Rays of Positive Electricity and Their Application to Chemical Analyses/Methods for Measuring the Number of the Positively Electric Fied Particles
Though the photographic plate furnishes an excellent means of detecting the existence of positively charged particles of different kinds it Is not suitable for comparing the number of particles present in a bundle of positive rays. For though the intensity of the lines on the photograph will vary with the number of particles, this number will not be the only factor In the expression for the intensity. As an example, consider the lines due (1) to very light particles like the atoms of hydrogen, and (2) to very, heavy ones like the atoms of mercury. If these particles have acquired the same amount of energy in the electric field before entering the cathode, the hydrogen atoms will have a velocity about fourteen times that of the mercury ones: they might therefore be expected to penetrate further into the film on the plate and produce a greater photographic effect than the mercury ones. If this expectation Is realized, and we shall see that it is, It Is evident that the photographic effect cannot be taken as a measure of the number of positively electrified particles.
A method which does give metrical results is founded on the following principle. Suppose that we replace the photographic plate in the preceding method by a metal plate in which there is a movable parabolic slit, then when this slit is moved into such a position that it coincides with one of the parabolas on the photographic plate, positively electrified particles would pass through the slit; if these particles are caught and their total charge measured we shall have a measure of the number of positively electrified particles. Thus if the slit were gradually moved up the plate there would be no charge coming through it, unless it coincided in position with one of the parabolas. As one parabola after another was passed, positive electricity would come abruptly through the slit, and the amount of the charge would be a measure of the number of particles passing through the slit. If instead of moving the parabolic slit we keep the slit fixed and gradually increase the magnetic field used to deflect the particles, we shall in this way drive one parabola after another on to the slit, beginning with the parabola due to the hydrogen atom and ending with that due to the mercury one, and the charges passing the slits will be proportional to the number of particles.
The apparatus used to carry this idea into practice is represented in Fig. 34. After passing through the electric and magnetic fields the particles, instead of falling on a photographic plate, fall on the end of a closed cylindrical metal box B. In the end of this box nearest the cathode a parabolic slit about 1 mm. in width is cut, the vertex of the parabola being the point where the undeflected rays would strike the box, and the tangent at the vertex the line along which the particles would be deflected by the magnetic force alone. This slit Is the only entry into the box. Inside the box and immediately behind the slit there is an insulated long, narrow vessel placed so that every particle passing through the slit falls into this vessel. This vessel is connected with a Wilson tilted electroscope by which the charge it receives can be measured.
From the front face of the box a portion was cut away, and the opening closed by a willemite screen. The positive rays could be deflected on to this screen and the brightness of the fluorescence observed; in this way one can make sure that the tube is in the proper state for giving positive rays before attempting to make the measurements.
The impact on the face of the box of the rays which do not pass through the slit gives rise to the emission of slowly moving cathode rays; if precautions are not taken these diffuse through the slit, enter the Faraday cylinder, and confuse the measurements. This diffusion can be avoided by placing a small permanent magnet near the slit The force due to this is strong enough to deflect the more mobile cathode rays without producing any appreciable effect on the positively charged atoms. The pressure of the gas between this box and the cathode should be made as small as possible: the best way of reducing the pressure is to absorb the gas by means of charcoal cooled with liquid air. This method will not produce a good vacuum when the gas in the tube is helium; with hydrogen, too, the vacuum is not so good as for heavier gases, for then the pressure can by this means easily be reduced to 3/1000 of a millimeter.
The method of observing with this apparatus is as follows: The positive rays are deflected by a constant electric field of such a magnitude that the heads of the parabolas are in line with one end of the slit The magnetic field is then increased by small increments and the deflection of the Wilson electroscope in ten seconds measured, Unless a parabola comes on the slit there Is practically no deflection; as soon, however, as the magnetic force is such that a parabola comes on the slit, there Is a considerable deflection which disappears when the magnetic force Is increased so as to drive the parabola past the slit The appearance and disappearance of the deflection of the electroscope are surprisingly sharp, so that lines quite near each other can be detected and separated. An example of the results obtained by this method Is given in Fig. 35. The abscissae are the values of the magnetic force used to deflect the rays, and the ordinates the deflection of the Wilson electroscope in 10 seconds. The gas in the tube was carbon monoxide.
