Rays of Positive Electricity and Their Application to Chemical Analyses/Atoms Carrying Two or More Positive Charges
Though the heads of most of the parabolic arcs are situated in the same vertical line, in many cases some of the parabolas, especially those corresponding to the atoms of oxygen and carbon, are prolonged towards the vertical axis. The prolongations do not reach right up to this axis but in many cases, as in the line a in Fig. 26, Plate II., which is due to the atom of oxygen, stop after going half-way. These prolongations of the parabolas are also parabolic and are continuations of the primary parabola. They are therefore due to particles which, when they are in the deflecting fields, have the same value of e/m as the particles which produce the primary parabolas. The fact that the smallest horizontal deflection of the prolongation is just half that of the corresponding deflection of the primary shows (see p. 12) that the swiftest of the particles in the prolongation has twice the kinetic energy of the swiftest,in the primary. Thus these particles when in the electric field in the discharge tube acquire twice the kinetic energy of the normal particle; they must therefore when in the discharge tube have had twice the normal charge. They must, after passing through the cathode and before getting into the deflecting fields, have had their charge reduced to the normal value. For as we have seen, the value of e/m in these fields Is normal, hence if they have retained the double charge they must have double the mass. If, however, they had retained the double charge the electrostatic deflection would have been normal: for though the kinetic energy Is doubled, which halves the deflection for normal charge, the charge and therefore the electrostatic deflection for given kinetic energy Is doubled too, and hence the result would be the normal deflection, while the actual deflection Is only onehalf of this. We conclude, therefore, that the prolongation Is due to particles which had a double charge when in the discharge tube, but which have lost one of these charges after passing through the cathode.
It is a strong confirmation of this view that when we find these prolongations we generally find on the same plate lines having a value of e/m twice that of the prolongation and beginning In the normal place; these are due to particles which have retained their double charge after passing through the cathode. And conversely when we find these lines corresponding to the double value of e/m we find a tail or prolongation to the line corresponding to the normal value. This would not necessarily be true at pressures so low that the particles did not make any collisions after passing through the face of the cathode, but I have not been able to reduce the pressure to this point.
The prolongations of the parabolas in some cases extend much more than half-way to the vertical axis; this is especially the case with the parabola produced by the positively charged atom of mercury. Fig. 27, Plate II, shows that even when the electric and magnetic fields are strong enough to produce several millimetres deflection in the heads of the parabolas corresponding to the other elements, the head of the mercury parabola is so little deflected that at first sight it seems to coincide with the origin. When exceedingly large electric fields are used it can be seen, however, that the head of the mercury parabola is distinctly displaced, and on measuring the amount of the de-flection it is found to be one-eighth of the normal displacement of the heads of the parabolas corresponding to the other elements.
This, as we have seen, implies that the particles which produce the head of the parabola corresponding to the atom of mercury must have eight times the maximum amount of energy possessed by the normal atom; if the theory given above is true, this means that some of the mercury atoms had, before passing through the cathode, eight times the normal charge, i.e. had lost eight corpuscles. Eight corpuscles is a very large number for an atom to lose, so that if in this case we can obtain independent evidence of such a loss it will be a strong confirmation of the theory.
A study of plates taken with large electrostatic deflections has revealed the existence of seven parabolas due to mercury, corresponding to the mercury atom with 1, 2, 3, 4, 5, 6, 7 charges respectively. The parabola corresponding to eight charges has not been detected, but as the parabolas in general get fainter for each additional charge, it is probably on the plate although not intense enough to be visible. Fig. 28, Plate II., taken from a photograph when the gas in the tube was the residual gas left after exhaustion by the Gaede pump, shows these lines very well. The measurements of m/e for the parabolas on this plate give the following value m/e is taken as unity for the atom of hydrogen) : —
m/e 200 200/1 102 200/2 50.4 200/4 44 this Is not a mercury line but is due to CO2 39.8 200/5 33.7 200/6 28.6 200/7
It will be noticed that the heads of the parabolas corres- ponding to 1, 2, 3 . . . charges respectively lie on a straight line passing through the origin. This shows (p. 12) that the particles which produce these heads are all moving with the same velocity, and therefore, since each particle is an atom of mercury, that the kinetic energy of the particles at the heads of the parabolas is constant. This is in agreement with the theory, for the heads of all the parabolas are due to particles which before passing through the cathode had lost eight corpuscles. The particles at the head of the parabola corresponding to one charge (m/e= 200) has regained seven of these after passing through the cathode; the one at the head of the parabola corresponding to two charges (m/e= 100) has regained six and so on ; as the charge on these particles when they were in the discharge tube was eight units in each case, they would naturally acquire the same amount of kinetic energy before passing through the cathode.
