Rays of Positive Electricity and Their Application to Chemical Analyses/On The Use of the Positive Rays for Chemical Analysis
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On The Use of the Positive Rays for Chemical Analysis
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1. We shall now proceed to show how the method of positive rays supplies us with a very powerful method of chemical analysis, and how from the study of the positive ray photographs we are able to determine the different kinds of atoms and molecules in the discharge tube. Each atom or molecule in the discharge tube produces a separate parabola on the photographic plate and if we measure these parabolas then by means of the formula
we can determine the value of e/m for the particles producing any of the parabolas which occur on the photographic plate. We know, too, that the charge e is either the ionic charge, whose value on the electrostatic system of units is 4.8 x 10-10, or some multiple of it. We have, too, as we shall see, the means of determining what this multiple is. As we can determine the value of e and since we know by the measurement of the parabola the value of e/m we can deduce the value of m and thus determine the masses of the particles forming the positive rays. As these particles are the atoms and molecules of the gases in the discharge tube it is evident that in this way we can determine the atomic or molecular weight of the gases in the positive rays. We can thus identify these gases as far as can be done by the knowledge of their atomic weight. The study of the photographs gives us in fact the atomic weights of the various gases in the tube and thus enables us to determine the nature of the contents of the tube. We can thus analyse a gas by putting a small quantity of it into a discharge tube and taking a photograph of the positive rays. This method of analysis has many advantages. In the first place only a very small quantity of gas Is required, the total amount of gas In the discharge tube of the size described on page 16 would only occupy about .01 c.c. at atmospheric pressure and a constituent present to the extent of only a very small percentage would give well defined parabolas. If there Is a new gas In the tube it is indicated by the presence of a new parabola, but this parabola does far more than show that something new is present, it tells us what is the atomic weight of the new constituent. Let us compare for a moment the method with that of spectrum analysis. We might detect a new gas by observing an unknown line in the spectrum when the electric discharge passed through the gas. This observation would, however, tell us nothing about the nature of the substance giving the new line, nor, indeed, whether it arose from a new substance at all: it might be a line given out by a well-known substance under new electrical conditions. Again, If a substance is only present to the extent of a few per cent It very often happens that its spectrum is completely swamped by that of the more abundant substance: thus, for example, in a mixture of helium and hydrogen we cannot observe the helium lines unless the helium is a considerable percentage of the mixture.
This is not the case with the positive rays, or at any rate not to anything like the same extent; the presence of one per cent of helium would be very easily detected by the positive rays. The method, too, is more sensitive than that of spectrum analysis. With the apparatus described above the helium in 1 c.c. of air, i.e. about 3 x106 c.c., could be detected with great ease when it formed only about one per cent of the mixed gas in the tube. No special attention was paid to making this particular apparatus specially sensitive. To get the best results the size of the tube running through the cathode has to be chosen with reference to the distance of the photographic plate from the cathode, and other circumstances; this was not done in the apparatus under discussion, nor were the photographic plates used of any special sensitiveness; by attention to these points the sensitiveness of the method could very materially be increased.
Again the method of the positive rays enables us when we have found the substance to say whether the molecule is monatomic or diatomic; if it is diatomic we shall have two new parabolas, one indicating particles with a mass twice that of the picartles producing the other; if the molecule is monatomic there will be only one parabola unless the particle acquire a double charge and the presence of this extra charge can be recognized by the tests previously described. The method of the positive rays has the advantage of revealing the presence of the molecules of compound gas as well as the atoms and molecules of elementary substances. Since different compounds may have the same molecular weight there is sometimes ambiguity in interpreting the photographs produced by the positive rays; for example, CO2 and N2O produce the same parabolas as also do CO and N2 In such cases to find out the origin of such a parabola we must repeat the experiment under different conditions; for example, if we put something in the tube which absorbs CO2 and not N2O and find that the parabola disappears, we conclude that it was due to CO2; If it does not disappear it is not due to CO2, but to N2O or some other compound with the same molecular weight.
2. The ambiguity as to whether a line with a value of m/e equal say to 8 (m/e for the hydrogen atom being taken as unity) is to be ascribed to an atom with atomic weight 8 carrying a single charge or to one with an atomic weight 16 carrying two charges or to one with atomic weight 24 with three charges may be removed by the considerations given on page 47. For example, If the particle producing this parabola A carries a double charge there will be another and more intense parabola B for which the value of e/m is twice that for A, and the parabola B will have a prolongation towards the vertical axis, the distance of the head of this prolongation from the vertical axis being half the distance of the heads of the normal para- bolas (see p. 47). If A represents a particle with a threefold charge there will be another stronger parabola B for which e/m has three times the value corresponding to the parabola A, and B will have a prolongation towards the vertical axis extending to one third of the normal distance.
3. For the purposes of Chemical Analysis It is not necessary to use the elaborate apparatus shown in Fig, 10, the simpler one shown in Fig. 6 is all that Is required for this purpose. The more elaborate apparatus is only required when we require to know accurately the values of the quantities A and B which occur In the expression for e/m.
