i66 



NATURE 



[June 14, 1888 



and filled from time to time with various gases. The arc was 

 an instantaneous flash which could not be repeated more than 

 twice without rendering the sides of the vessel opaque with a 

 complete coating of magnesium. It was therefore analogous to 

 an explosion of magnesium vapour. The strong blue line 

 A4481, two pairs about A3S95, 3893, and A3855, 3848, the strong 

 pair about A2935, 2927, and the two weaker lines of the quad- 

 ruple group, namely A2789'9 and 2797, all come out in the arc 

 given by a Siemens dynamo between magnesium electrodes in 

 air, in nitrogen, and in hydrogen. We have observed most of 

 them also when the arc is taken in carbonic acid, in ammonia, 

 in steam, in hydrochloric acid, in chlorine, and in oxygen. The 

 observations render doubtful the correctness of the received 

 opinion that the temperature of the spark discharge is much 

 higher than that of the arc. Heat, however, is not the only 

 form of energy which may give rise to vibrations, and it is 

 probable that the energy of the electric discharge, as well as 

 that due to chemical change, may directly impart to the matter 

 affected vibrations which are more intense than the temperature 

 alone would produce. 



The Bands of the Oxide. 



The set of seven bands in the green, beginning at about 

 A50o6 - 4 and fading towards the violet side of the spectrum, 

 which we have before attributed to the oxide of magnesium, have 

 been subjected to further observation, and we have no reason to 

 doubt the correctness of our former conclusion that they are due 

 either to magnesia or to the chemical action of oxidation. On 

 repeating our experiments with the spark of an induction coil 

 between magnesium electrodes in different gases at atmospheric 

 pressure, we could see no trace of these bands in hydrogen, 

 nitrogen, or ammonia, whether a Leyden jar was used or not. 

 Nor could we see them at all in carbonic oxide, but in this case 

 the brightness of the lines due to the gas might prevent the 

 bands being seen if they were only feebly developed. On the 

 other hand, the bands come out brilliantly when the gas is 

 oxygen or carbonic acid, both with and without the use of a 

 Leyden jar. In air and in steam they are less brilliant, but 

 may be well seen when no jar is used. When a jar is used they 

 are less conspicuous, because in air the lines of nitrogen come 

 out strongly in the same region, and in steam the F line of 

 hydrogen becomes both very bright and much expanded. 1 It 

 seems, therefore, that it is not the character of the electric dis- 

 charge, but the nature of the gas which determines the appear- 

 ance of the bands ; and the absence of the bands in the absence 

 of oxygen, and their increased brilliance in that gas, leave little 

 room for doubt that they are due to the oxide, or to the process 

 of oxidation. If a very small piece of magnesia, such as a 

 fragment of the ash of burnt magnesium ribbon, be held 

 in an oxy-hydrogen jet, most of the spectrum of burning 

 magnesium is developed in the flame for a short distance 

 from the piece of magnesia. Under these circumstances, the 

 flame shows the b group and the magnesium-hydrogen series 

 close to it, the bands in the green, the triplet near L, the 

 triplet near M of the flame of burning magnesium, with the 

 group of bands in that region, and the line A2852. It is remark- 

 able that the proportions in which the oxygen and hydrogen are 

 mixed affect the relative intensities of different parts of the 

 spectrum. In general, both the metallic lines of the b group 

 and the bands of the oxide are easily seen ; but if the oxygen 

 be in excess the bands of the oxide come out with increased 

 brightness, while the b group fades or sometimes becomes in- 

 visible. On the other hand, if the hydrogen be in excess the 

 bands fade, and the b group shows increased brilliance. There 

 can hardly be much difference in the temperature of the flame 

 according as one gas or the other is in excess, but the excess of 

 oxygen is favourable to the formation and stability of the oxide, 

 while excess of hydrogen facilitates the reduction of magnesium 

 and its maintenance in the metallic state. As regards tempera- 

 ture, it should be observed that while substances merely heated 

 by the flame, and not undergoing chemical change, are not likely 

 to rise to a temperature above the average temperature of the 

 flame, it will be otherwise with the materials of the flame itself 

 and other substances in it which are undergoing chemical change, 



1 Neither the arc of a Siemens dynamo, nor that of a De Meritens 

 magneto-electric machine, when taken in a crucible of magnesia, shows these 

 bands, even if metallic magnesium be dropped into it. A stream of hydro- 

 gen led into the crucible with a view to cool it does not elicit them. When 

 the arc is taken in the open air, and metallic magnesium dropped through it, 

 the bands appear momentarily, tut that is probably the result of the burning 

 cf the magnesium vapour outside the arc. (May 23.) 



and have at the instant of such change the kinetic energy due to 

 the change. 



