IV PREFACE. 



for different bodies of air, the February maximum at least can not be 

 of local origin ; and it is thus in a measure probable that the December 

 maximum also is due to non-local causes. But whether these are the 

 aggregated effects of remote terrestrial sources, or whether they repre- 

 sent an actual invasion of the atmosphere on the part of some cosmic 

 agency, remains to be seen. 



The air to be treated in the present series of experiments is, therefore, 

 continually approaching a state of purity so far as foreign admixtures 

 are concerned. Hence the properties of dust-free air, or in practice of 

 filtered air, became increasingly important. It may be proved by aid 

 of the inclosed steam- jet 1 that even dust-free air must be an aggregate 

 of nuclei, whose number grows rapidly larger as their diameter de- 

 creases, with a maximum for molecular dimensions ; or that size is 

 distributed among the molecules and quasi-molecules of dust-free air 

 in a way somewhat recalling the distribution of velocity among mole- 

 cules, and that among particles larger and smaller the air molecule 

 represents a condition of maximum occurrence. The fog-limit of dust- 

 free air is thus a variable quantity, depending eventually on the details 

 of the method of filtration, or other process for rendering the air dust- 

 free. In fact, as the fog-limit rises, the coronas for a given exhaustion 

 above the fog-limit increase in aperture, up to a limit. It should always 

 be remembered that the particles or nuclei here in question are very 

 small, even in comparison with ions. 



With the object of meeting the state of things in question, systemat- 

 ically, the data in Chapters I, II, and III were investigated, while 

 Chapter VI contains a summary of the work as a whole. The method 

 employed is believed to be a new departure, inasmuch as all results are 

 expressed in terms of the number of nuclei observed per cubic centi- 

 meter, so that the nucleations produced are the criteria throughout. To 

 offer conditions sufficiently varied for the experimental work, the nucle- 

 ation of dust-free air is in these chapters coarsened by ionizing it, either 

 by the X-rays or by a weak sample of radium acting through a sealed 

 tube. It thus appears that the ions or fleeting nuclei resulting are also 

 pronouncedly of all sizes within limits and that the increment of nuclea- 

 tion between two definite degrees of exhaustion (i. e., degrees of sudden 

 cooling) above the fog-limit but not too far from it, is greater as the 

 radiation applied from without is more intense. Virtually the grada- 

 tion of particles is thus more fine-grained or more nearly continuous 

 with the efficient nuclei lying within closer limits of size, as the ioniza- 

 tion is more intense. 



J Barus: Bulletin U. S. Weather Bureau, No. 12, 1893. 