For the determination of the masses of the particles producing the different parabolas the measurement of the quantities A and B Is unnecessary if we can recognize the particle which produces any particular parabola. For since A and B are the same for all the parabolas, then for any two parabolas we have by the equation on page 106
when e/m1 and e/m2 are the values of e/m, for the particles producing the parabolas (1) and (2) respectively, (x1y1) (x2y2) are the co-ordinates of any point on the first and second parabolas respectively.
If the points on the two parabolas have the same values of x so that x1= x2
if the charges are the same
As the line corresponding to the atom of hydrogen occurs on al! the plates and can at once be recognized by being the most deflected line on the plate, the value of (e/m} for the particles producing any parabola can be at once, by the aid of this formula, compared with the value of this quantity for an atom of hydrogen and the masses of the various particles thereby determined,
A convenient instrument for making the necessary measurements is shown in Fig. 14. The plate is inserted in the holder A. The camera is arranged so that the direction in which the rays are deflected by the magnetic force alone (the vertical axis in the preceding figures) is parallel to the longer side of the photographic plate. The deflection due to the electrostatic field is at right angles to this and parallel to the shorter side of the plate. The plate is placed in the holder so that the axis of no electrostatic deflection is parallel to B, and that of no magnetic deflection perpendicular to BB. A needle NN whose point comes close to the plate is placed in the carrier C which can move parallel to BB by sliding along BB, and perpendicular to it by means of the screw S, the position of the carrier is read by two verniers V1 and V2. There is always a circular patch of some size on the plate at the place, where the undeflected particles hit the plate: the zero is at the centre of the spot. By putting the needle first at the centre of the spot, then moving the carrier through a certain distance perpendicular to BB by the screw S, and sliding the carrier parallel to BB until the needle comes on the parabolas In turn, the values of y for the different parabolas corresponding to a constant value of x can be measured.
The equation page 110 then enables us to find the ratio of the of masses of the particles producing the different parabolas. We can avoid any uncertainty as to the position of the zero by taking two photographs, the electrostatic field remaining the same in the two, while the magnetic field in the first photograph is equal in magnitude but opposite in direction to that in the second. Thus each kind of particle will now give two parabolic arcs, as in Fig. 38, and the distance between two points AB situated on the same vertical line will be twice the vertical deflection due to either magnetic field. As these arcs are much finer than the central spot, the distant AB can be measured with greater accuracy than either deflection separately.
The advantages of the method are illustrated by the photo- graphs reproduced in Figs. 47 and 48, Plate IV. These re- ' present the parabolas obtained when the discharge passes through the residues of liquid air; Fig. 47 represents the curve for the residues which had been treated so as to include the heavier constituents of the atmosphere; Figs. 48, Plate IV. and 49, Plate V. when the treatment had been such as to retain the lighter constituents.
When the plate for the heavier gases is measured up, it shows a faint line corresponding to the atomic weight 128 (xenon), a very strong line corresponding to an atomic weight 82 (krypton), a strong argon line 40, and the neon line 20. There are no lines on the plate which cannot be ascribed to known elements, and hence we may conclude that In the atmosphere there are no unknown gases of large atomic weight occurring In quantities comparable with those of xenon and krypton. This is a good example of the convenience of this method of analysis, for a single photograph reveals at a glance all the gases In the sample analysed. This photograph shows very plainly the existence of multiply charged atoms of the monatomic gases. The neon parabola extends towards the vertical to within half the normal distance of the heads of the parabolas; this shows that some of the neon atoms carry a double charge, and this is confirmed by the presence of a line on the plate for which e/m has twice the value corresponding to the neon line. The argon parabola approaches the vertical even more closely than that representing neon, as it begins at a distance from the vertical only one-third the normal distance, showing the argon atom can have as many as three charges. The krypton line approaches to within one-quarter of the normal distance showing that the krypton atom may have as many as four charges. We see from this how the maximum charge acquired by atoms of elements belonging to the same group increases with the atomic weight of the element
Let us now consider the photograph taken with the lighter constituents (Fig. 48, Plate IV.): here we find the line corresponding to helium; to neon, this is very strong and there is also a line corresponding to the neon atom with a double charge; to argon, and in addition there is a line corresponding to an element with an atomic weight 22. A molecule of carbonic acid with a double charge would give a line in this position, but this cannot be the origin of the line as the carbonic acid can be removed from the gas without producing any change in the brightness of the line. This line is much fainter than the neon line so that in the atmosphere the quantity of the gas which is the source of this line is small compared with that of neon.
The origin of this line presents many points of interest: there is no recognized element with this atomic weight, nor are there any compounds of recognized elements which would satisfy these conditions. It must, I think, be a new element.
For though the compound NeH2 would have the required mass, there is strong evidence that the Is due to an element. Thus we find on the plate another line for which m/e =11, and the line for which m/e = 22 has a prolongation half way to the axls, showing that the particle exists with a double charge; this Is a frequent occurrence with an atom of an element but I do not know of any case of a molecule of a compound possessing more than one charge.
