CONDITION OF THE LARGER PLANETS. 195 



however, could only happen because of the existence of 

 very deep cloud-layers entirely shrouding the glowing 

 planet, and in this case, as the clouds would probably 

 like our own clouds have a much higher reflective 

 capacity than rock surfaces have, we should expect to 

 find the planets Jupiter and Saturn shining much more 

 brightly, though only by reflected light, than they 

 would if their surface resembled that of our own earth, 

 or Mars, or Jupiter. Now the following table from 

 Zollner's * Grundziige einer allgemeinen Photometric des 

 Himmels ' gives very interesting evidence on this point : 



Snow just fallen reflects about . . 783 parts of 1000 of incident light ; 



White paper 700 



Jupiter's surface 



Saturn's surface 



Uranus's surface 



Neptune's surface 



whereas 



624 

 498 

 640 

 465 



White sandstone reflects only about 237 parts of 1000 of incident light ; 

 Clay marl ,, 156 ,, 



Mars's surface 267 ,, 



The Moon's surface ,, 174 



We may take Jupiter and Saturn together, and 

 Mars and the moon ; getting average reflective power 

 of giant planets : that of small planets :: 561 I 220 ; or 

 ' the giant planets, if they owe their light entirely to 

 reflection, have a reflective power more than 2 times 

 greater than that of the small bodies, Mars and the 

 moon. As the sea regions of Mars are observably 

 darker than his land regions, it is probable that our 

 earth, if her light could be estimated in the same way 



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