A comparison of this curve with a photograph of the discharge through the same gas shows many interesting features. On the photograph the strongest lines are those corresponding to the atom and molecules of hydrogen. The curve on the other hand shows that the number of hydrogen particles is only a small fraction of the number of CO particles. The extraordinary sensitiveness of the photographic plate for the hydrogen atom in comparison with that for atoms and molecules of other gases is shown in all the curves taken by this method. But great as is the discrepancy in the case of the photographic plate between the effects produced by hydrogen atoms and an equal number of heavier atoms, it is not nearly so great as It is for a willemite screen: such a screen may show the hydrogen, lines very brightly while the CO line is hardly visible, when measurements made with the electroscope In the way just described show that the number of particles of hydrogen is only a few per cent of the number of the CO particles.
It is difficult to get from the photographs any estimate of the relative amount of the different gases in the discharge tube when it contains a mixture of several gases; for example, if the tube is filled with a mixture of hydrogen and oxygen the relative quantities of these gases may be varied within wide limits without producing any very marked effect on the relative brightness of the hydrogen and oxygen lines in the photograph. This electroscope method is much more metrical as will be seen from Figs. 36 and 37, the first of which represents the curve when the gas in the tube was a mixture of one-third hydrogen and two-thirds oxygen, while in the second, the gas was one-third oxygen and two-thirds hydrogen,
The negatively charged hydrogen atoms seem to have the same preponderance in their effect on the photographic plate over other negative atoms as positive hydrogen atoms have over other positive atoms. Thus on all the plates the line corresponding to the negatively electrified hydrogen atoms is well marked, often being comparable with the negatively electrified oxygen atom. With the electroscopic method the negative hydrogen atom can only just be detected, while the negatively electrified oxygen atoms produce a large negative deflection. A curve showing the comparative numbers of different kinds of negatively electrified atoms is shown in the curve
Fig. 38: the gas in the tube was phosgene, COCl2; the curve at the top of the figure represents the number of negatively electrified particles, the one at the bottom the positively electrified ones. It will be seen that the negative atoms detected by the electroscopic method were carbon, oxygen, and chlorine, and that the chlorine atoms were by far the most numerous. On the photographs taken with this gas the line due to negatively electrified hydrogen seemed comparable in intensity with that due to negative chlorine. An interesting point about the curve representing the distribution of positively electrified, atoms is the great variety of atoms and molecules in this case, thus we find atoms of carbon, oxygen, and chlorine, and the molecules CO, Cl2, CCl and COCl2. It will be noticed that only a small fraction of the current is carried by free carbon and oxygen atoms, showing that in phosgene the carbon and oxygen atoms are so firmly united that the greater part of them remain together even when the gas is dissociated.
Are the atoms in a molecule of a compound gas charged with electricity of opposite signs?
The study of the curves obtained by the electroscopic method throws some light on the electrical states of the two atoms in a diatomic molecule of an elementary or compound gas. If we regard the forces which keep the atoms together as electrical in their origin, the question naturally arises, are the two atoms in a molecule of hydrogen, for example, charged one with positive the other with negative electricity; or in a molecule of hydrochloric acid gas is the hydrogen atom positively charged, the chlorine negatively, or in a compound like ammonia NH3 does the nitrogen atom carry three negative charges and each of the hydrogen atoms one positive one ?
Let us consider the case of CO for which we have in Fig. 35 the curve which represents the relative numbers of the different kinds of positively charged atoms. If the carbon atom in the molecule were positively, the oxygen atom negatively electrified, then we should expect that if a molecule of CO were split into atoms by the impact of a rapidly moving positively electrified particle, there would be a tendency for the carbon atoms to have a positive charge and for the oxygen ones to have a negative, so that in the positive rays we should expect to find more carbon atoms than oxygen ones. The curve, Fig. 36, shows that the number of positively electrified carbon atoms exceeds that of the positively charged oxygen ones in the proportion of 11 to 7. These figures, however, underrate the number of oxygen atoms which came through the cathode, for some of them after passing through the cathode acquired a negative charge. The charges given to the electroscope show that the proportion between negatively and positively charged oxygen atoms was as 2 to 7, while the number of carbon atoms which were negatively charged was very small in comparison with that of the positively charged atoms. Taking the negative atoms into account as well as the positive we find that the proportion between the number of carbon and oxygen atoms passing through the cathode is as 11 to 9; the numbers are too nearly equal to allow us to suppose that in the molecule one of the atoms is positively, the other negatively charged
The curve for COCl2, Fig. 38, shows that the proportion of positively electrified chlorine atoms in the positive rays Is not very different from the proportion of chlorine atoms in the normal gas. If the atoms in the molecule COCl2 had individually carried electric charges we should have expected the atoms of the strongly electro-negative element chlorine to have carried a negative charge and to have been relatively deficient in the positive rays.
