Table XIII

MERCURY DROPLET OF RADIUS DISCHARGING ELECTRONS UNDER THE INFLUENCE OF ULTRA-VIOLET LIGHT

Volts Drop No. 1 Sec. per Cm. Sec. per Cm. Time Interval between Discharges in Seconds Charge in No. Electrons Emitted

2.260 11.0 - 1200} {49.4}

3.070 11.0 + 32.8} {50.5}

11 4.4 1

1.960 + 194 54.4

12.8

1.960 + 190 60.8 6.4 1

23

1.820 11.2 + 220 65.0 4.2 1

40.0

1.690 + 230 69.8 4.8 1

15.2

1.550 + 332 75.1 5.3 1

3.040 Drop No. 2 10.4 + 98 43.5

5.6

2.540 + 200 49.4 5.9 1

18.6

2.230 + 300 55.2 5.8 1

35.0

2.230 + 76 60.7 5.5 1

42.0

1.930 + 200 65.0 4.3 1

54

1.810 + 176 69.6 4.6 1

70

1.650 + 250 75.2 5.6 1

45

1.520 + 500 79.4 4.2 1

9.8

1.520 + 119 85.1 5.5 1

Mean 29 Mean 5.1

So long, then, as we are considering the ionization of neutral atoms through the absorption of an ether wave of any kind, the evidence at present available indicates that the act always consists in the detachment from the atom of one single negative electron, the energy with which this electron is ejected from the atom depending, as we shall see in chap. X, in a very definite and simple way upon the frequency of the ether wave which ejects it.

IV. IONIZATION BY -RAYS

When the ionization is due to the passage of -rays through matter, the evidence of the oil-drop experiments as well as that of C. T. R. Wilson’s experiments (see chap. IX) on the photographing of the tracks of the -rays is that here, too, the act consists in the detachment of one single electron from a single atom. This experimental result is easy to understand in the case of the -rays, when it is remembered that Wilson’s photographs prove directly the fact, long known from other types of evidence, that a -ray, in general, ionizes but a very minute fraction of the number of atoms through which it shoots before its energy is expended. If, then, its chance, in shooting through an atom, of coming near enough to one of the electronic constituents of that atom to knock it out is only one in one thousand, or one in one million, then its chance of getting near enough to two electronic constituents of the same atom to knock them both out is likely to be negligibly small. The argument here rests, however, on the assumption that the electrons within the atom are independent of one another, which is not necessarily the case, so that the matter must be decided after all solely by experiment.

The difference between the act of ionization when produced by a -ray and when produced by an ether wave seems, then, to consist wholly in the difference in the energy with which the two agencies hurl the electron from its mother atom. Wilson’s photographs show that -rays do not eject electrons from atoms with appreciable speeds, while ether waves may eject them with tremendous energy. Some of Wilson’s photographs showing the effect of passing X-rays through air are shown in the most interesting plate opposite p. 190. The original X-rays have ejected electrons with great speeds from a certain few of the atoms of the gas, and it is the tracks of these electrons as they shoot through the atoms of the gas, ionizing here and there as they go, which constitute the wiggly lines shown in the photograph. Most of the ionization, then, which is produced by X-rays is a secondary effect due to the negative electrons, i.e., the -rays which the X-rays eject. If these -rays could in turn eject electrons with ionizing speeds, each of the dots in one of these -ray tracks would be the starting-point of a new wiggly line like the original one. But such is not the case. We may think, then, of the -rays as simply shaking loose electronic dust from some of the atoms through which they pass while we think of the X-rays as taking hold in some way of the negative electrons within an atom and hurling them out with enormous energy.

V. IONIZATION BY -RAYS

But what happens to the electronic constituents of an atom when an -particle, that is, a helium atom, shoots through it? Some of Bragg’s experiments and Wilson’s photographs show that the -particles shoot in straight lines through from 3 to 7 cm. of air before they are brought to rest. We must conclude, then, that an atom has so loose a structure that another atom, if endowed with enough speed, can shoot straight through it without doing anything more than, in some instances, to shake off from that atom an electron or two. The tracks shown in Figs. 14 and 15, facing p. 190, are Wilson’s photographs of the tracks of the -particles of radium. They ionize so many of the atoms through which they pass that the individual droplets of water which form about the ions produced along the path of the ray, and which are the objects really photographed, are not distinguishable as individuals. The sharp changes in the direction of the ray toward the end of the path are convincing evidence that the -particle actually goes through the atoms instead of pushing them aside as does a bullet. For if one solar system, for example, endowed with a stupendous speed, were to shoot straight through another similar system, but without an actual impact of their central bodies, the deflection from its straight path which the first system experienced might be negligibly small if its speed were high enough, and that for the simple reason that the two systems would not be in each other’s vicinity long enough to produce a deflecting effect. In technical terms the time integral of the force would be negligibly small. The slower the speed, however, the longer this time, and hence the greater the deflection. Thus it is only when the -particle shown in Fig. 15 has lost most of its velocity—i.e., it is only toward the end of its path—that the nuclei of the atoms through which it passes are able to deflect it from its straight path. If it pushed the molecules aside as a bullet does, instead of going through them, the resistance to its motion would be greatest when the speed is highest. Now, the facts are just the opposite of this. The -particle ionizes several times more violently toward the end of its path than toward the beginning, and it therefore loses energy more rapidly when it is going slowly than when it is going rapidly. Further, it is deflected more readily, then, as the photograph shows. All of this is just as it should be if the -particle shoots straight through the molecules in its path instead of pushing them aside.

