passed through a layer of 10 millionths of a millimetre in thickness, but not through one twice as thick. A layer of silver 16 millionths of a millimetre in thickness would be equivalent in weight to a layer of 0.12 millimetre of air at atmospheric pressure.

tricity exists in definite units or atoms called electrons, but whether there are corresponding positive electrons or not is not known. It has often been the custom to regard positive electricity as distributea uniformly throughout a spherical volume of very large dimension as compared with the negative electron.

RADIUM HALOS. When a particle of radium remains enclosed in glass or in some forms of Professor Sir J. J. Thomson has approached mica for a considerable time it causes a dis- the question of the positive electricity in the coloration of the surrounding material. Ex- atom in a very ingenious manner. He amination of this "pleochroic halo" has shown showed that the mean probable deviation from that the color effect extends out from the its path experienced by a B particle in passing radium particle for a distance just equal to through an atom of matter would depend on that which the a particle can travel in the the distribution of the electric charges in the material before losing its ionizing power. Cor- atom. In particular he compared the deviation responding to the emission of a particles with calculated on the hypothesis that the positive different velocities and ranges of ionization by electricity in the atom is localized in definite different radioactive products, there are found units with that calculated on the hypothesis of halos of different sizes, and often multiple a uniform volume distribution of positive elechalos, the radius of each darkened portion cor- tricity throughout the atom. The theory was responding to the range of the a particle put to experimental test by J. A. Crowther, which produced it. The amount of radium whose results favor the hypothesis of uniform associated with, and producing, one of these halos is estimated at about 10-16 grams. Forming thus a very sensitive indication of the action of a particles at any time, the phenomenon may prove of considerable value in the investigations of radio-mineralogy.

VELOCITY AND IONIZATION. Dr. H. Geiger has made careful measurements of the change in velocity and in ionization produced by an a particle at points along its path. He found the velocity to be proportional to the cube root of the remaining range of ionization instead of the square root as formerly believed. The results agree well with the earlier ones of Rutherford except near the end of the range. The ionization produced at any point by an a particle is proportional to the rate at which it loses energy, which, in turn, inversely varies as the velocity, so that the ionization is inversely proportional to the velocity, or to the cube root of the remaining range. This explains the increase in ionization observed near the end of the range.

Careful measurements sustain the view (previously adopted on rather insufficient evidence) that all they particles from a single radioactive product have the same initial velocity, although when passing through air they acquire small differences in velocity and direction because of collision with the molecules.

DEVIATION OF PARTICLES. The scattering of a particles by matter has also been investigated by Geiger. Placing various thicknesses of metal foil in the path of a small parallel beam of a particles he observed the scintillation pattern produced by these particles on a zinc sulphide screen beyond the foil: As thicker foils were used the mean distance of the scintillations from the axis of the beam increased. By careful observation of the scintillation pattern he was able to determine the relation between the scattering of the beam and the nature and thickness of the material traversed. These results he was able to state in terms of the most probable angular deviation experienced by an a particle in passing through an atom. Thus a single atom of gold produces an angular deviation of about one two-hundredth of a degree. The deviation is proportional to the atomic weight of the metal.

distribution.

Thomson's theory with the experimental results obtained by Crowther also affords an opportunity to compute the number of negative electrons in the atom. The number was found to be three times the atomic weight.

ABSORPTION OF RAYS. Investigating the absorption of homogeneous B rays, Crowther found that, after the initial scattering, the particles were absorbed according to an exponential law, that is the number of particles stopped in any thin layer of the material was proportional to the number incident upon it. The deviations from the exponential law, observed with thin layers of absorbing material, were shown to be due to the scattering.

