248 Mr. J. N. Lockyer. On the Classification [Jan. 10, 



variance with. Angstrom's observation, for we should expect the 

 spectrum to be somewhat variable.* It is probable that the observa- 

 tions showing nothing but continuous spectrum were made when the 

 temperature was only sufficient to render the meteoric particles red 

 hot. That the zodiacal light does consist of solid particles, or at all 

 events of particles capable of reflecting light, is shown by the 

 polariscope. 



No one has ever gone so far as to suggest that the zodiacal light is 

 an atmospheric phenomenon, and yet the principal line in its spectrum 

 is identical with that in the spectrum of the aurora. We have, there- 

 fore, an additional reason, if one be required, for discarding any 

 atmospheric origin which has been suggested for the auroral spectrum. 



PART II. FALLEN DUST. 



We have now complete evidence of the existence of meteoric dust 

 in the atmosphere, first, from the known number of meteorites which 

 enter the atmosphere, and secondly, from the spectroscopic observa- 

 tions of aurorae. This dust will finally reach the earth's surface, and 

 it is exceedingly interesting to trace its subsequent history as far as 

 possible. 



The detection of such dust which falls on the general surface of the 

 earth is almost hopeless, but that which falls on the sea will have a 

 chance of accumulating where the water is quietest. The researches 

 of Messrs. Murray and Renardf during the " Challenger " Expedition 

 seem to indicate that such an accumulation really takes place. 



An examination of the deep-sea deposits collected during the expe- 

 dition has led them to believe that certain small "magnetic spherules " 

 are totally unlike particles of iron derived from basaltic rocks or from 

 furnaces, and that their origin is probably meteoritic. In addition to 

 these, great numbers of the so-called "manganese nodules" were 

 found in the red muds from deep-sea bottoms. Messrs. Murray and 

 Renard incline to the belief that these owe their origin chiefly to the 

 decomposition of volcanic rocks, but my own researches seem to show 

 that they may be at least partly formed by the accumulation of altered 

 meteoric dust. 



An analysis of one of these nodules by Professor Renard (' " Chal- 

 lenger" Report, Narrative,' vol. 1, Part II, p. 1048), gives the follow- 

 ing : 



* Since the above was written I have received a letter from Mr. T. Sherman, 

 stating that he has reason to believe that the appearance of the 558 line in the 

 zodiacal light lias a regular period. 



f " On the Microscopic Characters of Volcanic Ashes and Cosmic Dust, and 

 their Distribution in Deep-sea Deposits," ' Edinb. Koy. Soc. Proc.' and ' Nature/ 

 vol. 29, p. 585. 



