430 



Profs. G. D. Liveing and J. Dewar. [Mar. 16, 



magneto-electric machine, see "Proc. Roy. Soc," vol. 30, p. 175) between 

 magnesinm electrodes wliicli have had fonr or five minutes' exposure. 

 These observations tend to confirm the theoretical view that alterations 

 of temperature cannot put a stop to any of the fundamental vibrations 

 of a molecule ; at the same time we cannot be sure that the impulses 

 communicated by an electric discharge may not be in some respects 

 different from those resulting from mere increment of temperature. 



There is, however, a further point for consideration, which is, 

 how far the presence of a mixture of molecules of different elements 

 affects the respective vibrations. This is a condition which obtains in 

 most or all of our observations of the arc in crucibles, as well as in 

 the solar atmosphere, so that it is important to see if any effects can be 

 traced to such a condition of matter. Indeed, in order to arrive at 

 any probable explanation of the variations observed in the spectra 

 of sun-spots and of the chromosphere, we require to study the pheno- 

 mena produced by such mixtures of vapours as exist in our crucibles, 

 and not merely the spectra produced by the isolated elements, either in 

 arc, spark, or flame. 



It is only on some such supposition as that above suggested that we 

 can account for the absorption lines produced by admixtures of mag- 

 nesium with sodium and potassium respectively (" Proc. Roy. Soc," 

 vol. 27, p. 353) ; and it is possible that the very remarkable effect 

 of hydrogen in producing the reversal of chromium hues (ib., 

 vol. 32, p. 405) and of other lines (ib., vol. 28, p. 472) is a result 

 of analogous action. We have more particularly observed the 

 effect of a current of hydrogen on the iron lines at wave-lengths 

 4918, 4919' 7, and 4923. These lines, as seen in the arc in a mag- 

 nesia crucible, usually have about the same relative strengths as are 



o 



shown in Angstrom's map of the solar spectrum; Tbalen gives their 

 intensities as 2, 1, 3 respectively. They are all developed simultane- 

 ously when iron is dropped into the crucible, the first being sometimes 

 reversed, the second frequently reversed for some time, the third much 

 strengthened but not reversed. After a time these effects die out, but 

 if now a very gentle current of hydrogen is led in through one of the 

 carbons perforated for the purpose, the line at 4919*7 is again strongly 

 reversed, that at 4918 expanded, while that at 4923 becomes very 

 bright but remains sharply defined. These effects of the hydrogen 

 were observed several times. In all cases the line at wave-length 4923 

 seemed to maintain about the same relative strength compared with 

 the other two lines, and never showed any variation at all correspond- 

 ing to the prominence it holds in Young's catalogue of chromospheric 

 lines, where it has a frequency of forty, while that at 4918 has only 

 half that frequency, and the strongest line of the three does not figure 

 at all * 



* Mr. Lockyer's figure (" Proc. Eoy. Soc," to]. 32, p. 205) accompanying his 



