the Weights of Atoms. 



297 



§ 73. To estimate the values of e and e as defined in 

 §§ 70, 72, consider the albedos* of the earth as might be 

 seen from a balloon in the blue sky observed by Majorana and 

 Gr. Sella over Etna and over Monte Rosa respectively. These 

 might be about *2 and *4, the latter much the greater because 

 of the great amount of snow contributing to illuminate the 

 sky over Monte Rosa. With so much of guess-work in our 

 data we need not enter on the full theory of the contribution 

 to sky-light by earthshine from below according to the 

 principle of §§ 67, 6S, interesting as it is ; and we may take 

 as very rough estimates *2 and *4 as the values of e and e r . 

 Thus (31) becomes 



T=^.10 19 =^P.10 19 . . . (32). 



§ 74. Now it would only be if the whole light of the sky 

 were due to the ultimate molecules on which the refractivity 

 depends that/ or f could have so great a value as unity. If 

 this were the case for the blue sky seen over Monte Rosa by 

 G. Sella in 1900, we should have / / = 1, and therefore 

 iV=5*73 . 10 19 . But it is most probable that even in the 

 very clearest weather on the highest mountain, a consider- 

 able portion of the light of the sky is due to suspended par- 

 ticles much larger than the ultimate molecules N 2 , 2 , of the 

 atmosphere ; and therefore the observations of the luminosity 

 of the sky over Monte Rosa in the summer of 1900 render it 

 probable that 3" is greater than 5*73 . 10 19 . If now we take 

 our estimate of § 50, for the number of molecules in a 

 cubic cm. of air at 0°, and normal pressure, iV=10 2() , we 

 have 1— /='699 and 1— / v = , 427; that is to say, according 

 to the several assumptions we have made, '699 of the whole 

 light of the portion of sky observed over Etna by Majorana 

 was due to dust, and only '427 of that observed by Sella on 



* Albedo is a word introduced by Lambert 150 years ago to signify 

 the ratio of the total light emitted by a thoroughly unpolished solid or a 

 mass of cloud to the total amount of the incident light. The albedo of an 

 ideal perfectly white body is 1. My friend Professor Becker has kindly 

 given me the following table of albedos from Miiller's book Die Photo- 

 metrie der Gestime (Leipsic, 1897) as determined by observers and 

 experimenter 



Mercury 044 



Uranus 060 



Venus 076 



Neptune 0-52 



Moon 0o4 



Snow 078 



Mars 022 



White Paper 070 



Jupiter 062 



White Sandstone 0-24 



Saturn 072 



Damp Soil 0"08 



.Phil. Ma</. S. 6. Vol. 4. Xo. 21. Sept. 1902, 



