18 EEPOET OP THE SECRETARY. 



the nucleus, as published in •" Experiments with Ionized Air " (Smith- 

 sonian Contributions to Knowledge, vol. xxix, 1901) and in "Struc- 

 ture of the Nucleus," issued as part of the same volume in 1903. The 

 investigation was carried on with the aid of a grant from the Hodg- 

 kins fund. Doctor Barus describes the nucleus as a dust particle 

 small enough to float in the air but larger than the order of molec- 

 ular size, and shoAvs that such a particle precipitates condensation in 

 an atmosphere saturated Avith water in its immediate vicinity. He 

 uses the term " nucleation " to denote the number of nuclei per cubic 

 centimeter regardless of their source — which ma}^ be from mechan- 

 ical, thermal, chemical, high potential, or radio-active processes — or 

 their special joroijerties. By far the greater number are initiallj'^ 

 ionized, or at least carry an electric charge, and where they occur in 

 thousands and millions of approximately uniform size they give 

 rise to condensational phenomena of transcendent beauty and im- 

 I^ortance. 



The articles printed in the Miscellaneous Collections are, as the 

 name of the series would indicate, miscellaneous in character. The 

 past year has added 38 numbers to this series of papers, including 

 several relating to the bibliography of chemistry ; an exhaustive 

 catalogue of Diptera, or two-winged flies, showing the geographical 

 distribution of several species knoAvn to spread disease; accounts of 

 explorations in Alaska and among the Canadian glaciers; some in- 

 teresting popular papers by Doctor Gill on the sculpin, the tarpon, 

 the ladyfish, and the carp, and a lecture by Dr. Andrew D. ^Vllite on 

 the diplomatic service of the United States, the first of the series of 

 lectures under the Hamilton bequest. 



The Contributions and Miscellaneous Collections just spoken of are 

 published at the expense of the Institution fund in editions of 1,500 

 copies, and are intended solely for distribution to the large libraries 

 and institutions of learning throughout the world. 



The Annual Report, on the other hand, is published by a Govern- 

 ment appropriation in an edition of some 12,000, and is intended 

 for a more general distribution. Primarily, this volume is a report 

 to Congress on the o]3erations of the Institution during the year, but 

 its popular interest lies largely in its General Appendix, contain- 

 ing a number of papers showing the recent progress of the world's 

 scientific Avork. To the selection of appropriate papers for this Ap- 

 pendix I have given much personal attention for several years past. 



The manuscript of the Avork of the late Dr. G. Brown Goode, on 

 some contributions of America to the progress of science, Avhich dur- 

 ing the past year has been worked over by Mr. Kenneth M. Goode, 

 editorial assistant, is noAv almost in its final shape, and Avill, I hope, 

 appear as a Smithsonian publication during the coming year. 



