June 14, 1 888] 



NATURE 



167 



helped to bring them out. They seem to come out more 

 strongly under the conditions which make both the green bands 

 of the oxide and the b group show well. 



Wo have not noticed the more refrangible triplet (A36337 to 

 3620 "6 about) under other circumstances, but the triplet near M 

 is produced when magnesia is held in the flame of cyanogen 

 burning in oxygen, in the flash of pyroxylin with which 

 magnesium filings have been mixed, or which has been treated 

 with an alcoholic solution of magnesium chloride. 



It is not only very strongly developed, but shows strongly re- 

 versed on our photographic plates, in the spectrum of the arc 

 from a Siemens dynamo taken between electrodes of magnesium 

 in oxygen ; and most of the accompanying ultra-violet bands of 

 the magnesium flame spectrum are at the same time reversed. 

 It is less strongly, but distinctly, reversed in the spectrum of the 

 same arc taken in air, in carbonic acid gas, and in sulphurous 

 acid gas. It appears also if the arc is taken in ordinary nitrogen 

 unless great precautions are taken to exclude all traces of oxygen 

 or carbonic acid, when it completely disappears. It is developed 

 also in the flash produced when a piece of magnesium ribbon is 

 dissipated in air by the discharge through it of the current from 

 50 cells of a storage battery. Also in the spark in air at atmo- 

 spheric pressure between magnesium electrodes connected with 

 the secondary wire of an induction coil when the alternating 

 current of a De Meritens magneto-electric machine is passed 

 through the primary. 



In two cases, but only two, we have found this triplet, or 

 what looks like one or both of the more refrangible of its lines, 

 developed in vacuous tubes. In both tubes the gas was air. 

 One had platinum electrodes and a strip of magnesia from burnt 

 magnesium disposed along the tube ; the other had fragments of 

 the Dhurmsala meteorite attached to the platinum electrodes. 

 The discharge was that of an induction coil worked in the usual 

 way without a Ley den jar. In each case it is only in one photo- 

 graph of the spectrum that the lines in question appear. In 

 other photographs taken with the same tubes they do not show. 



On the other hand, this triplet does not make its appearance 

 in the arc from a dynamo between magnesium electrodes in 

 hydrogen, coal gas, cyanogen, 1 chlorine, hydrochloric acid, or 

 ammonia ; nor in the arc from a De Meritens machine in hydro- 

 gen or nitrogen. It does not show in the spark between mag- 

 nesium electrodes of an induction coil used in the ordinary way, 

 either with or without a Leyden jar, in hydrogen or in air at 

 atmospheric pressure ; nor in the glow discharge in vacuous tubes 

 with magnesium electrodes when the residual gas is either air, 

 oxygen, hydrogen, carbonic acid gas, or cyanogen. Nor does 

 it appear, except in the one instance above-mentioned, in the 

 glow discharge in highly rarefied air in a tube containing either 

 magnesia or a strip of metallic magnesium. 



A review of all the circumstances under which the triplet near 

 M and its associated bands appear, and of those under which 

 they fail to appear, leads pretty conclusively to the inference 

 that they are due not to merely heated magnesium but to the 

 oxide, or to vibrations set up by the process of oxidation. 



We have expended a vast amount of time and trouble over 

 vacuous tubes, and our later experiments do but confirm the 

 opinion which we had previously formed that there is an un- 

 certainty about them, their contents and condition, which makes 

 us distrustful of conclusions which depend on them. Photo- 

 graphs of the ultra-violet spectra given by such tubes tell tales of 

 impurities as unexpected as they are difficult to avoid. Every 

 tube of hydrogen which we have examined exhibits the water 

 spectrum more or less, even if metallic sodium has been heated 

 in the tube, or the gas dried by prolonged contact with phos- 

 phoric oxide. Indeed the only tubes which do not show the 

 water spectrum have been filled wij.h gases from anhydrous 

 materials contained in a part of the tube itself; and even when 

 tubes have been filled with carbonic acid gas from previously 

 fused sodium carbonate and boracic anhydride the water spec- 

 trum is hardly ever absent. The last traces of the ultra-violet 

 bands of nitrogen are almost as difficult to be rid of with 

 certainty. Frequently, unknown lines or bands make their 



1 In taking the arc in this way in cyanogen our photographs show the 

 whole of the five bands of cyanogen between K and L well reversed. We 

 have before noticed (Roy. Soc. Proc, vol. xxxiii. p. 4) the reversal of the 

 more refrangible three of these bands against the bright background of the 

 expanded lines of magnesium when some of that metal was dropped into the 

 arc between carbon electrodes, but in taking the arc between magnesium 

 electrodes in an atmosphere of cyanogen the bright wings of the expanded 

 magnesium lines near L extend beyond the cyanogen bands, and the whole 

 series of the latter are well reversed. (May 23.) 



appearance, and the same tube will at different times exhibit 

 wholly different spectra. This is especially the case with tubes 

 of rarefied gases which oppose much resistance to the passage of 

 the electric discharge, such as oxygen. 



