PREFACE. 



The present researcbes .ire a coBtinuation of tbe work begun in 1893, while I 

 heia a professorship of meteorology in the United States Weather Bureau, and 

 completed in 1895 in a private laboratory, Avhose facilities and maintenance i 

 owed to the generosity of Mr. Clarence King and Prof. Alexander Graham Bell. 

 For reasons of no moment here, the work was abrui>t]y discontinued at a very 

 promisino- sta-e of progress and I have had no opportunities to return to it since. 



In the meantime tlie whole subject underwent an astounding transformation, 

 due to the discovery of the X-rays and tlie brilliant achievements reached with 

 these new means, chiefly at the University of Cambridge, by Prot. J. J. ihomson 



and his pupils. ■ -j. .■ f fUo 



It was therefore with great pleasure that I welcomed an invitation from the 

 Secretary of the Smithsonian Institution, empowering me to begin the work anew, 

 along lines but little cultivated with this end in view by the other physicists. 



The present work, as a glance at the following pages will show, has been 

 laborious throughout, and the interpretations difficult and precarious It the 

 steam let is to be used as an instrument of research, the evidence is o the nature 

 of color criteria, unsatisfactory at best. The ionizer chiefly used, though admirable 

 in all other respects, has shown a degree of subtle variability, for which I was at the 

 outset, altogether unprepared. It ^vas essential, therefore, that the results reached, to 

 be warrantable, should be looked at from many points of view, and this has given 

 rise to a greater variety in the chapters than was originally planned. The peculia, 

 value of the steam jet in spite of all disadvantages, remains: it is a non-electrical 

 instrument giving evidence with a direct bearing on obscure electrical processes 

 and from this pit of view I believe that extensive work with it is eventually 



bound to be fruitful. , „ ,, 



In work so alive and well known as that to which the following pages a, e 

 tributary, I have thought it unnecessary to enter into a full ^>'W-g-l%- ^ost 

 of it has issued from the Cavendish Laboratory (Thomson, C. 1. R- ^d «n 

 Rutherford, Townsend, Zeleny, H. A. Wilson, McClelland, and others; to whom 



■Cf. E. Merritt: Science, xH, pp. 4-48, 98-4, 1900; H. Becquerel iV^./.jr, Ixni, PP^ 396- 

 ,08100. • N E Dorsey • U. S. Monthly Weather Review, Sept., 1900 ; above all the monographs 

 t^R^^^^^^^^s au Con„,s iLnational ,e Physi.ue, Pans, ,900, vol. u, by Becquerel, 

 Curie, J. J. Thomson, Villari, and Villard. 