It has been shown that there is a small but appreciable decrease in velocity of the B particles in passing thin matter. Crowther found, for example, that a certain beam of B rays decreased in velocity from 2.735 X 1010 centimetres per second to 2.690 X 1010 centimetres per second in passing through 0.47 millimetres of aluminum. Allowing for the decrease in the mass of the 6 particles as their velocity decreases, this would amount to a loss of only 10 per cent. of their energy, although 71 per cent. of the rays were "absorbed " in passing through this sheet of aluminum. This shows that the main cause of the absorption of the B rays does not, as in the case of the a rays, lie in the decrease in their velocity.

Although the matter has not been very decisively settled, it seems most probable that the B rays emitted by a single radioactive product have all the same velocity, and that the results in opposition to this view obtained by some observers are probably due to the scattering of the rays by air before they reached the apparatus used to determine their ve locity.

ELECTRON. The elementary electric charge (e) carried by an electron, which is supposed to be the ultimate limit of divisibility of elec trical quantity, is one of the most important physical constants. A knowledge of this constant makes possible the computation of the actual masses of atoms and molecules, the number of molecules in unit volume of any gas, or in a given mass of any substance, and other imPOSITIVE ELECTRICITY. The nature of posi- portant physical quantities. Determinations tive electricity is still an open question. We of its value made in recent years have shown are well enough assured that negative elec- that the earlier values of about 3 X 10-10

PHYSICS

electrostatic units is probably too low, and that it is more nearly 4.6 X 10-10.

deep-seated in the metal. Investigations on the positive ions emitted by the alkali sulphates furnish additional evidence that the positive ions are due to the presence of alkaline impurities in the metals.

Early in the year 1910 Professor R. A. Millikan published the results of a series of meas. urements of the elementary charge. In a chamber containing air saturated with water or al THE MAGNETIC RAYS. It was observed many cohol vapor, and ionized by the radiation from years ago that when a discharge tube was a large quantity of radium, a cloud of fine drops placed longitudinally in a magnetic field the was produced by a sudden expansion of the cathode rays were bent in a helix, but when the saturated air. The water (or alcohol) drops magnetic field was increased beyond a certain formed about the ions in the air and so behaved value this helix changed to a continuous band like fairly large charged spheres. It was usually of light, and the potential across the tube possible to fix the attention on an individual changed abruptly. Attempts which were made drop and determine which electric field would to detect any electric charge carried by this just prevent it from falling under the action of radiation failed. The radiation was found to gravity. From this field strength, and the be deflected by an electric field in a direction velocity with which the drop fell when the field was removed, using the results of careful determinations of the viscosity of moist air, they were able to compute the electric charge carried by the drop. The charges carried by the many drops observed were all found to be multiples of the same number, so that it was at once evident how many elementary charges each drop carried. The result of this work was given as 4.65 X 10-10 electrostatic units of quantity, which agreed very well with the best results previously obtained.

In later work Professor Millikan has used drops of oil, mercury and glycerine, and has been able to keep a single drop in the field of view for an indefinite length of time. Changes in the motion of the drop indicating that it had taken up additional charges were observed, and from the measurements made each charge could be determined. It was found that the charges thus taken from the ions in the air were usually of the magnitude of a single elementary charge, although sometimes multiple charges were caught. Furthermore, every charge detected on any of the drops was within one-half of one per cent. of some multiple of the same quantity, 4.917 X 10-10 electrostatic units. This then is the value of the elementary charge as found from Millikan's later experiments.

From the fact that an ion present in the air could join one of the drops bearing a charge of the same sign, which would therefore exercise an electrical repulsion upon it, Millikan computed that the ion must have been in motion with a kinetic energy of about 5 X 10-14 ergs, which agrees with the value computed from the kinetic theory of gases.

perpendicular to the lines of electric force, although radiations of charged particles are ordinarily deflected along the lines of force. These considerations led to the belief that this was some new form of radiation, and various explanations of it were offered.

