ADVERTISEMENT. 



In the present memoir, entitled "A Continuous Record of Atmospheric 

 Nucleation," the author further discusses his researches on the nucleus, as 

 published in Experiments with Ionized Air, Smiths.onian Contributions to Know- 

 ledge, vol. xxix, 1901, and in Structure of the Nucleus, issued as part of the 

 same volume in 1903. The investigation has been carried on with the aid of 

 a grant from the Hodgkins Fund of the Smithsonian Institution. 



Doctor Barus describes the nucleus as a dust particle small enough to float 

 in the air, but larger than the order of molecular size. Such a particle pre- 

 cipitates condensation in an atmosphere saturated with water vapor in its 

 immediate vicinity. When these nuclei occur approximately of uniform size in 

 thousands and millions, they give rise to condensational phenomena of trans- 

 cendent beauty and importance. By far the greater number of nuclei are 

 initially ionized, or at least carry electric charge. 



In addition to mechanical, thermal, and chemical processes, high potential 

 is shown to be a fruitful source of nuclei. Certain kinds of radiation, like ultra- 

 violet light, or the X-rays, or radioactive bodies, may also generate nuclei in the 

 dust -free air through which the radiation passes. 



The term "nucleation" is here used to denote the number of nuclei per 

 cubic centimeter regardless of their source or special properties. 



The scope of the present memoir is summarized by the author in his preface. 



In accordance with the rule adopted by the Smithsonian Institution, the 

 manuscript has been submitted for examination to a Committee consisting of 

 Professor Wilder D. Bancroft, of Cornell University, and Professor Edgar F. 

 Smith, of the University of Pennsylvania, and, having been recommended for 

 publication, it is herewith presented in the series of Contributions to Knowledge, 



S. P. LANGLEY, 



SECRETARY. 



Smithsonian Institution, 



Washington, May, 1905. 



