S66 NORMAN LOCKYER LECTURE 



whole volume corresponding to the visible disk ; the ice-coating is 

 16,000 miles in thickness and the depth of the atmosphere is about 

 6,000 miles. The rocky core of Saturn is about 14,000 miles in radius ; 

 it is covered with a layer of ice some 6,000 miles thick, over which is an 

 atmosphere extending to a height of 16,000 miles. The total weight of 

 the atmosphere of Saturn is about equal to that of the rocky core. Saturn 

 has the most extensive atmosphere of any of the planets, which explains 

 why it has the lowest mean density and the most flattened disk of any 

 planet. 



The pressures of these extensive atmospheres are very great ; at the 

 bottom of Jupiter's atmosphere, for instance, the pressure is fully a 

 million times the pressure at the bottom of the Earth's atmosphere. At 

 a relatively small depth in the atmosphere, the pressure is great enough 

 to compress the gas to a density nearly equal to that of the corresponding 

 liquid. It is stated by Wildt that at the bottom of the atmospheres the 

 pressure is great enough to solidify even the permanent gases. 



The densities of the atmospheres are low ; according to Wildt's cal- 

 culations they are 0-78 for Jupiter and 0-41 for Saturn. This enables 

 most of the possible constituents to be excluded, for all known gases, in 

 the liquid or solid state, have densities exceeding 0-3, with the exceptions 

 of hydrogen and helium. Frozen oxygen, for instance, has a density of 

 1-45; nitrogen, 1-02; ammonia, 0-82. In addition to helium and 

 hydrogen, the only gases whose densities in the liquid or solid state are 

 less than the density of the greater portion of the atmosphere of 

 Jupiter are the hydrocarbons, methane and ethane. There seems to 

 be no escape from the conclusion that the atmospheres of the major 

 planets must contain large quantities of free hydrogen and helium. This 

 conclusion is in accordance with expectation. The planets are believed 

 to have been formed in some way or other from the Sun, which is known 

 to contain a large amount of hydrogen, to the extent of about one-third 

 part by weight. Helium, oxygen, carbon and nitrogen are abundant in 

 its outer layers. Massive planets, like the four major planets, would 

 retain their light constituents ; hydrogen and helium are therefore to be 

 expected to be present in large amount in their atmospheres. 



The spectra of the major planets are of great interest. In the early 

 days of spectroscopy Huggins discovered visually a strong absorption 

 band in the orange and several weaker bands in the green in the spectrum 

 of Jupiter. These bands appear more strongly in the spectrum of 

 Saturn, but are not found in the spectrum of the rings — a conclusive 

 proof that they originate in the atmosphere of Saturn. Uranus and 

 Neptune show for the most part the same bands with still greater intensity, 

 together with some additional ones. The great increase in the selective 

 absorption from the yellow into the red and infra-red from Jupiter to 

 Neptune accounts for the green colour of Uranus and Neptune ; most 

 of the red and yellow regions of their spectra are lost by absorption. 

 The investigations of Slipher during recent years have extended the 

 spectra far into the infra-red to beyond A 10,500 and have revealed several 

 intense bands in that region. 



The origin of these bands remained unknown until a few years ago. 

 They had never been observed in the laboratory. Then Wildt succeeded 



