POSITIVE RAYS. In 1886 Goldstein while experimenting with the discharge in gases at low pressures observed luminous streamers passing through perforations in the cathode and illumi nating the gas behind it. The luminosity, he assumed, was due to rays of some sort which travelled in the opposite direction to the cathode rays (q.v.) and so passed through the aperture in the cathode. On account of the method by which they were obtained he called them "Canalstrahlen." Subsequently Wien showed that they could be deflected by means of a magnetic field. They have been very fully investigated by Sir J. J. Thomson who called them "Positive Rays" on account of the fact that normally they are positively electrified. Recently they have been included in the general term "Mass Rays" which covers all swiftly moving par ticles of matter of atomic or molecular size whether charged elec trically or not. The rays may be developed in many different ways, the most general being ionization of a gas at low pressure in a strong electric field. Ionization, which may be due to collision or radiation, means in its simplest case the detachment of one electron from a neutral atom. The two resulting fragments carry charges of electricity of equal quantity but of opposite sign. The negatively charged portion is the electron, the atomic unit of neg ative electricity itself, and is the same whatever the atom ionized. It is extremely light and theref ore in the strong electric field rapidly attains a high velocity and becomes a cathode ray. The remaining fragment is clearly dependent on the nature of the atom ionized. It is immensely more massive than the electron, for the mass of the lightest atom, that of hydrogen, is about 1,845 times that of the electron, and so will attain a much lower veloci ty under the action of the electric field. However, if the field is strong and the pressure so low that it does not collide with other atoms too frequently it will ultimately attain a high speed in a direction opposite to that of the detached electron, and become a "positive ray." The simplest form of positive ray is, therefore, an atom of matter carrying a positive charge and endowed, as a result of falling through a high potential, with sufficient energy to make its presence detectable. Positive rays can be formed from molecules as well as atoms, so that it will at once be seen that any measurement of their mass will give us direct information as to the masses of atoms of elements and molecules of compounds, and that this information will refer to the atoms and molecules indi vidually, not, as in chemistry, to the mean of an immense aggre gate. It is on this account that accurate analysis of positive rays
is of such fundamental importance in research on the structure of atoms.
For visual effects the rays are best detected by a screen made of powdered willemite, which glows a faint green when bombarded by them. When permanent effects are required this screen is replaced by a photographic plate. The sensitivity of the plate to positive rays bears no particular relation to its sensitivity to light; so far the best results have been obtained from comparatively slow process plates of the type known as "Half Tone." Thomson's "Parabola" Method of Analysis.—The method by which Sir J. J. Thomson investigated the properties of positive rays, and which still remains pre-eminent in respect to the variety of information it supplies, consists essentially in allowing the rays to pass through a very narrow tube and then analysing the fine beam so produced by electric and magnetic fields.
The construction of one of the types of apparatus used is indi cated in fig. 1. The discharge by which the rays are generated takes place in the bulb A. The cathode B is placed in the neck of the bulb. Its face is of aluminium and so shaped as to present a hemispherical front provided with a funnel-shaped depression. This hole through which the rays pass is continued as an extremely fine-bore tube, mounted in a thick iron tube forming the continua tion of the cathode as indicated. The finer the bore of this tube the more accurate are the results obtained, and tubes have been made as narrow as one-tenth of a millimetre. but as the intensity of the beam of rays falls off with the inverse fourth power of the diameter a practical limit is soon reached. The cathode is cooled by the water-jacket C. The anode D may be placed in a side tube. The gas to be examined is led in through a fine leak E and pumped off at F. The pressure is usually adjusted so that the dis charge potential is 30,00o to so,000 volts.