158 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 35 



unknown molecule involved must have a moment of inertia of 70.5 X 

 10"*'^ c. g. s. units. This agreed almost exactly with that of the mole- 

 cule of carbon dioxide — already known from laboratory observations 

 in the infrared. All doubt regarding this identification was removed 

 when Dunham, passing light through 40 meters of CO2 at a pressure 

 of 10 atmospheres, found that the strongest of the bands found in 

 Venus was faintly absorbed. Recently Adel and Slipher, using a 

 path of 45 meters through gas at 47 atmospheres' pressure, have 

 found the bands considerably weaker than they appear in the planet. 

 They conclude that the amount of carbon dioxide above the visible 

 surface of Venus is at least 2 mile-atmospheres — that is equivalent 

 to a layer 2 miles thick at standard atmospheric pressure and tem- 

 perature. The whole amount above the planet's solid crust may be 

 much greater. For comparison it may be noted that the whole at- 

 mosphere of the earth amounts to 5 mile-atmospheres, and the oxy- 

 gen in it to one and a quarter. 



These bands do not show in the solar spectrum, even when the 

 sun is setting. But there is very little CO2 in the earth's atmosphere, 

 and the whole amount in the path, even at sunset, amounts to only 

 30 feet under standard conditions. 



The weak absorption in these bands, like that in the visible bands 

 of water vapor, arises because they involve high harmonics of the 

 fundamental vibration-frequencies — in this case the fifth. 



So far we have had to do with bands of familiar and readily 

 identified molecules; but the major planets have been much more 

 puzzling. 



Jupiter shows a conspicuous band in the orange, which was dis- 

 covered visually by Huggins in the earliest days of spectroscopy, and 

 fainter ones in the green. These appear more strongly in Saturn, 

 but only in the spectrum of the ball of the planet, and not at all in 

 that of the ring — which might be anticipated, since the ring consists 

 of a multitude of tiny isolated satellites, and should be quite devoid 

 of atmosphere. Uranus, though its light is faint, shows the same 

 bands, much more strongly, and many others in addition. One of 

 these, which closely coincides with the F line of hydrogen (A 4861) 

 led Huggins to conclude that the planet's atmosphere was rich in 

 hydrogen. 



This interpretation, though quite permissible at the time, was 

 erroneous, for the line is absorbed only by dissociated atoms of hy- 

 drogen, which will not be present except at very high temperatures. 



The bands cut out so much of the red and orange light that the 

 whole disk of Uranus appears decidedly green — an unusual color, 

 noticed from the time of the planet's discovery. 



In Neptune's spectrum, the bands are of enormous strength, cut- 

 ting out the red almost entirely and making the planet look still 



