ATMOSPHERES OF THE PLANETS — RUSSELL 159 



gfreener. They are hard to observe visually in so faint an object, and 

 the full realization of their intensity came only with the admirable 

 photograph of V. M. Slipher, in 1907. In later years, and with 

 modern plates, Slipher has extended his observations far into the 

 red, finding bands of ever-increasing strength — up to A 10000 for 

 Jupiter, where there is light enough to follow the spectrum farthest. 



For more than 60 years after their first discovery, and 25 after 

 Slipher's spectrograms, these bands presented one of the principal 

 unsolved puzzles of spectroscopy — for no one had duplicated them in 

 the laboratory. To be sure, one group, near A 7200, agrees fairly well 

 with a band of water vapor — but the still stronger water-bands 

 deeper in the red are absent, so that this must be a chance coincidence. 



When the radiometric measures of Coblentz and Lampland, and of 

 Nicholson and Pettit, showed that the temperature of the visible sur- 

 faces of Jupiter and Saturn must be well below — 100° Centigrade^ — 

 while Uranus and Neptune are doubtless colder — the range of pos- 

 sibilities was very much narrowed. But it was not imtil 1932 that a 

 young and brilliant German physicist, Kupert Wildt, realized the 

 solution of the problem. 



Other gases, like water vapor and carbon dioxide, have strong 

 fundamental absorptions in the infrared, and fainter harmonics in 

 the more accessible part of the spectrum, which demand a long 

 absorbing path in the laboratory to bring them out. Utilizing ob- 

 servations of this sort, Wildt showed that certain bands in the 

 spectrum of Jupiter near A 6470 and A 7920 agreed with those of 

 ammonia, and others, at A 6190, A 7260, and A 8860, with bands of 

 methane. The original comparison was not quite conclusive, for 

 with the moderate dispersion then employed the planetary bands 

 had not been adequately resolved into their component lines. This 

 was soon accomplished by Dunham, who found so complete a coinci- 

 dence of the accurately measured individual lines that both identi- 

 fications were put beyond all question. For ammonia more than 60 

 lines were found to agree, and for methane 18 lines in part of one 

 band. Some expected band lines were naturally blended with solar 

 lines, but not one of importance failed to appear. 



From these comparisons Dunham estimates that the quantity of 

 ammonia gas above the visible surface of Jupiter is equivalent to a 

 layer 10 meters thick under standard conditions. In Saturn it is 

 less. 



The climax of the tale came this year, when Adel and Slipher an- 

 nounced that practically all the bands had been identified, and were 

 due to methane. The 45-meter path and the 40-atmosphere pressure 

 got enough of the gas into the way of the light to produce bands 

 intermediate in intensity between those in Jupiter and in Saturn. 



