12 PLEASANT WAYS IN SCIENCE. 



We might in like manner fail to obtain any spectro- 

 scopic evidence of the presence of particular elements in the 

 sun, because they do not exist in sufficient quantity in the 

 vaporous form in those outer layers which the spectroscope 

 can alone deal with. 



In passing, I must note a circumstance in which some of 

 those who have dealt with this special part of the spectro- 

 scopic evidence have erred. It is true in one sense that 

 some elements may be of such a nature that their vapours 

 cannot rise so high in the solar atmosphere as those of 

 other elements. But it must not be supposed that the 

 denser vapours seek a lower level, the lighter vapours 

 rising higher. According to the known laws of gaseous 

 diffusion, a gas or vapour diffuses itself throughout a space 

 occupied by another gas or several other gases, in the same 

 way as though the space were not occupied at all If we 

 introduce into a vessel full of common air a quantity of 

 carbonic acid gas (I follow the older and more familiar 

 nomenclature), this gas, although of much higher specific 

 gravity than either oxygen or nitrogen, does not take its 

 place at the bottom of the vessel, but so diffuses itself that 

 the air of the upper part of the vessel contains exactly the 

 same quantity of carbonic acid gas as the air of the lower 



made by some spectroscopists in the case of the planets Jupiter and 

 Saturn. In considering the spectroscopic evidence respecting the con- 

 dition of these planets' atmospheres, they have overlooked the circum- 

 stance that we can judge only of the condition of the outermost and 

 coolest layers, for the lower layers are concealed from view by the 

 enormous cloud masses, floating, as the telescope shows, in the atmos- 

 pheric envelopes of the giant planets. Thus the German spectroscopist 

 Vogel argues that because in the spectrum of Jupiter dark lines are seen 

 which are known to belong to the absorption-spectrum of aqueous 

 vapour, the planet's surface cannot be intensely hot. But Jupiter's 

 absorption-spectrum belongs to layers of his atmosphere lying far above 

 his surface. We can no more infer the actual temperature of Jupiter's 

 surface from the temperature of the layers which produce his absorption- 

 spectrum, than a visitor who should view our earth from outer space, 

 observing the low temperature of the air ten or twelve miles above the 

 sea-level, could infer thence the actual temperature of the earth's surface 