The ultra-violet magnesium lines which we have observed in 

 vacuous tubes with magnesium electrodes, when the induction 

 coil, without jar, is employed, are the triplets at A.3837, and the 

 lines A.2852, 2802, and 2795. These appear whether the residual 

 gas be air, oxygen, hydrogen, or carbonic acid. When a jar is 

 used we have obtained also the triplets at P and S, the pair 

 about A.2935 and 2927, and the quadruple group near A.2802 and 

 the quintuple group beyond, and in one case only, in oxygen, the 

 group near s, described below, and the flame-triplet near M. 

 When no jar is used sometimes only A2852 is to be seen, some- 

 times A2852 and the strong pair near A.2802, and sometimes also 

 the triplet near L. We infer, therefore, that this is the order of 

 persistency of these lines under the circumstances. 



Group near " s." 



In their list of lines in the spectrum of magnesium (Phil. 

 Trans., 1884, p. 95) Messrs. Hartley and Adeney have given 

 two lines, A307i"6and A. 3046*0, which we had not heretofore 

 observed either in the spectrum of the flame, arc, or spark of 

 magnesium ; but in our recent observations we have noticed in 

 many cases a well-marked line which, by interpolation between 

 neighbouring iron lines, appears to have a wave-length about 

 3073 '5, and a pair of narrow bands sharply defined on their less 

 refrangible sides at wave lengths about 3050-6 and 30467, and 

 fading away on their more refrangible sides. 



The circumstances under which this group is seen and is not 

 seen, do not seem to indicate that its emission is connected with 

 any particular temperatures so much as with the character of the 

 electric discharge, and perhaps aho with the density of the 

 magnesium vapour. 



Royal Microscopical Society, May 9. — Dr. C. T. Hud- 

 son, President, in the chair. — The President said that on the 

 occasion of his taking the chair for the first time, he desired, 

 before beginning the business of the evening, to thank the 

 Fellows very heartily for the honour which they had done him 

 in electing him their President. — Mr. Crisp exhibited a form of 

 camera lucida by M. Dumaige, of Paris, fitted in a box with a 

 cover, which, when closed, kept the prism and mirror free from 

 dust ; also by the same maker, an adapter with spiral springs, 

 for rapidly changing objectives, and a portable microscope in 

 which the foot and stage were in one piece. — Dr. Kibbler ex- 

 hibited and described a new stand and camera, which, he 

 believed, would be found very useful for photomicrography. It 

 had been made to his design by Mr. Bailey, his idea being that 

 it was best not to take negatives upon a large plate, but on a 

 quarter-plate first, and afterwards to enlarge the pictures from the 

 original negatives. The great advantage of this method was in 

 the amount of light gained for the purpose of focussing. The 

 quarter-plate size was also the proper one for lantern slides. The 

 ordinary diaphragm plate placed immediately below the stage 

 he had found entirely useless, but by removing it a certain 

 distance from the object it then ceased to cut off the field, and 

 began to reduce the light and to improve the penetration and 

 definition. With high powers this answered very well, but it 

 would not work with low powers unless the diaphragm was 

 removed to a distance too great to be convenient in practice. 

 He had therefore devised the plan of introducing a short I^inch 

 condenser behind the stage, and about 3 inches in front of the 

 diaphragm plate, in this way throwing it out of focus. The effect 

 of this was that the same improvement in penetration and 

 definition was obtained, but on a much shorter distance. Atten- 

 tion was also called to a method of clamping the object in 

 position when the focus had been obtained ; also to a plan for 

 obtaining a fine adjustment by a tangent screw. — Mr. Mills's 

 note on a sponge with stelliform spicules was read. — Mr. Crisp 

 referred to some comments which had recently been made in 

 America upon the advantages of the method of tilting the stage 

 of the microscope as a means of obtaining a very economical and 

 simple fine adjustment, on which some discussion took place. — 

 Dr. A. C. Stokes's paper on new Infusoria Flagellata from 

 American fresh-waters, containing descriptions of twenty new 

 species, was read. — A paper on the Foraminifera of the Red 

 Chalk, by Messrs. H. W. Burrows, C. D. Sherborn, and Rev. 

 G. Bailey, was also read. 



