38o 



NA TURE 



[February i8, 1892 



the Eustachian tubes. Difference-tones, on the other hand, 

 seem to be only perceptible when the tones of both forks affect 

 the same ear. The same number of the Studien contains the 

 first instalment of an elaborate article by Dr. J. Merkel upon 

 the psychophysical error-methods. 



If we may judge from the progress of the Photographic 

 Society of India, photography is rapidly becoming more 

 popular among Anglo-Indians. In January 1891, there were 

 277 members on the Society's books ; now there are over 310. 



The Oesterreichische Botanhche Zeitschrift for February con- 

 tains a very full account of the results of Porta and Rigo's visit 

 to Spain in 1891, and of the species gathered by them on their 

 journey. 



We have received Parts 10 and 11 of the " Illustrations of 

 the Flora of Japan," published at Tokyo. The drawings con- 

 tinue excellent ; the diagnoses are unfortunately in Japanese. 



Messrs. Dulau and Co. have issued a catalogue of botanical 

 works which they offer for sale. It contains the titles of about 

 3000 writings relating to geographical botany. 



The "Electrical Trades Directory," issued by Mr. George 

 Tucker on behalf of the Electrician, has made its tenth annual 

 appearance ; and no effort has been spared to bring it up to 

 date. The biographical division of the work contains sketches 

 of the careers of 260 men who are well known in the electrical 

 world. These sketches have all been revised by the subjects of 

 them. No fewer than 28 of these notices are accompanied by 

 portraits, among which is a portrait of Prof. Ayrton from a 

 specially engraved steel plate. 



The Royal University of Ii-eland has issued its Calender for 

 the year 1892. 



An experiment, illustrating the remarkable power possessed 

 by palladium of occluding hydrogen, is described by Prof. Wilm, 

 of St. Petersburg, in the current number of the Berichte of the 

 German Chemical Society. The experiment is so simple, and 

 requires so short a time to exhibit, that it would appear to be 

 eminently suitable for lecture demonstration. The metallic 

 palladium is employed in the finely divided state obtained by 

 heating the easily prepared yellow crystals of the compound 

 PdClj . 2NH3, first in the open air, and subsequently for a 

 short time in an atmosphere of hydrogen. A small quantity, 

 about four grams in weight, of the palladium so obtained is 

 placed in a bulb blown at the bend of a U-shaped tube. The 

 extremity of one limb of the U-tube is bent round at right angles, 

 andconnected with a wash-bottle containingsulphuric acid, which 

 in its turn is connected with a Kipp's apparatus generating 

 hydrogen from zinc and dilute sulphuric acid. The wash-bottle 

 serves not only to dry the hydrogen, but also to indicate the 

 speed of the current of gas. The extremity of the other limb 

 of the U-tube is narrowed to a capillary, and terminates with a 

 tightly-fitting stopcock and jet. In commencing the experiment, 

 the hydrogen current is started, and then, first the metal, and 

 afterwards the whole U-tube, is carefully heated with a Bunsen 

 flame in order to remove the moisture formed by the action of 

 the hydrogen, under the influence of the palladium, upon the 

 oxygen of the air contained in the apparatus. When all 

 the air and moisture are thus driven out of the apparatus, 

 an attempt may be made to ignite the issuing hydrogen at 

 the jet above the open stopcock. It will be found, how- 

 ever, that even while the metal is hot and the stream 

 of hydrogen very rapid, a constantly burning flame can- 

 not be maintained at the jet with the stopcock fully open ; 

 instead, a series of somewhat explosive ignitions and sudden 

 extinctions occurs. It is only when the stopcock is turned so as 

 to reduce the exit of the gas to a minimum ihal a constantly 

 NO. II 64, VOL. 45] 



burning jet can be obtained, the hydrogen in contact with the 

 palladium being then subjected to a certain amount of compres- 

 sion. The palladium is now heated a little more strongly, just 

 above bright redness, when it is no longer capable of occluding 

 hydrogen, and then the lamp is withdrawn, and after a few 

 seconds the stopcock closed. The occlusion is then demon- 

 strated in a most striking manner, for the stream of hydrogen 

 continues to bubble through the sulphuric acid bottle and into 

 the U-tube for several minutes with its original rapidity, al- 

 though all exit is prevented by the closing of the stopcock. At 

 length, however, the occlusion diminishes, and the stream of 

 hydrogen gradually becomes slower and slower, until it entirely 

 ceases, the palladium having regained the temperature of the 

 room, and become saturated with hydrogen at this temperature. 

 If now the stopcock is opened, and the metal again heated, 

 upon applying a^ flame to the jet, the issuing hydrogen evolved 

 from the palladium takes fire, and burns with a tall flame which 

 remains constant for some minutes, then, as the hydrogen 

 stored in the palladium becomes exhausted, diminishes in size, 

 and finally disappears. The moment the flame is removed 

 occlusion instantly commences again, and the experiment may 

 be repeated any number of times with undiminished effect. 



The additions to the Zoological Society's Gardens during the 

 past week include a Herring Gull {Larus argentatus), six 

 Common Gulls {Lams canus), five Black-headed Gulls {Lams 

 ridibundus), British, presented by Mr. T. A. Cotton, F.Z.S. ; a 

 Roseate Cockatoo (Cacatua roseicapilla) from Australia, a Grey 

 Parrot {Psiltactis erithacus) from West Africa, presented by 

 Mrs. Hennah ; a Red-winged Parrakeet {Aprosinictus erythro- 

 pterus) from Australia, presented by Lieut. -Colonel R. J. H. 

 Parker, R.E. ; a Cape Dove {CSna capcnsis) from South Africa, 

 presented by the Rev. George Smith. 



OUR ASTRONOMICAL COLUMN. 



Photographic Magnitudes of Stars. — The Astronomer- 

 Royal communicated a paper " On the Relation between Dia- 

 meter of Image, Duration of Exposure, and Brightness of 

 Objects in Photographs of Stars taken at the Royal Ob- 

 servatory, Greenwich," to the Royal Astronomical Society in 

 January. First, with regard to the relation between the diameter 

 of the image of a star on the photographic plate and the time 

 of exposure. Measures of a large number of stars have led to 

 the development of the following empirical formula for the 

 same star with different exposures — 



sjd = I '03 log / + const., 

 where d = the diameter of a stellar image in seconds of arc, 

 and i the exposure in seconds of time, the magnitude, m, being 

 expressed on Pogson's scale. This relation represents the 

 observations through a range of eight magnitudes, with a mean 

 apparent ei'ror of less than one-twentieth of a magnitude. By 

 using the measures from which the above formula was deduced, 

 and photometric determinations of magnitude made at Bonn 

 and Oxford, data were obtained for determining whether there 

 was a constant relation between duration of exposure and 

 brightness of star photographed, whether, in fact, for equal 

 diameters of images — 



Exposure x brightness = const. 



The relation found from the comparison was 

 o"4 X m — 0'97 log i + const. 

 This agrees very well with 



0'4 X ;;/ = log ( f const., 

 in which 0*4 is the logarithm of the number expressing the 

 magnitude-ratio. By combining the two relations developed, it 

 follows that, for the same exposure — 



Magnitude of star = const. - 243 x V diameter. 

 And finally the three formulre are expressed by the following : — 



m — 2"5(l0ii i - og-j ,^'d} + con^t., 

 connecting magnitude, oiametcr of image, and time of exposure. 



