1.The methods of investigating the motion of negative ions in gases at low pressure that have been explained in some previous papers may be extended to cases in which larger variations are made in the electric force and pressure. In order to find the kinetic energy of the motion of agitation of the ions, the velocity in the direction of an electric force, and the value of e/m for different forces and pressures, it is necessary to investigate experimentally two properties which are characteristic of the motion of electrons. These are the abnormal lateral diffusion of a stream of ions moving in a uniform electric field, and the deflection of the stream produced by a small transverse magnetic force. In the previous experiments the two phenomena were investigated separately and in each case with apparatus which gave satisfactory results when small electric forces were used and the pressures were limited to a certain range. In order to investigate the motion under larger forces an apparatus of more suitable dimensions was constructed, by means of which both the required sets of experiments may be made. 2. The negative action of ultra-violet light on the plate A, fig. 1, and after traversing the distance from A to B some of the ions passed through a narrow slit S, 2 mm. wide and 15 mm. long, in the centre of the metal sheet B. The electric force was in the same direction on the two sides of B, so that the ions, after passing through the slit, continue their motion towards the plane electrodes C, which were parallel to the plane of B. The electrodes C wT ere 4 cm. from B, and three flat rings, Ri, R2, £3, 7 cm. internal diameter, were fixed at distances of 1, 2, and 3 cm. respectively from the plane of the electrode C. A separate connection for each ring and for the plates A and B was brought out through a large ebonite plug fitted in the brass cover of the apparatus, and was maintained at a potential proportional to the distance of the corresponding ring, or plate, from the electrodes C. The stream of ions ------R* ------Ra -R, C, C£ Cj F ig.
An account was recently given of a method by which the velocities of both positive and negative ions produced by Rontgen rays could be measured in very dry gases,* and some measurements were given for ions in air. These measurements have now been extended to the cases of carbon dioxide and hydrogen, and some calculations, based on the work of Townsend,! show that, while the positive ions retain a large envelope of m atter even under the influence of a fairly large field of force, the negative ions become considerably diminished in size when their velocity is increased either by an increased force or by diminished pressure.W hile these results are in general accord with those of other observers, yet the point which has hitherto escaped notice is th a t the negative ions are far more readily deprived of their customary envelope in a very dry gas than in a comparatively moist atmosphere.In the paper already cited it was shown th at the velocity of ions in air is a function of X/^>, where X is the potential gradient, and p the pressure of air in the apparatus, and th at this function is independent of the actual values of either X or p, but is considerably affected by minute traces of moisture. Ibis result has been confirmed for ions in hydrogen and in carbon dioxide, and evidence has been obtained that the velocities of negative ions in air are affected by traces of carbon dioxide, while those in hydrogen are consider ably lowered by traces of air.These facts may be compared with the observations of Franck and Pohl,! who have shown that the velocities of negative ions in nitrogen and argon are very considerably lowered by moisture and other impurities. Unfor tunately these workers used Rutherford's method of estimating the mobilities of ions, and this involves the use of an alternating E.M.F. which is a sine function of the time. As our experiments have shown that the velocity of a negative ion is not a linear function of
IT is well known t.hat the colour of an indicator in solution depends, within certain limits, on the concentration of the hydrogen ion (or hydroxyl ion) in the solution. These limits, which vary greatly with the nature of the indicator? have been determined for a large number of indicators by Fels (Zeitsch. Elektrochem., 1904, 10, 208) and Salessky (ibid., p. 205), and a knowledge of them enables us t o choose, for any given volumetric operation, the indicator that will give the best results. Conversely, by testing a solution with a large number of indicators, we can arrive at an estimate of the concentration of hydrogen ions in it. Friedenthal (Zeitsch. EZeEtrochem., 1904, 10, 114) and Salm (Zeitsch. physikal. Chem., 1906, 57, 471) have measured in this way the degree of dissociation of weak acids and weak bases, and have obtained results agreeing often to within a few per cent. of those obtained by conductivity measurements; but it can hardly be seriously proposed to supersede the latter, more accurate, although more laborious, method by the former, except when the acid or base under investigation is extremely weak. On the other hand, the determination of the degree of hydrolysis of salts in aqueous solution is often both laborious and at the same time subject t o large percentage errors; a simple colorimetric method would therefore be of considerable importance, provided that it could be made as accurate as, or more so than, the methods usually employed. Friedenthal's method cannot be considered accurate enough for this purpose; the difficulty of judging colours without employing a suitable apparatus is very great, and, moreover, it is impossible t o keep weakly acidic standard solutions of indicators (for comparison) unchanged for any length of time.It is far better to determine once and for all the relation between depth of colour and the concentration of the hydrogen ion.
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