196 



KN()\\ij:i)r,i:. 



Mav. 1912. 



objects for the microscope, especially when illiiiTiiiiiited by 

 reflected liRht. but arc scarcely satisfactory as an addition to 

 the photograph, to say nothing of the ufTect they may have on 

 the print itself. Some idea of their appearance may be gained 

 from the ilhislr.ition (I'ignre 228), which is a photomicrograph 

 taken by ri'llectcil light. The little white dots arc the spores. 

 The want of definition in parts is dne to differences of distanc<,' 

 of the im.ige planes, for although a narrow angle objective was 

 employed in order to obtain depth of definition, the distances 

 apart of different parts of the object were too great. When this 

 fungus was removed from the print by brushing or careful rub- 

 bing with a piece of cotton wool, the picture was in many cases 

 found to be nniiijured ; in others spots remained. Far more 

 serious is that effect of damp which causes a fading of the 

 print geiiemlly. brought about most probably by decomposition 

 products of the mountant itself in many cases, aided by small 

 cpiantities of the fixing .igent remaining in the print. Obviously 

 the best way of protecting prints from the troubles arising 

 from mildew is to Ueep them as far as possible in a dry and 

 pure atmosphere, as the fungus of which we have been writing 

 flourishes in a damp and unwholesome one. 



EXPOSUKE TABLE FOR MAY.— The calculations 

 are made with the actinograph for plates of speed 200 H and 

 D, the subject a near one. and lens aperture F16. 



PHYSICS. 



By .Alfred C. G. EoERroN, B.Sc. 



CLOW IN HYDROGEN.— Hertz had observed a glow in 

 hydrogen after the passage of an electric discharge from a 

 Leyden jar, the hydrogen being at a pressure of one hundred 

 millimetres of mercury. Professor Strutt, in continuation of 

 his work on the glow produced by electric discharges in 

 various gases, which has led to such important results in the 

 case of nitrogen, has investigated this glow in hydrogen. He 

 finds that it is due probably to traces of sulphuretted hydrogen, 

 which are decomposed by the discharge, and subsequently the 

 sulphur and hydrogen re-combinc and in doing so emit a 

 bluish light. 



THALLIUM FLAME.— Dr. Lowry has described an 

 ingenious method of obtaining the monochromatic green 

 thallium flame, for use in polarimetric measurements. 

 Thallium chloride is heated in a silica bulb, which is 

 connected by a metal tube to the tube of a Bunsen burner 

 and passes up within it, terminating in a vertical jet. A 

 current of oxvgen passes over the heated thallium chloride 

 and carries the vapour over into the flame, giving a steady 

 lasting colour to it. 



G. E. Gibson has heated metallic thallium to 1300°C in an 

 evacuated quartz tube. The vapour emits the green line 

 strongly, and it has been shown that this is a true temperature 

 radiation and not merely a luminescence eft'ect. The light 

 from the tube was focused on the slit of a spectroscope : after 

 the introduction of the cold tube containing the thallium, the 

 dark thallium line crosses the continuous spectrum of the 

 radiation from the furnace, but this gets fainter and disappears 

 as the temperature of the thallium vapour reaches that of the 

 furnace. In three minutes the black absorption line has 

 disappeared altogether with the temperature of the furnace 

 standing at 1450°, .and if tlie tube is suddenly removed from 



the furnace, the thallium line appears bright against a dark 

 background. It is not easy to obtain direct evidence of the 

 production of bright line spectra by the effect of temperature 

 alone, but this experiment shows the effect very beautifully. 

 Professor Wood has de.scribed an experiment, already men- 

 tioned in these coluiims, in which iodine is dropped into a hot 

 (juarlz bulb, the vapour of the iodine giving out a reddish 

 light and making the quartz bulb appear red hot. 



The nature of the light emitted by a flame depends on the 

 nature of the flame itself. A. Harnack has investigated the 

 spectra of various metals in the oxyhydnigen flame, and has 

 compared them with the spectra obtained in the hydrogen- 

 chlorine flame, which has a temperature of 2300°. Only 

 chloride bands are connnon to both flames : thus the copper 

 chloride bauds in the oxyhvdrogen flame would correspond to 

 the copper bands in the hydrogen chlorine flame. The 

 hydrogen-chlorine flame spectra of the metals gives rise to 

 comparatively few lines. The chloride band spectra are more 

 fully developed in the hydrogen-chlorine flame than in the 

 oxyhydrogen flame in which the chlorides of the metals are 

 burnt. 



RADIOACTIVITY.- There is no well-established case 

 where radioactivity is caused by artificial means. In 

 speaking of flames we may refer to the work of Carter, 

 published in The Philosophical Magazine of November 

 last in this connection. Searching tests have been made 

 to find whether any ^ rays are emitted when large changes of 

 energy occur during the coinbustion of substances giving rise 

 to high temperature flames, but no positive result has been 

 obtained. Special precautions had to be taken during these 

 experiments to prevent elTccts of temperature on the electro- 

 scope used for detecting the presence of the rays. In the 

 flame itself enormous ionisation is occurring — and no doubt 

 electrons are set free within the flame, but these are still part 

 of the processes going on within the flame, and no electrons 

 are projected out with great velocity, and lost to the body 

 projecting them, as in radioactive processes. The author 

 investigated the radiation from the electric arc, the spark, and 

 the oxyacetylene flame to search for an effect, but none was 

 obtained. 



Mention may be made here of a negative result obtained by 

 Dr. J. Vincent; he investigated whether the period of radio- 

 active change could be altered in anv way by the application 

 of powerful electric fields. No change of any kind was 

 observed. The spontaneous processes occurring within the 

 atom giving rise to radioactivity are still out of human control, 

 but every week adds know-ledge to the nature of these 

 processes. Professor Rutherford's, and his collaborators', 

 work on the counting of the a particles emitted from radio- 

 active substances by scintillations produced on zinc sulphide 

 screens, and the scattering of a and fi p.articles by various 

 substances is giving rise to further knowledge of the internal 

 structure of the atom. 



The a rays from different substances are characterised by 

 their range or the average distance through which they travel 

 in air at the standard temperature and pressure — 0°C. 

 and seven hundred and sixty millimetres. Geiger and Nuttall 

 have investigated the ranges of a particles from uranium and 

 thorium and have found that they are proportional to the 

 transformation constants — the constants of the time of change 

 from the one radioactive substance into its successor — and 

 since no o ray products are known of very short range the 

 explanation is probably to be sought here, for the life of a 

 substance emitting a rays of one centimetre range would be 

 so long that its period of change would be very slow and its 

 activity would escape detection. 



FOG. — Dr. Aitken has shown that the sun causes the pro- 

 duction of fog when a light wind brings an impure and damp 

 air into the neighbourhood : while when the wind comes from 

 a pure direction, the sun causes no fog to appear. He has 

 come to the conclusion that the fogs are due to the action of 

 the sun on the sulphur products in the air produced by the 

 combustion of coal, and also to the sunshine forming hydrogen 

 peroxide in the air. In this way particles are formed which 

 can condense water vapour in air unsaturated with moisture. 