The view that each of the atoms in a molecule of a compound contains as much positive as negative electricity is supported by considerations drawn from other branches of physics. If the atoms in a molecule of a gas carried separate charges so that one kind of atom was positively, another negatively, charged, then if the gas were dissociated into these atoms the atoms would be charged and the dissociated gas would be a good conductor of electricity. Now there are several gases which are dissociated at low temperatures, nickel carbonyl, for example, is at 100° C split up into nickel and CO to a very large extent; If these atoms were charged the electrical conductivity of the gas might be expected to begin to show marked increase at a temperature of about 70° C. when the dissociation first becomes appreciable. The variation of the conductivity of nickel carbonyl with temperature is, however, as Prof. Smith has shown, quite normal, following the same laws as for an undissociated gas. L. Bloch,1 too, has shown that the dissociation of arseniuretted hydrogen which also takes place at low temperatures Is not accompanied by any Increase in electrical conductivity. He also showed that many chemical reactions between gases which go on at low tempera- tures such as the oxidation of nitrogen bioxidCj the action of chlorine on arsenic, the oxidation of ether vapour, have little or no effect on the conductivity.
Chemical action, unless accompanied by high temperature, has not been shown to give conductivity. The very vigorous combination of hydrogen and chlorine under sunlight seems to have absolutely no effect on the conductivity of the mixture, and this Is a strong reason for supposing that the atoms In the molecule are not charged.
It Is true that chemical actions vigorous enough to raise the gases to a very high temperature, such as, for example, the combination of hydrogen and oxygen In the oxy-hydrogen flame, the oxidation in a Bunsen flame, the burning of CO and so on, make the reacting gases good conductors of electricity. This conductivity seems, however, from the result of recent experiments, to be due to the high temperatures produced by the chemical action rather than to that action itself. The conductivity cannot be due to the molecule being dissociated into positively and negatively electrified atoms, for the determinations of the mobility of the negatively electrified particles in flames and gases at a very high temperature show that it is much larger than would be possible If these particles had masses comparable with that of even the lightest atom, These negatively electrified particles are corpuscles, not atoms, and the electrical conductivity of gases at high temperatures is due to the dissociation of the molecules and atoms Into positively charged molecules and atoms and negatively electrified corpuscles, and not to a dissociation such as occurs In solution when the electrified bodies are positively and negatively electrified atoms. The conductivity of hot gases seems to be an example of the emission of corpuscles from hot bodies, rather than to be directly connected with chemical combination. We know that when we raise solids such as metals, or still better, certain oxides to a high temperature they give out copious streams of corpuscles, and the conductivity of flames is better explained by supposing that gases possess this property to some extent than by attributing It to chemical action alone.
We are led by these results to regard the electrical forces which keep the atoms In a molecule together as due not to one atom being charged positively and the other negatively but to the displacement of the positive and negative electricity In each atom. Thus each atom acts like an electrical doublet, and attracts another atom In much the same way that two magnets attract each other,
On the Information Afforded by the Positive Rays as to the Constitution of a Gas, the Nature and Properties of the Molecules, and the Process of Ionization in a Discharge Tube
The results we have given above enable us to appreciate the importance of the positive rays In Investigating the con- stitution of gases and the properties of atoms and molecules.
In the first place they give a very direct and simple proof of the molecular constitution of gases. We have seen that the positive rays from a gas produce on the photographic plate a finite number of distinct and sharp parabolas. As each parabola corresponds to a different kind of charged particle this shows that there are only a limited number of different kinds of particles in the discharge tube. The sharpness of the lines shows that all the particles of the same chemical element have to a very high degree of accuracy the same mass. If the atoms of hydrogen, for example, differed appreciably in mass it would not be possible to get on the photographic plates parabolas as sharply defined as those which can be obtained when the tube through the cathode has a very fine bore. As far as we know at present the fineness of the line depends only on the bore of the tube. This would not be the case if there were any variation in the mass of the atoms, for then instead of a line we should get a band bounded on one side by the parabolas corresponding to the heaviest atom and on the other by that corresponding to the lightest.
Again we see that in some gases we have both atoms and molecules, in others only atoms. We can infer from the study of the curves produced by the positive rays that helium, for example, Is a monatomic gas, hydrogen and oxygen diatomic.
The rays show too that the atoms and molecules of the gases can be charged with electricity; all of them, as far as we know, with positive electricity, some of the atoms with negative as well. The circumstances are very unfavourable for a particle in the positive rays to get a negative charge, and we must not conclude that because an atom or molecule has not been observed to acquire a negative charge when in the positive rays it is incapable of doing so under more favourable circumstances.
