242 



Profs. G. D. Liveing and J. Dewar. [May 31, 



the correctness of the received opinion that the temperature of the 

 spark discharge is much higher than that of the arc. The greater 

 mass of the incandescent matter in the arc may be expected to give 

 a greater number of lines, because the gradations of temperature 

 will be less steep than in a smaller mass, and we shall have from 

 the outer part of the mass the light which is emitted at compara- 

 tively low temperatures, while from the inner part we shall get those 

 rays which are only produced by the highest temperatures. More- 

 over, compounds which may be dissociated in the interior of the mass 

 may be re-formed in the outer part, and produce their characteristic 

 emission or, in some cases, absorption spectra. 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 \5006'4 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.* It seems, therefore, that it is not 

 the character of the electric discharge, but the nature of the gas 

 which determines the appearance of the bands ; and the absence of 



* Neither the arc of a Siemens' dynamo, nor that of a De Meritens' magneto- 

 electric machine, when taken in a crncible of magnesia, shows these bands, even if 

 metallic magnesium be dropped into it. A stream of hydrogen 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 moment- 

 arily, but that is probably the result of the burning of the magnesium vapour out- 

 side the arc. — May 23. 



