12 REPORT OF THE SECRETARY. 



in an unbounded region of the atmosplnu'e .saturated witli water this 

 nucleus must be a persistent structure. This he linds is strikingly 

 apparent even when the air is saturated with ver}^ volatile liquids other 

 than water. 



In conclusion, the author points out that the size of the nucleus must 

 vary with the medium in which it is suspended, and that water nuclei 

 in particular will depend for their dimensions on the meteorological 

 status of the atmosphere. Finall}', the importance of correlating this 

 variation of nuclear diameter with the electrical activity of the water 

 nucleus is insisted on, with a view to its possible application to atmos- 

 pheric electricity. 



A memoir liy Dr. Victor Schumann on the absorption and emis- 

 sion of air and its ingredients for light of wave lengths from 250 /<// 

 to 100 yUyW was put in type during the year, but the presswork was not 

 completed. 



This memoir, which forms the concluding part of Volume XXIX 

 of the Smithsonian Contributions to Knowledge, gives an account of 

 researches, aided by grants from the Hodgkins fund, on the emission 

 and absor2)tion of the gases of atmospheric air in the ultraviolet 

 spectrum. Within the last fifteen years our knowledge of I'adiation 

 has been greatly increased, and now eml)races wide ranges of the 

 spectrum heretofore unknown. Without assigning an}" place to the 

 numerous kinds of "rays" whose discovery has been associated in the 

 public mind first with the work of Kxintgen and later with that of the 

 Curies, I am speaking here rather of the extensions of the spectrum 

 in wave lengths which are actually measurable and known. Thus 

 beyond the red the spectrum has now been studied in practical conti- 

 nuity to a wave length of nearly 100 microns; and at a great remove 

 be3'ond this is another known region embracing the so-called Hertzian 

 or electric waves now employed in wireless telegraphy. Be3^ond the 

 violet progress has been, relativeh^ speaking, less rapid, unless, indeed, 

 it shall prove that the liontgen and other radiations fall in this region. 

 But a great step in advance has been made by the unwearied investi- 

 gations of the author of the present work. Doctor Schumann. 



The difiiculties hindering research in the ultraviolet are great and 

 consist chiefiy in the opacity of the usual optical media to the short 

 wave-length rays. Quartz, for a long time considered best in this 

 part of the spectrum, is found to be too opaque, and has been largely 

 superseded in Doctor Schumann's investigations ])y fluorspar for 

 prisms and plates. Air, even in layers of a few millimeters' thick- 

 ness, is almost wholly opaque, and other gases absorb strongly. It 

 has, therefore, been necessary to emplo}^ a spectroscope from which 

 the air is exhausted' to the highest practicable degree; and this and 

 other necessary apparatus Doctor Schumann has designed and con- 

 structed with his own hands, though aided ly grants from the Hodg- 

 kins fund of the Smithsonian Institution. 