H. Thirkill has shown that this phenomenon may be explained as the effect of cathode rays, coiled up into a very close helix by the action of the magnetic field. The sudden appearance of the band of light when the magnetic field is increased is due to the fact that the cathode rays are concentrated by the field, increasing the current density in the strongest part of the field, and, as Thomson has shown, such an increase in current density is often accompanied by an abrupt increase in the luminosity. As the luminosity is accompanied by increase in ionization the difficulty found in detecting the negative charge accompanying the rays is explained by the increased leakage of charge from the metal cylinder. The negative charge carried by the rays Thirkill has detected (using a very sensitive galvanometre, and taking precautions against this leaking away of the charge). The unusual direction of deflection in the electric field can also be explained on this theory. It is thus concluded that the magnetic rays are slowly moving negative particles whose paths are coiled up into helices by the strong magnetic field along which they travel.

LIGHT PRESSURE. The pressure exerted by light on solid reflecting and absorbing surfaces was proven experimentally several years ago by Lebedew, Nichols and Hull, and others. During the past year Lebedew has experimentally demonstrated the pressure exerted by light on a partially absorbing gas. This pressure is proportional to the coefficient of absorption of the gas for the light used, and to the amount of radiant energy falling on the gas per second. Several different gases were used, and in all cases the pressures measured agreed with those theoretically computed within less than 30 per cent., which, considering the difficulties of the experimental work, is as good an agreement as could be expected. It may be noted that the highest pressure observed was about 4 microdynes (approximately 4 X 10-9 grams) per square centimetre.

NATURE OF THE THERMIONS. The charges carried by the ions emitted by hot bodies (thermions) have been further investigated by Professor O. W. Richardson and Mr. Hurlbirt. In earlier work it had been found that the negative ions had the same characteristics as the electrons, as was expected. It was also found that the positive ions from platinum, carbon, and iron had a value of e/m (the ratio of the electrical charge carried by the ion to its mass) corresponding to an atomic weight of about 26 if they carried the elementary charge. Seven other metals and three alloys recently investigated also produce positive ions having Theoretically, when a beam of light is very nearly the same value of e/m. This emitted there should be a backward pressure makes it seem probable that it is sodium exerted upon the source (the "recoil from atoms, present in the metal as an impurity, light," from analogy with the recoil of a gun which carry the positive charge. However, as which "emits" a projectile). The measuresevere chemical treatment of the metals did not ment of this recoil in the case of a body in cause any change in the phenomenon, it is cer- which heat is developed by some internal tain that any impurity which accounts for the change (for example, by electrical heating or carriers of the positive charge must be very by combustion) is beset with many experi

mental difficulties and uncertainties. But when radiation falls on an absorbing body, that body is heated by it and itself emits radiation in the form of very long heat waves, sending out, in the steady (final) state, as much energy as it receives. The back pressure exerted by this heat radiation will combine with the direct pressure due to the incident radiation, and the resultant can be measured. The pressures on discs with reflecting or absorbing surfaces and light incident on one side or the other can be theoretically calculated. If P is the pressure exerted by the incident light on an absorbing disc, it can be shown that for a disc which is perfectly absorbing on both sides the resultant pressure will be P; if black on receiving side but polished on the other, 5/3 P; if polished on receiving side, or on both sides, 2 P. Poynting and Barlow have measured the pressures on various discs, and found them in good agree ment with the values calculated from the amount of incident energy and the character of the discs investigated. Thus the "backward pressure" of radiation on the source may be considered as experimentally proven.

tribution of the energy and the groove-form can be experimentally confirmed. Further experiments with the use of various groove-forms are in progress.

LONG HEAT WAVES. The fact that by repeated reflection from a selectively reflecting surface nearly monochromatic radiation can be obtained has for several years been used to locate certain lines in the infra-red. During the past year Rubens and Hollnagel have obtained in this way the "residual rays (after multiple reflection) from rock salt, sylvine, potassium bromide, and potassium iodide. Their measurements of the wave-length were made with an interferometer of the Fabry-Perot type, with quartz plates. Wave-lengths longer than any before obtained in the infra-red were measured, the longest being those of potassium iodide, whose "residual rays have a wave-length of 96.7u, or nearly one-tenth of a millimetre. This brings the known region of heat radiation a little nearer to that of short electrical waves in which radiation of about 2 millimetres wave-length has been obtained. On the other hand, the lowest wave-lengths yet obtained in the ultra-violet are about 0.12, about one eight-hundredth as long as these new waves in the infra-red.

