary tube containing mercury is bent to a narrow rectangle at one end, and is drawn out to a fine point at the other. This thermometer is inverted, and the point dips into a small dish of mercury on the scale-pan of a delicate balance. On the