Chemistry and Physics. 325 



well as observations on the capillary spectrum-tube of ordinary 

 form, at low pressure, but in no case does the development of 

 the negative bands by cathode rays imitate the aurora as well as 

 the atomic rays. As far as it goes this tends to support the idea 

 that the aurora arises from rays from the sun of an atomic nature 

 but our author has never been able to secure experimentally 

 quite so simple a spectrum as that of the aurora. The negative 

 nitrogen bands are accompanied by other details such as bands 

 of the positive group or by the line 3995. Traces of hydro- 

 gen also are always present in the spectrum, the source of this 

 gas evidently being the material of the electrodes. 



It has been presumed on theoretical grounds that the upper 

 air in which the aurora occurs is rich in helium and hydrogen but 

 neither the present nor previous photographs show any traces of 

 these gases in the spectrum of the aurora. The only alternatives 

 are either that these gases are absent or that the conditions of 

 excitation are not such as to develop their spectrum. As to 

 Irydrogen, all laboratory experience shows that any kind of elec- 

 tric discharge through air will reveal the smallest trace of hydro- 

 gen by its spectrum. The difficulty of explaining the absence of 

 helium lines is not so great but it is still serious. On the diffu- 

 sion theory the composition of the atmosphere well below the 

 measured height of the auroral discharge should be at least five 

 volumes of helium to one of nitrogen. If these figures are 

 accepted it is very difficult on the hypothesis of atomic ray excita- 

 tion to explain the absence of both the helium and the nitrogen 

 lines from the spectrum. On the hypothesis of cathode ray 

 excitation the helium lines might be vanishingly feeble but a 

 new difficulty appears, namely that the intensity distribution in 

 the nitrogen band spectrum of the aurora is not that character- 

 istic of cathode rays but rather of atomic rays. The author dis- 

 misses the matter with the suggestion that the .true mode of 

 excitation may be something entirely different from either. — 

 Proc. Roy. Soc., 101, 114, 1922. f. e. b. 



6. Suspended Impurity in the Air. — Some of the methods 

 hitherto used for the trapping and examination of suspended 

 impurities in the air are the following: (a) The filter-paper 

 method, in which a given volume of air is filtered through white 

 paper leaving a smudge upon it. From the darkening of the 

 paper some estimate of the amount of impurity present is 

 afforded by comparison with a properly constructed scale of 

 shades. It is applicable only to the case of dark colored particles 

 and their quantitative estimation is difficult as the amounts col- 

 lected are hardly weighable. It has the further drawback that 

 the sediment thus entangled in the meshes of the paper cannot be 

 subjected to microscopic examination. 



(b) The condensation method. When air not too dry and con- 