In fact, when chemical changes are occurring in a flame it 

 cannot be taken for granted that the temperatures of the mole- 

 cules are all alike, or that the vibrations which they assume are 

 the result of heat alone. On the other hand, the temperature 

 of the metal separated from magnesia by the oxyhydrogen flame 

 cannot, we suppose, be at a temperature higher than that of the 

 hottest part of the flame. We are therefore inclined to think 

 that the metallic lines (b) are manifested at a lower temperature 

 than the bands of the oxide ; and the appearance of a line in the 

 position of the first band without any trace of the second band 

 (which is nearly as bright as the first), and without any trace of 

 the b group, is quite sufficient to create a suspicion of mistaken 

 identity when Mr. Lockyer ascribes the sharp green line in the 

 spectrum of nebulae to this band of magnesia. This suspicion 

 will be strengthened when it is noticed that the line in question 

 is usually in the nebulae associated with the F line of hydrogen, 

 if it be borne in mind that the spark of magnesium in hydrogen 

 does not give the bands, and that the oxyhydrogen flame hardly 

 produces them from magnesia when the hydrogen is in excess. 



In Mr. Lockyer's map of the spectrum of the nebula in Orion 

 (Roy. Soc. Proc. vol. xliii. p. 134), he has 'represented three 

 lines in the position of the edges of the first three of these 

 bands. If these three lines were really seen in the nebula, there 

 would be less room to doubt the identity of the spectra ; but the 

 authorities quoted for the map {loc. cit., p. 142) mention only a 

 single line in this position. 



When the flame of burning magnesium is viewed with a high 

 dispersion, these bands are resolved into series of fine, closely 

 set lines. Seven such series may be counted, beginning at the 

 approximate wave-lengths 5006*4, 4995 '6, 4985*4, 4973*6, 

 4961*6, 4948*6, 4934*4, respectively. When a condensed spark 

 is taken between magnesium electrodes in oxygen mixed with a 

 little air, the pair of strong nitrogen lines may be seen simul- 

 taneously with the bands, and lying within the first band, the 

 bright edge of the band being somewhat less refrangible than 

 the less refrangible of the two nitrogen lines. 



When the bands are produced by the spark discharge between 

 magnesium electrodes in oxygen or other gas, we have not been 

 able to resolve them into lines, but the whole amount of light 

 from the spark is small compared with that from the flame, and 

 besides it is possible that the several lines forming the shading 

 may be expanded in the spark, and thus obliterate the darker 

 spaces between them. 



Triplet near M and adjacent Bands. 



Our former account of the spectrum of the flame of burning 

 magnesium included a description of a triplet near the solar line 

 M, and a series of bands extending from it beyond the well- 

 known triplet near L. As we had not observed these features in 

 the spectrum of the spark or arc, and could not trace their con- 

 nection with any compound, we concluded that they were pro- 

 duced by magnesium only at the comparatively low temperature 

 of the flame. We have since found that they are not produced 

 by the metal at that temperature only, but are exhibited as 

 strongly, or even more strongly, in the arc between electrodes 

 of magnesium. In the latter case they appear concurrently 

 with the line at 4481 and other lines which seem to belong to 

 high temperatures. We must therefore regard them as not only 

 produced at the temperature of flames, but as persistent at 

 temperatures very much higher. 



The different circumstances under which we have observed this 

 triplet are as follow : — 



In the oxyhydrogen flame when a very small piece of mag- 

 nesia is held in it. In this case the outer two lines of the triplet 

 are much stronger than the middle line (A3724 about), which in 

 some of our photographs does not show at all. It should be 

 noticed that the least refrangible of the three lines (A3730 about) 

 is in general more diffuse and not quite so bright as the two 

 more refrangible lines. Magnesia in the oxyhydrogen flame 

 also gives rise to some bands close to and more refrangible than 

 the triplet, and to another still more refrangible but less bright 

 triplet, in which the lines are set at nearly equal distances from 

 each other, with the approximate wave-lengths 3633*7, 3626*2, 

 3620*6. These additional bands and triplets are not really 

 absent from the flame spectrum, for traces of them may be seen in 

 some of our photographs of the magnesium flame, but they seem 

 relatively brighter in the oxyhydrogen flame with magnesia, and 

 the longer exposure of the photographic plate in the latter case 



