JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. IV NOVEMBER 19, 1914 No. 19 



PHYSICS. — Avogadro's constant and atmospheric transparency. 

 F. E. FowLE, Astrophysical Observatory, Smithsonian 

 Institution. 1 



In this communication are given coefficients for the trans- 

 missibility of radiation through the dry air vertically above 

 Mount Wilson together with factors and a formula for comput- 

 ing the transmissibility for moist air for other zenith distances 

 and altitudes above sea-level where dust has become a negli- 

 gable quantity (above 1000 meters). No account is, however, 

 taken in this formula of selective absorption. 



Ey means of Eajdeigh's formula connecting the scattering of 

 light passing through a gas with the number of molecules, these 

 coefficients have been used to compute the number of molecules 

 under standard conditions. The merits of the present reductions 

 over those of an earlier paper by the author lie in using the index 

 of refraction proper to each wave-length in place of a mean value, 

 in better values for the transmission coefficients and in a more 

 accurate value for the barometric pressure, the value before 

 used being taken at an appreciable higher altitude on the moun- 

 tain than our observatory. The mean results give for Loschmidt's 

 number or the number of molecules in a cubic centimeter of a 

 gas at 0°C. and 76 cm. pressure 



n, = (2.70 ± 0.02) X lO^^ 



^ This paper, read before the Philosophical Society, October 10, 1914, will 

 appear in full elsewhere, probably in the Astrophysical Journal. 



529 



