PREFACE. IX 



corresponding; cases for nuclei produced by the X-rays are given in the earher 

 volume and elsewhere. 



The following chapters, III, IV, V, VI, have been written to draw a variety 

 of conclusions from the data in my work on the Structure of the Nucleus, which 

 escaped me in the earlier volume, as well as to correct certain errors relating to 

 the quantity of water precipitated under given conditions, and to the diffusion 

 of nuclei, to which I have already called attention elsewhere. Chapter VI, 

 relating to periodic distribution in the colors of successive coronas, shows under 

 what conditions the angular diam.eter of a ring of a given color may be used 

 for the estimation of the number of particles producing the observed diffraction 

 pattern. In Chapter VIII a definite practical application is made, for use in 

 the last chapter of the book. 



Chapter VII shows a method by which fog particles, even of minutest size, 

 may be meastired under the microscope, or microscopically photographed. 

 The peculiar difficvilties encountered in the interpretation of these results, in 

 spite of the fact that fog particles are obtained in definite sizes and numbers, 

 are considered critically. 



The chief results of the book, however, are given in the last chapter, which 

 is a record of over two years of observation of the number of nuclei present per 

 cubic centimeter of the atmosphere of the city of Providence. The nuclei are 

 abundantly represented, particularly in the winter months. Curiouslv enough, 

 the maxima and minima appear at about the time of the winter and summer 

 solstices respectively. The reason for this cannot be sought in the astronomical 

 circumstances involved, but must be atmospheric in character. I have supposed 

 that in addition to rain, light pressure, which must be more effective as the 

 days are longer, may have something to do with this. Under any circum- 

 stances a highly nucleated medium is an interesting medium. Since much of 

 the nucleation must be of local origin and referable to combustion, the question 

 arises, what has become of the ionization which was simultaneously generated ? 

 It has probably vanished as does the ionization in the experimental condensa- 

 tion chamber in the laboratory. 



To reply to these questions systematically, observations have now for 

 nearly a year been taken at Providence and at Block Island simultaneously. 

 The latter station has many of the meteorological elements of Providence, but 

 Block Island lies sufficiently in the sea. and is in winter at least sufficiently 

 free from human habitation to present entirely different conditions as to nuclea- 

 tion. I have also in progress a continuous series of observations on the changes 

 in the lapse of time of the nucleation of dust-free (filtered) air, i. e., of air free 

 from foreign nuclear ingredients. The results will be reported in due course 



elsewhere. 



Carl Barus. 



Brown University, January q, 1905. 