We have seen too that the atoms of all the elements except hydrogen can acquire more than one unit of positive charge. The maximum number of such units seems to depend on the atomic weight of the atom, for mercury, the heaviest atom yet investigated, it Is as large as eight, for krypton four or five, for oxygen two, and so on. No undoubted case of a double charge on a molecule, whether of an element or a compound, has yet been observed in addition to the atoms and molecules of recognized elements these rays reveal the existence of and other combinations which are not known to exist permanently in the free state. Thus the positive rays from marsh gas CH4 show, in addition to the atom and molecule of hydrogen and the atom of carbon, the systems CH1, CH2, CH3, CH4. The radicle OH with a negative charge has also been found when water vapour was in the tube. We can detect by this method systems which have a very transitory existence, for they need only last long enough to travel from the discharge tube to the photographic plate, a journey which takes less than the millionth of a second.
Again (see p. 45) the rays show that with those compounds of carbon, which contain two or more carbon atoms united by bonds, two carbon atoms connected together are found in the positive rays, and since they are found with a negative as well as with a positive charge, the two carbon atoms united In this way cannot be saturated.
Let us now consider the evidence given by these rays on the question of ionlzatlon. We see from the inspection of the photographs that ionlzatlon in the discharge tube Is not exclusively nor even mainly the detachment of a corpuscle from a neutral molecule: this process would produce merely positively electrified molecules. The photographs show that the products of ionlzatlon are much more complex than this: for though we do find the positively electrified molecules of the gas through which the discharge Is passing, whether that gas be an elementary one like hydrogen or oxygen or a compound one like carbon monoxide or carbonic acid, these molecules are by no means the only kind of electrified particles in the rays, instead of there being only one kind of carrier for the positive electricity there are always considerable number of kinds : in an experiment with benzene vapour I counted seventeen distinct kinds of positive carriers. Indeed the splitting up of the gas by the discharge is sometlmes so complete that the photographs of the positive rays in different hydrocarbons may be almost identical. Thus, for example, the prominent lines for the vapours of methyl alcohoI, ether ((C2H5)2O) and dimethyl ether are identical, showing that they are due to the products of dissociation of the molecules of these substances and not to the molecules themselves; the lines due to these are very faint.
In the discharge tube then we have dissociation — the splitting up of molecules into atoms — as well as the detachment of a corpuscle from the molecules.
The agents present In the tube which are known by independent experiments to produce ionlzation are : —
- 1. Cathode rays of varying velocities moving away from the cathode.
- 2. Positive rays moving towards the cathode.
- 3. Rays analogous to Röntgen rays, due to the impact of cathodic and positive rays against the molecules of the gas through which they are passing and also to the recombination of the ions of opposite signs.
The primary effect produced by the impact of a cathode ray with a molecule would be to detach a corpuscle from this molecule. If this corpuscle is one of the structural ones, i.e. one of those causing the attractions which hold the atoms In a molecule together, then the ionizatlon will be accompanied by the separation of the atoms In the molecule: we shall have dissociation as well as Ionlzation and the result will be a positively electrified atom. If, however, the corpuscle detached Is not a structural one the atoms In the molecule will not be separated and a positively electrified molecule will be the result. If this were the only method of ionization we should expect a considerable excess of positively electrified molecules over positively electrified atoms. It Is not, however, probable that any large fraction of the Ionization In the tube Is due to the direct action of the faster cathode rays. The amount of ionization due to such rays has been measured by Glasson2 who found, as Is Indicated by theory, that the number of ions produced by cathode ray per unit length of path varies inversely as the kinetic energy of the ray. For rays moving with a velocity of 4.7 x 109 cm./sec. through air at a pressure of 1 mm. of mercury, he found that I.5 pairs of Ions were produced by each ray In travelling over 1 cm. In the experiments with positive rays the velocity of the faster cathode rays was considerably greater than 5 x 109 cm./sec. This would reduce the ionization if the pressure In the gas remained the same, but the pressure in the gas in our experiments was less than .01 mm. of mercury, so that even If we neglect the diminution in ionization due to increase in velocity, a cathode ray would only produce 1.5 pairs of ions when it had travelled over a metre, a distance much greater than the length of the tube. We conclude that the ionization on the gas is not therefore in the main due to the fast cathode rays: It arises more probably from slow cathode rays and from positive particles. The positive ions from the negative glow when they get into the dark space soon acquire sufficient energy to ionize the gas, producing corpuscles and other positive ions. These secondary corpuscles will at first be moving slowly as they are in a region In the dark space where the electric field Is comparatively weak; they will be efficient ionizers as their velocity is small, and will produce other corpuscles by collision, these corpuscles will be in a still weaker electric field and therefore still more efficient ionizers, as it is not until the velocity of the cathode particles sinks below that due to a fall through about 200 volts, that the ionization due to these