212 Professor Deicar [June 6, 



mentioned band, between wave-lengths about 5,700 and BJIIB. This 

 band could not be seen when hydrogen was substituted for carbonic 

 oxide. A mixture of sodium carbonate and charred sugar, heated in 

 an iron tube, gave only the same absorption as sodium in hydrogen. 

 There were also no indications of any absorption due to a compound of 

 rubidium or of caesium with carbonic oxide. 



A mixture of barium carbonate, aluminium filings, and lamp-black, 

 heated in a porcelain tube, gave two absorption lines in the green, 

 corresponding in position to bright lines seen when sparks are taken 

 from a solution of barium chloride, at wave-lengths 5,242 and 5,136, 

 marked a and /5 by Lecoq de Boisbaudran. These two absorptions 

 were very persistent, and were produced on several occasions. A 

 third absorption line, corresponding to line 8 of Boisbaudran, was 

 sometimes seen ; and on one occasion, when the temperature was as 

 high as could be obtained in the furnace fed with Welsh coal, and a 

 mixture of charred barium tartrate with aluminium was used, a fourth 

 dark line was seen with wave-length 5,535. This line was very fine 

 and sharply defined, whereas the other thi-ee lines were ill-defined at 

 the edges ; it is, moreover, the only one of the four which corresponds 

 to a bright line of metallic barium. 



Repeated experiments with charred tartrates of calcium and of 

 strontium mixed with aluminium gave no results ; but on one occasion, 

 when sodium carbonate was used along with the charred tartrate 

 of strontium and aluminium, the blue line of strontium was seen re- 

 versed : and on another occasion, when a mixture of charred potassium, 

 calcium, and strontium tartrates, and aluminium was used, the calcium 

 line, with wave-length 4,226, was seen reversed. 



In order to command higher temperatures, experiments were made 

 with the electric arc enclosed in lime, magnesia, or carbon crucibles. 

 The diflerent forms used are represented in Plate II. Figs. 1, 2, 3, 4, 

 and 5 ; and the plan for projecting reversals in Plate III. 



In the first experiments thirty cells of Grove were employed ; in 

 the later ones the Siemens arc from the powerful dynamo-machine 

 belonging to the Royal Institution. 



The electric arc in lime crucibles gives a very brilliant spectrum 

 of bright lines, a copious stream of vapours ascending the tube. On 

 drawing apart the poles, which could be done for nearly an inch with- 

 out stopping the current, the calcium line with wave-length 4,226 

 almost always appears more or less expanded with a dark line in the 

 middle, both in the lime crucibles and in carbon crucibles into which 

 some lime has been introduced ; the remaining bright lines of calcium 

 are also frequently seen in the like condition, but sometimes the dark 

 line appears in the middle of K (the more refrangible of Frauuhofer's 

 lines H), when there is none in the middle of H. On throwing some 

 aluminium filings into the crucible, the line 4,226 appears as a broad 

 dark band, and both H and K, as well as the two aluminium lines 

 between them, appear for a second as dark bands on a continuous 

 background. Soon they appear as bright bands with dark middles ; 



