474 On the Reversal of the Lines of Metallic Vapours. [Apr. 3, 



4302 was reversed, this being the only one of the well-marked group 

 to which it belongs which appeared reversed. On another occasion, 

 when charred strontium tartrate was used, the line wave-length 

 4877 was seen reversed, as well as the strontium line near it. Also 

 the lines wave-lengths 6161, 6121, have again been seen momentarily 

 reversed. 



With magnesium, no new reversals of the lines of the metal have been 

 observed by us ; but when a stream of hydrogen or of coal-gas was 

 led into the crucible, the line wave-length 5210, previously seen by 

 us in iron tubes, and ascribed by us to a combination of magnesium 

 with hydrogen, was regularly seen, usually as a dark line, sometimes 

 with a tail of fine dark lines on the more refrangible side similar to 

 the tail of bright lines seen in the sparks taken in hydrogen between 

 magnesium points. Sometimes, however, this line (5210) was 

 seen bright. It always disappeared when the gas was discontinued, 

 and appeared again sharply on readmitting the hydrogen. These 

 effects were, however, only well-defined in crucibles having a height 

 of at least 3 inches above the arc. 



On putting a fragment of metallic gallium into a crucible, the less 

 refrangible line, wave-length 4170, came out bright, and soon a dark 

 line appeared in the middle of it. The other line, wave-length 4031, 

 showed the same effect, but less strongly. 



In the cases of cadmium and copper,, though we have made no 

 thorough examination of them, we can corroborate the results arrived 

 at by Cornu. We noticed particularly the disappearance in the arc 

 of the cadmium lines, with wave-lengths 5377 and 5336. 



On the addition of aluminium to either copper or silver in our lime 

 crucibles, we noticed that the copper or silver lines which had been 

 previously predominant, almost faded away, while the calcium lines 

 came out instead with marked brilliancy. In no case could we detect 

 the red lines of aluminium in the arc. 



With a view to re-introduce into the arc the magnesium line wave- 

 length 4481, we tried the action of an induction spark in a lime 

 crucible simultaneously with the arc, but without success; for the 

 conducting power of the hot walls of the crucible, and the highly 

 expanded gases within it, caused the resistance to be so much 

 diminished, that the spark passed as in a highly rarefied medium. In 

 order to succeed with this experiment, it seems plain that it must be 

 made in an apparatus which will allow of its being performed under 

 a pressure of several atmospheres. 



Reviewing the series of reversals which we have observed, we may 

 remark that in many cases the least refrangible of two lines near 

 together is the most easily reversed, as has been previously remarked 

 by Cornu. Thus, in the case of barium (though there is no very distinct 

 grouping of the lines of that metal) taking the rays in order, we have 