These photographs of Wilson’s are then the most convincing evidence that we have that the atom is a sort of miniature stellar system with constituents which are unquestionably just as minute with respect to the total volume occupied by the atom as are the sun and planets and other constituents of the solar system with respect to the whole volume inclosed within the confines of this system. When two molecules of a gas are going as slowly as they are in the ordinary motion of thermal agitation, say a mile a second, when their centers come to within a certain distance—about 0.2 . (millionths of a millimeter)—they repel one another and so the two systems do not interpenetrate. This is the case of an ordinary molecular collision. But endow one of these molecules with a large enough energy and it will shoot right through the other, sometimes doubtless without so much as knocking out a single electron. This is the case of an -particle shooting through air

But the question to which we are here seeking an answer is, does an individual -particle ever knock more than one electron from a single atom or molecule through which it passes, so as to leave that atom doubly or trebly charged? The oil-drop method used at low pressures[65] has given a very definite answer to this question. In no gas or vapor except helium, which we have as yet tried, is them any certain evidence that an individual -ray in shooting through an atom is able to remove from that atom more than one single electron at a time.

The foregoing result has been obtained by shooting the -rays from polonium through a rarefied gas in an oil-drop apparatus of the type sketched in Fig. 12, catching upon a balanced oil drop the positively charged residue of one of the atoms thus ionized, and counting, by the change in speed imparted to the droplet, the number of electrons which were detached from the captured atom by the passage of the -ray through or near it.[66]

This mode of experimenting extended to helium, however, has yielded the most interesting result[67] that every sixth one on the average of all the passages, or “shots,” which detached any electrons at all from the helium atom detached both of the two electrons which the neutral helium atom possesses. Since some of the ionization produced along the path of an -ray is probably due to slow-speed secondary -rays produced by the -ray, it is probable that the fraction of the actual passages through helium atoms of -rays themselves which detach both electrons is greater than the foregoing one in six. It has been estimated by Fowler at as high as three in four.

The foregoing experimental result of one in six was obtained only at the very end of the range of the -rays where they have their maximum ionizing power. When these rays were near the beginning of their range, and therefore were moving much more rapidly, the fraction of the number of double catches to total catches was only about half as much, i.e., the chance of getting both electrons at a single shot is much smaller with a high-speed bullet than with a slow-speed one. This is to be expected if the two electrons are independent of each other, i.e., if the removal of one does not carry the other out with it.

The foregoing is, I think, the only experiment which has yet been devised in which the act of ionization is isolated and studied as an individual thing.

Since 1913, however, very definite evidence has come in from two different sources that multiply-valent. ions are often produced in discharge tubes. The most unambiguous proof of this result has been furnished by the spectroscope. Indeed, Mr. Bowen and the author have recently found with great definiteness that high-voltage vacuum sparks give rise to spectral lines which are due to singly-, doubly-, trebly-, quadruply-, and quintuply- charged atoms of the elements from lithium to nitrogen, and even to sextuply-charged ones in the case of sulphur.[68] In view of the foregoing studies with X-rays, -rays, and -rays, it is probable that these spectroscopically discovered multiply-charged ions are produced by successive ionizations such as might be expected to take place in a region carrying a very dense electron current, such as must exist in our “hot-sparks.”

Again, J. J. Thomson has brought forward evidence[69] that the positive residues of atoms which shoot through discharge tubes in a direction opposite to that of the cathode rays have suffered multiple ionization. Indeed, he thinks he has evidence that the act of ionization of atoms of mercury consists either in the detachment of one negative electron or else in the detachment of eight. His evidence for the existence in the case of mercury of multiple charges from one up to eight is certainly very convincing, and it is possible, also, that under his conditions the act of ionization itself may consist in the detachment either of one or of eight electrons as he suggests. Further evidence upon this point must be sought.

VI. SUMMARY

The results of the studies reviewed in this chapter may be summarized thus:

1. The act of ionization by -rays seems to consist in the shaking off without any appreciable energy of one single electron from an occasional molecule through which the -ray passes. The faster the -ray the less frequently does it ionize.

2. The act of ionization by ether waves, i.e., by X-rays or light, seems to consist in the hurling out with an energy which may be very large, but which depends upon the frequency of the incident ether wave, of one single electron from an occasional molecule over which this wave passes.

3. The act of ionization by rapidly moving -particles consists generally in the shaking loose of one single electron from the atom through which it passes, though in the case of helium, two electrons are certainly sometimes removed at once. It may be, too, that a very slow-moving positive ray, such as J. J. Thomson used, may detach several electrons from a single atom.