The question now arises as to how the mercury atom acquires these very various charges. Can an atom of mercury when ionized lose any number of corpuscles from one to eight, or does it always lose a definite number? Take for example a mercury atom with five positive charges: has it got into this condition by losing five charges when it was ionized, or did it originally lose the maximum number eight and regain three subsequently ? The photographs suggest, I think that the second supposition is the correct one, and that in the discharge tube there are two and only two kinds of ionization. By one of these kinds the mercury atom loses one corpuscle, by the other eight The evidence for this is as follows: let us suppose for a moment that atoms with any charges from one to eight were produced by the ionlzation of the atoms of mercury in the discharge tube, and consider what effect this would have on the parabolas corresponding to the mercury atom with one charge. This would be due to atoms of the following kinds :—
Atoms which had lost
- (1) 8 corpuscles In the discharge tube and regained 7 subsequently.
- (2) 6 corpuscles In the discharge tube and regained 6 subsequently.
- (3) 6 corpuscles In the discharge tube and regained 5 subsequently.
and so on: the last member of the series being atoms which had lost one corpuscle on ionization and had not regained it.
The parabola seen on the plate would be due to the superposition of the eight parabolas due to these different types of atoms. The head of each of these parabolas would be separated from the head of any of the others: if d were the horizontal de- flection of the one due to the atom which had only lost one corpuscle in the discharge tube, d/2, d/3, d/4 d/5, d/6 d/7 d/8 would be the horizontal deflection of the heads of the parabolas due to the atoms which had lost 2, 3, 4, 5, 6, 7, 8 respectively. Thus the resultant parabola would for the part which had a horizontal deflection between d/8 and d/7 consist only of the parabola due to atoms of class (1); the part when the horizontal deflection was between d/7 and d/6 would consist of two parabolas due to the atoms of classes (1) and (2) ; the part with the horizontal deflection between d/6 and d/5 would be made up of the three parabolas corresponding to the atoms belonging to classes (1), (2), (3), and so on. Thus at the distance d/7, d/6, d/5, d/4, d/3, d/2 and d/1 we should expect an abrupt increase in the brightness of the curve, for at each of these places a new parabola is added to the old ones ; the intensity of the curve would thus not vary continuously but would have a beaded appearance. The abrupt increase in intensity at the distance d is very marked in the parabola; it is however, the only one to be detected. The intensity of this parabola is very great and it might be thought that the charges in the intensity might escape detection owing to the breadth of the curve. We may, however, apply the same reasoning to the parabolas which correspond to mercury atoms with three or four charges which are fine and well defined. The intensity of these curves is, however, perfectly continuous and there are no signs of the abrupt variations which ought to occur if the mercury atoms in the discharge tube had charges intermediate between one and eight. This result suggests that the ionization is mainly at any rate of two types, in the one type an atom of mercury loses a single corpuscle, In the other it loses eight There would thus seem to be two different agents producing ionization in the discharge tube. This is in accordance with another effect shown by many of the plates; on these plates there are well marked differences between the appearance of the lines due to charged atoms and those due to charged molecules. These differences are of various kinds: for example on the plate reproduced in Fig. 29, Plate III, the line α due to the hydrogen atom is of uniform intensity throughout, while β the one due to the hydrogen molecule, is very faint at the head but intense elsewhere; in others the line due to the atom is uniform while that due to the molecule has a beaded appearance. An example of this is shown in Fig. 30, Plate III. Perhaps the most important point shown by these plates is that when in a mixture of hydrogen and oxygen there are such differences between the lines due to the atoms and molecules of hydrogen, there are similar differences between the lines due to the atoms and molecules of oxygen. A similar result is obtained when the discharge passes through mixtures of nitrogen and hydrogen. It is interesting to observe that if we have along with these gases monatomic gases such as helium or mercury vapour, the lines corresponding to the atoms of these gases show the characteristics of both the and molecular lines of the diatomic gases, suggesting that some of the atoms of the monatomic gas have been Ionized by the process as the atoms of the diatomic gas, and others by the process which produced the charged molecules of this gas. There are in the discharge tube at least two kinds of ionizing agents, and it Is not unlikely that they produce different types of ionization. These agents are (1) rapidly moving cathode particles moving away from the cathode, and (2) positively charged atoms and molecules moving towards it; either of these agents can, as Is well known, produce ionization by collision. We should expect that the cathode particles, since they penetrate into the atom and come Into contact with the corpuscles Individually—the collision in favourable cases resulting In the detachment of a corpuscle—would give rise to singly charged systems, and If they struck a molecule might detach a corpuscle from one of the atoms without separating one atom from another, producing in this way a positively charged molecule. The collisions between the positively charged atoms and the atoms and molecules of the gas through which they are passing, might be expected to make the atom or molecule struck move off at a considerable velocity, which at first would not be shared by the corpuscles Inside the atom. The tendency of the corpuscles to leave the atom depends only upon the relative velocity of the atom and the corpuscles inside it, so that the ionizing effect produced by such a collision Is the same as If the atom were at rest, and all the corpuscles moving with the velocity acquired by the atom in the collision. Thus If there were several corpuscles connected with about the same firmness to the atom, the result of the atom acquiring a high velocity in a collision might be the liberation of all the corpuscles and the production of a multiply charged atom. Such a collision, since it would give one atom in a molecule a great velocity relatively to the other, would tend to dissociate the molecule Into atoms and produce positively charged atoms rather than molecules.