If we accept Mendeleefs table there is no room for an element with such an atomic weight as 22 unless we suppose that near neon we have a group of two or more elements with similar properties, just as in another part of the table there Is the group iron, nickel, and cobalt
Mr. F. W. Aston has made at the Cavendish Laboratory many attempts to separate this new gas from neon whose atomic weight Is 20. The method he first tried was to fractionate a mixture of the two gases by means of their absorption by cocoanut charcoal cooled by liquid air. This absorption in the case of most gases Increases with the atomic weight, and though the difference between 20 and 22 — the atomic weights of the two gases In the mixture — is but small, he devised an apparatus by means of which the absorption was repeated so frequently that If there had been as great a difference In the absorption as from the analogy with other gases we might have expected from the difference in their atomic weights, the proportion between neon and the new gas would have been appreciably altered by the treatment. He could, however, find no difference whatever In this proportion before and after fractlonation. To measure this proportion he used two methods, (1) by comparing the intensity of the two lines In the positive ray photograph and (2) by measuring the density of the mixture by means of a specially constructed quartz balance which would have detected an alteration of very few per cent in the proportion between the two.gases. We conclude, therefore, that the physical properties of the two gases are much more nearly equal than we should have expected from their atomic weights.
Another method of fractionation. used by Mr. Aston was more successful, this was to allow the mixed gases to diffuse through a porous substance such as the stem of a clay tobacco pipe. The lighter constituent diffuses faster than the heavier one and by this means he obtained sufficient alteration in the proportion between the two gases to produce appreciable changes in the relative brightness of the two lines on the positive ray photograph, and changes in the density large enough to be detected by the quartz balance. No difference, however, could be observed in the spectrum of the mixture, and this in conjunction with the failure of the cooled charcoal to produce any separation gives some grounds for the suspicion that the two gases, although of different atomic weights, may be indistinguishable in their chemical and spectroscopic properties. There are several products of radio-active transformations such as radio-lead and thorium which have different atomic weights and are supposed to be inseparable from each other by any chemical process.
As another example of the method we will take its application to the investigation of the gases given off when solids are bombarded by cathode rays. The apparatus used for this is shown in Fig 13. B is the vessel in which the positive rays are produced. A is a vessel communicating with B by two tubes, one of which is a very fine capillary tube while the other is 5 or 6 mm. in diameter; taps are inserted so that one or both of these tubes may be closed and the vessels isolated from each other. The vessel A contains a curved cathode such as are used for Röntgen ray focus tubes, and the cathode rays focus on the platform on which the substance to be bombarded is placed. After the metal or other solid under examination has been placed on the platform, the taps between A and B are turned and A is exhausted by a Gaede pump until the vacuum is low enough to give cathode rays. The electric discharge is then sent through A and the solid on the platform bombarded. The result of the bombardment is that in a few seconds so much gas, mainly CO2 and hydrogen, is driven out of the solid that the pressure gets too high for the cathode rays to be formed, and unless some precautions to lower the pressure were taken the bombardment would stop. To avoid this a tube containing charcoal cooled by liquid air is connected with A, the cooled charcoal absorbs the CO2, and enough of the hydrogen to keep the vacuum in A low enough to give cathode rays.
To see what gases are given off in consequence of the bombardment, a photograph of the positive rays is taken when the connexion between A and B is cut off. After this is finished,, and when the bombardment has gone on for several hours, the taps between A and B are turned and the gas from A is allowed to go into B; another photograph is taken. The lines in the second photograph which are not in the first represent the gases which have been liberated from the solid by the bombardment with cathode rays. Fig. 50, Plate V., represents two such photographs; (a) that taken before turning the tap and (b) after. In (b) there are the following lines which do not occur in (a): (1) a very strong line corresponding to a substance with atomic weight 3; (2) one corresponding to helium, atomic weight 4, generally much fainter than the 3 line, and (3) lines representing neon with one and two charges. The amount of helium and neon are so small that their spectral lines were not visible when the discharge through the tube was examined with the spectroscope. Nearly every substance I have tried, including platinum, palladium, aluminum, copper, zinc, Iron, nickel, silver, gold, lead, graphite, diamond dust, lithium chloride, two specimens of meteorites and a large number of metallic salts, give out, when first they are bombarded, helium as well as the substance giving the 3 line; the amount of helium given off generally falls off very considerably after the bombardment has been prolonged some hours; the substance giving the 3 line is, however, much more persistent, and In some cases, for example that of KHO, the bombardment may be prolonged for several weeks without any diminution In the rate of evolution of the gas.
The presence of mercury vapour In the vessel A diminishes very much the intensity of the 3 line; hence we may conclude, I think, that the substance which gives the 3 line combines with mercury vapour when an electric discharge passes through a mixture of the two gases. Another case where the presence of one gas causes the disappearance of the lines due to another is that of oxygen and mercury vapour; the mercury lines are not seen in the photographs of the positive rays when the gas in the tube Is mainly oxygen, although with most gases these are about the strongest lines on the photographic plate. The disappearance of the mercury lines In this case may easily be explained by the combination of the mercury vapour with the oxygen.