206 Professors Liveing and Dewar on the 



after which the liquid boiled slowly and quietly. Through 

 the length of the tube (that is, a thickness of about o inches 



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13 





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c 



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d 



of liquid oxygen) we viewed the hot pole of an electric arc 

 with a spectroscope having two calcite prisms of 30° and one 

 of 60°. As reference-rays we used the red potassium-lines, 

 of which the positions with reference to A and B were w r ell 

 determined by Kirchhoff, and confirmed by our own obser- 

 vations. These lines were easily obtained by dropping a little 

 of a potassium salt into the arc. 



The diffuse bands previously seen both in the gas and liquid 

 were all of exceptional strength, but we did not notice any 

 addition to their number except a faint band just above G. 

 In place of A we observed a band, but different from A in 

 the following remarkable particulars. Instead of having a 

 sharp edge on the more refrangible side and fading gradually 

 towards the less refrangible side, its position appeared to be 

 reversed ; the sharp edge was on the less refrangible side, and 

 it faded away gradually on the more refrangible side. More- 

 over its sharp, less refrangible edge did not coincide with the 

 sharp edge of A, but reached very nearly to the more refran- 

 gible of the two potassium-lines, that is, had a wave-length of 

 nearly 7660. At the same time the band extended beyond 

 the sharp edge of A on the more refrangible side. There 

 was no indication that it was resolvable into lines, or even 

 into two bands. Turning to the place of B in the spectrum 

 we were not able, with that thickness of oxygen, to detect 

 any band in that place. Olszewski (Wied. Ann. xlii. p. 663), 

 with a thickness of 30 millim. of liquid oxygen, observed a 

 somewhat faint band corresponding to A, which with a 

 Rutherford prism was not resolvable into lines, but he has 

 not noticed the reversed position of the band. 



Using a similar tube for the liquid oxygen, but six inches 

 long, the band at A came out very much stronger and ex- 

 tended much further on the more refrangible diffuse side, but 

 was not conspicuously expanded on the other side, and did 

 not hide the potassium-line. At the same time a fainter band 



