( 582 ) 



spectra for a future occasion, and here confine ourselves to the 

 principal characteristics. Tiie wave-lengths have been expressed in fin 

 with an uncertainty of no more than 0,3 mt. 



First series. Tiie well-known rather narrow absorption bands 

 of oxjgen (among- others 476,7 — 477,6) only played a secondary and 

 inconvenient part in this preliminary investigation; for when the 

 samples under observation are immersed in liquid air their absorption 

 spectra become impure, which should be duly taken into account. 

 We have not yet succeeded in observing the absorption spectrum of 

 the strongly paramagnetic liquid oxygen in a field of sufficient in- 

 tensity. 0,, NO2 and NO3 are also of importance ^). 



§ 5. T h i r d s e r i e s. Here chromium is of special importance. 

 It derives its name from its coloured compounds, which mostly 

 show dichroism and the well-known transmutation of colour with 

 change of temperature. We examined: 



Chroiniuni alum [CrK(S0j2l; diluted green aqueous solution. 

 At 18° light band 662,7—672,5; tainter band 688,1—726,4. A 

 plate of alum 2 m.m. thick exhibited pretty narrow bands in 

 liquid air, some of which were slightly affected by magnetism. 



Chromkim-potassmin ox<date [Or^ K^ (C^ Oj^ + 6 H^ 0] ; strongly 

 dichroitic (red-blue) small monoclinic crystals, which were cemented 

 on a covering glass and ground to a thickness of about ^l^ m.m. 



Plane of polarisation || long sides: at 18^ a bright band 698,1 — 

 703,7; at - 193° it lay 696,4—701,4. 



Plane of polarisation jl long sides: at 18' bright band 697,5 — 703,5; 

 at —193° it lay 696,4—701,4. 



An aqueous solution exhibited at 18° a broad band 693,2 — 702,3, 

 the maximum of which lay at 695,4 — 699,3; moreover a very faint 

 band 708,4—711,0. 



A solution in glycerin had this broad band at 18° from 694,9- 

 699,4. At a temperature higher than that of liquid air (roughly 

 estimated at — 130^): faint band 659,3 — 664,9 (possibly not simple), 

 faint band 669,0—671,2, stronger band 674,7— 676,8; halfshade limit 

 at 681,8. Very strong band 694,8—698,1, shade to 700,3; beginning 

 of region of absorption 706,0 ''). 



"Chroininni horax' obtained by melting together 5 — 15 7o chromium 

 fluorite with anhydrous borax, in the way of the borax-pearls used 

 in analytical chemistry; ground, polished and \arnished in order to 



1) E. Warburg & G. Leithauskr. Ann. d. Pliysik (4) 23, p. 209, 1907. 

 -) E. Wiedemann, Wied. Ann. 5 p. 515, 1878. W. Lapraik, Journ. f. prakt. 

 Chemie (2) 47 p. 307, 1893. G. B. Rizzo, Nuov. Gim. (3) 35 p. 132, 1894. 



