the Weights of Ah 



301. 



proportionate loss of light per centimetre due to particles 



small enough for KayleigiVs theory, whether " suspended 



particles " or ultimate molecules of air or both, we have 



6 -83oooo* = . 9 258 which gives *- 1 ='10-75 . 10 6 cms. Hence if, 



as in § 58, we suppose for a moment the want of perfect 



transparency thus defined to be wholly due to the ultimate 



molecules of air, we should have by the dynamics of refractivity 



TiB'—D) 

 n j ^ = '0006; and thence by (1) of § 55 with 



A = b* . 10 ~ 5 we should find for the number of molecules 

 per cubic centimetre n = 2*469 . 19 19 . But it is quite certain 

 that a part, and most probably a large part, of the want 

 of transparency produced by particles small enough for 

 Rayleiglr's theory is due to " suspended particles " larger 

 than the ultimate molecules : and we infer that the number 

 of ultimate molecules per cubic centimetre is greater than, 

 and probably very much greater than, 2*46;*' . 10 19 . Thus 

 from the surer and more complete data of Miiller regarding 

 extinction of light of different wave-lengths traversing the 

 air, we find an inferior limit for the number of molecules 

 per cubic centimetre nearly three times as great as that which 

 Rayleigh showed to be proved from Bouguer's datum. 



§ 80. Taking, somewhat arbitrarily, as the result of §§ 23-77 

 that the number of molecules in a cubic centimetre of a per- 

 fect gas at standard temperature and pressure is 10 20 , we have 

 the following interesting table of conclusions regarding the 

 weights of atoms and the molecular dimensions of liquefied 

 gases, of water, of ice, and of solid metals. 



Mass of atom 

 Substance, or of H 2 

 in grammes. 



Density. 



H 



O 



H,0 



H 2 



H o 



N 



Argon 

 Gold 

 Silver 



Copper 

 Iron 

 Zinc 



Number of 

 atoms in 

 cub. cm. 



0-45. 

 715 



8-05 



8-05 



8-05 



6-29 



1781 



88-52 



48-47 



28-43 



2515 



29-30 



10" 



-24 



Liquid at 17° absolute... "090 

 „ ,, freezing-point 127 



| Water 1-00 



I Ice -917 



Vapour at 0° C "487 . 10" ; 



Liquid 1047 



1-212 



Solid lil-32 



, 10-:3 



„ 895 



„ 7-8fi 



„ 7-15 



200 

 178 

 124 

 114 

 (305 

 166 

 68 

 218 

 217 

 311 

 313 

 244 



10 21 



. 10 15 

 .10 21 

 1„ 



Distance from 

 centre to 

 centre if 

 ranged in 

 cubic order 

 with actual 

 density. 



1-71 . 



1-78 

 2-00 

 206 

 118-2 

 1-82 

 2-45 

 1-66 

 1-66 

 1-475 

 1-47 

 1-60 



10" 



■8 



