RECENT ADVANCES IN SCIENCE 193 



to 1 91 6 have been analysed by F. E. Fowle (Smithsonian Miscel- 

 laneous Collections, 69, No. 3, 191 8) with some interesting 

 results. In order to be free from the effects of selective 

 absorption the discussion was limited to the spectral region 

 between 0-35 and 0-50/*. The coefficient of absorption, a X} 

 for wave-length \, can be expressed as the product of two 

 factors one, a a \ due to dry air and the other, a\ due to an 

 amount of water vapour above the station equivalent to 

 w cms. of water. a ak is practically constant, and writing 

 log a k = log a aK + w log a wX , and plotting a K against w, both a aK 

 and a„ x can be determined. Applying Rayleigh's law of 

 scattering of a beam of light in a gaseous medium to a a \, 

 which is found to vary inversely as the fourth power of the 

 wave-length as required by that law, a value is obtained for 

 N 0f the number of molecules of air per c.c. at 76 cm. pressure 

 and o°C. Fowle thus finds N = (272 ± o-oi)io 19 . The best 

 value otherwise obtained is (2*705 ± 0-003) io 19 . Hence the 

 term a a \ is due almost entirely to molecular scattering. This 

 result confirms the accuracy of the Mount Wilson estimations 

 of atmospheric losses affecting the radiation from the sun. 



Search was also made for a scattering the same for all 

 wave-lengths such as would be caused by large dust particles 

 and which might be called a " dry haziness " effect. In 

 1910-11 this amounted only to 0-5 per cent. After the Mount 

 Katmai eruption in 191 2 it became 25 per cent, but decreased 

 in 191 3 to 3 per cent, and in 1914-15 to 1 per cent. 



The transmission coefficients for atmospheric aqueous 

 vapour (a ttK ) were also found to vary with the inverse fourth 

 power of the wave-length. The scattering of radiation when 

 passing through liquid water was shown to be the same as 

 would be expected from the same number of molecules in a 

 gaseous state. The unexpected result was obtained, however, 

 that the same amount of water in the form of atmospheric 

 water vapour gave 50 times as much absorption as required 

 by theory. The explanation suggested for this result is that 

 molecular aggregates are formed which increase the diameter 

 of the scattering particles : such increase in diameter far 

 more than compensates for the reduction in the number of 

 separate particles as far as the amount of scattering is con- 

 cerned. This molecular condition may be connected with 

 ionisation phenomena, which suggests a connection with solar 



