October 13, 1923J 



NATURE 



557 



forming on the hand of a radiologist after too frequent 

 exposures to tubes of radium which he handled 

 during the course of clinical work. A new device 

 for the application of radium to the tonsils is described 

 by Stewart, a previous article in this issue upon the 

 treatment of neoplasms of the tonsil by Quick show- 

 ing that good results are obtained by methods which 

 ensure a thorough irradiation of the affected parts. 



Cretaceous Overfoeding in the Alpine Region. 

 — A detailed review of the results of recent observa- 

 tions on the Alpine overfolds, and particularly of 

 L. Kober's work on the deeply penetrating " Tauern- 

 fenster " in 1921, is given by A. Tornquist of Graz 

 in the Geologische Rundschau, vol. 14, pp. 110-145 

 (1923). The title, " Intrakretazische und alttertiare 

 Tektonik der ostlichen Zentralalpen," shows how 

 the movements that have produced successive over- 

 folded sheets have been traced back into the 

 Cretaceous period, the most striking evidence being 

 the unconformable deposition of the Gosau beds 

 upon the earliest overfolded series. The notice of 

 Kober's " Bau und Entstehung der Alpen " in 

 Nature, vol. 112, p. 322, gave some hint of these 

 conclusions. 



Meteorology in the East Indian Seas. — The 

 Meteorological Chart of the East Indian Seas for 

 September, recently issued by the Meteorological Office, 

 is of considerable interest. Winds and ocean currents 

 are dealt with in detail, together with the normal 

 atmospheric pressure and temperature of air and sea, 

 as well as other matter of importance to the navigator. 

 The chart comprises the Red Sea and covers the area 

 from the Cape of Good Hope to the China Sea and 

 Western Australia. It is well shown how under 

 normal conditions the ocean current responds to the 

 prevailing wind. The winds are under the direct 

 influence of the several areas of high and low baro- 

 meter, and in any position on the chart the seaman 

 can interpret the changes he is experiencing, in 

 normal circumstances, and can estimate how soon 

 he may expect a change of conditions. The observa- 

 tions used extend over a period of about sixty years. 

 On the back of the chart ocean currents are discussed 

 for the track between Honolulu and Fiji. Current- 

 roses are used on a system analogous to the wind- 

 roses on the face of the chart, a system somewhat 

 open to question, and for any extended alteration in 

 this direction expert knowledge is desirable, if 

 possible equal to that given to the general system, 

 hitherto used, of showing ocean currents. A com- 

 parison is given of temperature in fixed and in portable 

 screens on board ship. Probably the position of the 

 screen must be left chiefly to the commander of a 

 ship, with cautious suggestions. The usual form of 

 screen used at sea, supposing the single louvre screen 

 to be still in use, has to be screened itself, as single 

 louvres are not effective. 



Radioactivity and Solar Radiations. — It has 

 been asserted that radioactivity is independent of 

 all known physical agents ; but M. A. Nodon, in the 

 Comptes rendus of the Paris Academy of Sciences of 

 June II, describes additional experiments, which seem 

 to confirm his previously expressed view that the 

 process is greatly accelerated by very penetrating 

 radiations from the sun. These radiations are able 

 to penetrate a thin sheet of lead, the absorption 

 being greater the higher the atomic number of the 

 screen employed. The action is more marked during 

 periods of solar activity. 



Temperature of the Crookes Dark Space in 

 Glow Discharge. — Observations on the glow dis- 



NO. 2815, VOL. I 12] 



charge have recently been made in the Physikalisch- 

 Technischen Reichsanstalt at Charlottenburg by 

 Herr A. Giinther-Schulze. He measured the energy 

 delivered to the cathode, and there converted into 

 heat ; and found the ratio that it bore to the total 

 energy delivered to the cathode and the dark space ; 

 this amounted to 72 per cent, in argon at i'83 mm. 

 pressure, 39 per cent, in hydrogen at 2-37 mm., and as 

 much as 73-4 per cent, in nitrogen at 3-53 mm. If the 

 free path of the atoms corresponds to room tempera- 

 ture, or the dark space is cool, this ratio is about 

 20 per cent. The natural conclusion is that the dark 

 space is heated by the collisions taking place in it 

 between the positive ions and the gas molecules ; and 

 a calculation of the probable temperature, in the 

 case of one of the experiments with nitrogen, leads to 

 the figure 720° C. The electrical energy expended in 

 the dark space appears to be sufiicient to account for 

 this rise of temperature. The length of free path at 

 this temperature is such that a considerable propor- 

 tion of the ions pass through the dark space without 

 colliding with a molecule, and the number of average 

 free paths between the boundary of the dark space 

 and the cathode must be small. All this agrees with 

 the fact that, when the velocity of canal-rays is 

 measured, a marked proportion have the velocity 

 corresponding to the total cathode fall. The free 

 path of the electrons is four times as great as that of 

 the positive ions, so that most of them cross the dark 

 space without collision, and begin to produce ions 

 when they reach the negative glow, the maximum 

 number per electron being equal to cathode fall 

 divided by ionisation voltage. It follows that the 

 ratio of the electronic current to the ionic current in 

 the dark space is about i : 10. 



Free Path of Slow Electrons in Monatomic 

 Gases. — Using an incandescent cathode, a cylindrical 

 grid surrounding it, and a concentric cylindrical 

 anode at a voltage very slightly higher than the grid, 

 it is found that the form of the characteristic curve 

 showing the relation between anode current and grid 

 voltage is strongly influenced, in the case of argon, 

 by the abnormally long free path of very slow electrons 

 through this gas. Minkowski and Sponer in a paper 

 dated March 27, published in the Zeitschrift fur 

 Physik, give the curves obtained with argon, krypton, 

 xenon, neon, and helium. For the first three gases 

 there is a sudden rise in the current curve at zero 

 voltage, followed by a sudden drop ; with neon and 

 helium, there is a less marked sudden rise, followed by 

 a more gradual rise ; in all these cases, however, and 

 also in the case of mercury vapour, the shape of the 

 curve near zero volts is to be explained by the fact 

 that the free path of slow electrons is abnormally 

 long. In the case of the inert gases, certain sharp up- 

 ward bends in the succeeding portions of the curves 

 are interpreted as being due to the fact that the 

 electrons have reached the velocity required either to 

 excite the atoms or to ionise them ; and, making the 

 proper corrections, the voltages agree quite well with 

 those which direct measurement shows are needed to 

 produce these effects. When the electrons strike the 

 atoms in inelastic collision their velocities are reduced, 

 their free paths are increased, and, as a result, the 

 current becomes greater. With argon, a number of 

 these sudden jumps of current are observed, corre- 

 sponding to two different excitation voltages, to the 

 ionisation voltage and to twice one of the exciting 

 voltages or the sum of the two ; this implies that the 

 velocity of the electrons, at one of the last-mentioned 

 points, is such that it can collide with an atom 

 exciting it, and retain sufficient energy to excite 

 another atom. 