particles increases as the velocity decreases. Thus near the anode end of the dark space the number of slowly moving cathode rays will increase with very great rapidity, and the gas in this neighbourhood will be a mixture of molecules and comparatively slowly moving cathode rays. Though these rays are slow in comparison with those that have acquired the energy due to the fall in potential through the whole of the dark space, their kinetic energy is large enough to correspond to that due to the thermal agitation of a particle at a very high temperature. For example a corpuscle moving with a velocity 107 cm./sec. has energy corresponding to that due to thermal agitation at 0° C.: one moving with a velocity of 108 would have energy equal to that due to the thermal agitation at about 27000° C. A velocity of 108 cm./sec. would be acquired by a corpuscle through a fall of potential of between 3 and 4 volts, so that the velocity of even the slowest cathode particles in the discharge tube will be considerably greater than this. If there were anything approaching to equipartition of energy between these corpuscles and the structural corpuscles of the atoms in a molecule, the latter would acquire so much energy that they might wander away from the places they ought to occupy if they are to keep the atoms in the molecule together. The result of this would be that the atoms would separate and some of the corpuscles inside them would have considerable kinetic energy. This energy might be sufficient to carry them outside the atoms and thus produce positively charged atoms: either of the atoms in a diatomic molecule might be positively electrified in this way. The seat of ionization of this kind would be at the end of the dark space next the negative glow, so the positively electrified' particles produced in this way would fall through the whole of the potential difference in the space, and would acquire the maximum amount of kinetic energy, they would therefore hit the photographic plate at the of the parabola corresponding to this kind of particle. Some of the photographs,, such as the one reproduced in Fig. 39, Plate IV,, have the heads of the parabolas very much more intense than the rest of the arc, indicating that in this case the majority of the particles have fallen through the maximum potential difference and therefore have been produced at the end of the dark space. There are many cases, however, where concentration does not occur, and where the arcs of the parabolas are very long, indicating that there is a very considerable range in the kinetic energy of the particles, the uniform intensity of the lines shows that the number of particles is fairly equally distributed over a considerable range of kinetic energy.
We might account for this variation in the kinetic energy in. the following ways:—
- 1. By supposing that the charged particles all started from the same region, the end of the dark space, but before reaching the cathode got neutralized and so were only under the influence of the electric field for a fraction of the journey through the dark space.
- 2. By supposing that the ionization which produced these particles occurred not merely at the end of the dark space but to some extent throughout the whole of this space.
- 3. By supposing that the small amount of kinetic energy possessed by some of the particles is due to their colliding with the molecules of the gas whilst passing through the dark space and in this way losing some of their kinetic energy.
Let us begin with the first of these suggestions. We have seen (p. 32) that unless the pressure Is very low some of the particles, after they pass through the cathode, get neutralized and lose their charge for a time; in some cases they acquire it again by being ionized by collision with a corpuscle. If this process went on in front of the cathode as well as behind it we should get variations In the kinetic energy of the particles, as some of them would have passed a larger fraction of their time In the uncharged state than others. The corpuscles which produce neutralization and ionlzation must be those in the molecules of the gas through which the positive rays are passing and not the free corpuscles. For in the dark space in front of the cathode there is an intense electric field in which the free corpuscles are moving far faster than the particles so that the relative velocity of the particles and free corpuscles is much greater than that of the particles and the corpuscles in the molecules, and therefore the union of the positively electrified particles and the free negatively electrified corpuscles is not likely to occur; there may, however, be some neutralization owing to a positively charged particle uniting with a corpuscle from an uncharged molecule, through which the positive particle is passing.
The third suggestion that the loss of kinetic energy is due to collisions between the particles and the molecules in the discharge tube through which they are moving is open to the objection that It would produce effects of the same general character on all the lines. Thus If all the particles started from the end of the dark space and the reduction In their velocity was due to these collisions we should expect all the lines to show a general resemblance in the way the intensity varied along the parabola. We find, however, on the same plate lines which are quite short, with all the intensity concentrated at the head, and others which are long and of equal intensity throughout The second suggestion, that positive particles are produced at different parts of the field by other positively electrified particles in rapid motion, seems to me to Indicate an effect which must undoubtedly be largely instrumental In increasing the length of the parabolas. The particles which are produced near the cathode will not, when they reach the cathode, have fallen through as great a potential difference as those produced farther away and will therefore have a smaller velocity.
1 "Annajes de Chimte et de Physique," XXII, pp. 370, 441; XXIII, p. 28.
2 Phil. Mag.," October, 1911.