The maximum number of charges carried by a multiply charged atom does not seem to be related to any chemical property of the atom such as Its valency, but to depend mainly on the atomic weight; thus mercury, the most massive atom on which observations have been made, can have as many as eight charges, crypton atomic weight (82) four or five, argon atomic weight (40) three, neon atomic weight (20) two, nitrogen atomic weight (14), and oxygen (16) two, perhaps in rare cases three, helium also occurs with two charges; the multiple charge has been found on the atoms of all the elements tested with the very suggestive exception of hydrogen: no hydrogen atom with more than one charge has ever been observed, though as the hydrogen lines occur practically on every plate more observations have been made on the hydrogen lines than on those of any other element.
When there are on the plates lines corresponding to atoms of the same element with one, two, three charges, then the larger the number of charges the fainter the line. Judging from the intensity of the lines we should conclude that the number of multiply charged atoms is only a small fraction of the number with one charge. The ratio of the number of atoms which have only one charge to that of those which have two or more charges is very variable and depends on conditions which are not yet fully understood. For example in the case of the carbon atom this ratio seems to depend to a very great extent on the type of gaseous carbon compound in the discharge tube. With some hydrocarbons the doubly charged carbon atoms are relatively much brighter than with others. Again, in the case of oxygen I have found that the purer the oxygen the fainter was the line due to the doubly charged oxygen atom in comparison with that due to the atom with only one charge. It would thus seem that atoms torn from chemical compounds were more likely to have a double charge than those obtained from a molecule of the element. Chemical combination can not, however, be the only means by which the atoms acquire multiple charges, for the atoms of the inert monatomic gases, neon, argon and crypton, are remarkable for the ease with which they acquire multiple charges.
I have not been able to find any case in which a molecule of either an elementary or compound gas carries a double charge. The line corresponding to the molecule of nitrogen appears on some plates to have a prolongation towards the vertical axis; this would imply a double charge on the nitrogen molecule. I am inclined to think that this prolongation is not really due to the nitrogen molecule, but to the atom of aluminium, as m/e for this atom is 27.5, and for the nitrogen molecule 28, the lines would be so close together that It would be difficult to differentiate them.
Charged atoms on the view we have been discussing are in general produced by the impact of other charged atoms or molecules, while charged molecules are produced by the impact of cathode rays. This must not be taken to imply that cathode rays never produce charged atoms; it is probable that they would do so if they hit one of those corpuscles in the molecule which help, by the forces they exert, to bind the two atoms in the molecule together. There is direct evidence that in some cases charged atoms are produced by cathode rays, for Fulcher ("Astrophysical Journal," 34, p. 388) has shown that the passage of cathode rays through a gas generates in some cases the line spectrum of the gas, and line spectra are regarded as arising from atoms and not from molecules.
But though cathode rays may produce some charged atoms they more frequently produce charged molecules, the chief source of the charged atoms being positive rays, i.e. rapidly moving charged molecules or atoms. The view that the charged atoms and molecules are produced by different agents helps us to understand the remarkable variations which occur in the relative intensities of the lines due to the atoms and molecules of the same element. To take the case of hydrogen, sometimes the line due to the atom is stronger than that due to the molecule, at others it is weaker. Examples of this are shown in Figs. 31 and 32, Plate III.
Very small variations in the conditions of the discharge are sufficient to produce wide variations in the relative intensities of the atomic and molecular lines. If, for example, the cathode is placed so that its face comes inside the neck of the discharge tube as in Fig. 33a, the atomic line of hydrogen is stronger than the molecular: it is weaker, however, when the face protrudes beyond the mouth of the neck as in Fig. 33b. When the cathode is in the position indicated by Fig. 33a the pressure, when the positive rays are at their best, is higher than when the cathode is placed as in Fig. 33b ; so that this result suggests that, as the pressure increases, the intensity of the lines due to the atoms as compared with that of those due to the molecules increases also.