THE " ECHELETTE GRATING. The diffraction grating, consisting of a great number of very narrow grooves ruled very close together on a THE LIGHT OF THE FIREFLY. Coblentz and reflecting surface, has long been of the greatest Ives have made an interesting study of service in forming spectra for investigation. It the spectral distribution of energy in the possesses the great advantage of giving a regu- light emitted by the common firefly. As the lar and long spectrum, accompanied by the dis- radiation is, of course, discontinuous and of advantage of distributing the light through very low intensity, it was found impossible to several spectra, thereby giving a less intensity apply any of the more ordinary methods, so the in any one spectrum. The manufacture of spectrum was photographed and the intensities these gratings was brought to its greatest per- of different portions compared with those in a fection by the late Professor Rowland at the photograph of the spectrum of an electric lamp, Johns Hopkins University. Professor Wood of whose spectral distribution was determined. Johns Hopkins has brought out a grating de- The firefly radiation has a luminous efficiency signed particularly for forming the spectrum of of 96.5 per cent., while that of the carbon lamp the infra-red radiations, which are of so much is 0.4 per cent., and that of the most efficient importance in the optical researches of to-day. artificial light 4 per cent. It is probable that The theory of the grating shows that with the firefly gives out light as a result of some grooves of the proper shape, a large part of physiological-chemical process. The high effithe energy falling upon the grating can be concentrated into a single spectrum. It is mechanically impracticable to give a very definitely known shape to the groove in the ordinary grating because of the exceeding fineness of the rulings, but for work in the infra-red, where the wave-lengths are many times longer than in the visible spectrum, the grooves may be made correspondingly larger. On account of this Professor Wood has been able to rule grooves of definitely known shape, which are yet fine enough for infra-red work. Ruling with carborundum crystals on gold-plated copper plates which had been made as nearly plane as possible, he has secured some very good gratings, in which the metal was not cut at all, but simply compressed to form the groove.

The action of these gratings has been investigated by Professor A. Trowbridge and Professor Wood, using the vacuum spectrometre recently devised by the former. With infra-red radiation they throw practically all the energy into one or two spectra. They give considerably higher "resolving power," that is, greater angu lar separation of radiations having a given difference in wave-length, than any other apparatus that has been used in the remote infra-red portion of the spectrum. The results obtained with the gratings agree with the theory and show that by the use of these echelette gratings the theoretical relation between the dis

ciency stated refers only to the comparison of the visible radiation with the total, and nothing is known about the efficiency of the process which causes the emission of the light.

PHYSIOGRAPHY. See GEOLOGY. PHYSIOLOGY. Of perennial interest is the question of the physiological effect of alcohol on the animal body. Mendel, working on men and dogs, to the former of which he gave small doses, to the latter doses sufficient to produce intoxication, found that alcohol increased the output of uric acid, though he was unable to give a satisfactory explanation of this effect. Even with the dogs there was no indication of any pronounced alteration indicating disturbed protein metabolism. This seemed to indicate some regulating mechanism of the body. Mackensie and Hill found that alcohol increased the ability to work, and to hold the breath, and Hamill showed that an isolated heart will utilize limited amounts of alcohol given to it in its nutrient fluids. Burridge stated that fatigue is not due to an effect on the central nervous system, but is rather due to the action, probably of potassium salts, upon the nerve endings. It seemed certain that the nerve endings were the parts affected, and potassium salts were shown to be capable of producing this effect. Lactic acid in the blood would give the same results. Jacobson reported results indicating that there are no specific "trophic" nerve fibres governing the