VI 



PREFACE. 



diameter from a siiperior limit indefinitely. They may also be arranged in New- 

 ton's series at a phase difference IVoni the former. They aie particularly vivid for 

 particles so small that the coronas have become vague or a colorless fog. They 

 last until the cloud particles merge into individual molecules, though the last to 

 ap[)ear before the air becomes pei'fectly cleai', can only be produced with the steam 

 jet. Indeed this last groujt of colors, seen without coronas in the color tube, \vas 

 fully treated in the eailier volume, though their apparently complementary rela- 

 tions to the colois of the discs of the coronas did not then appear. 



In determining not only the size but the number of particles piwlucitig these 

 splendid i)henomena as a whole, I have answered another of the outstanding ques- 

 tions there proposed. It has not been practicable to reach more tban a first 

 api)ro.\imation, :ind on overhauling the work after about a year's experience with 

 investigations of the same general character, T am left in some doubt whether I 

 have successfully coped with the subsidence erroi'. Nevertheless the results par- 

 ticularly with reference to a.xial colois aie noteworthy. They represent a separate 

 r)henomenon superimposed upon and coloring the diffraction phenomenon for rea- 

 sons wliich it is difficult to make out. To invoke interferences for these small 

 spherules, one must first e.vplaiu how the light gets in ; Ijeing in how it gets out 

 again, and thei'eafter, why particles are needed which at first l)lush seem about ten 

 times too large. 



Tile remaining chapters (IV, V, VI) bear directly on the subject of this volume. 

 I have long been in search of a method for pioducing nuclei which shall have the 

 luunblest origin possible; for when one operates with the powerful instruments of 

 modern lesearch and intei'prets the results by similarly recondite electrical methods 

 one is perhaps less apt to confront tlie nude facts. This quest culminated natu- 

 rally in the method of producing nuclei by shaking solutions. Afterwards I found 

 that Lenard' in his thorough fashion had incidentally traveled along a similai'road 

 befoie. Lenai'd's interests, however, were centered in his electrical investigation, 

 and though he discovered the astonishing [)ersistence ("27 hours) of nuclei obtained 

 from a jet of rivei' watei- and their corona-producing quality, he was interested 

 chiefly in the bearing of this result on the electricity of waterfalls and did not 

 contrast the persistence in question with the fleeting nuclei of pure water and 

 weakest solutions, nor enter into the other si)ecial lines of investigation in tiiis 

 memoii-. The reader will find tliat 1 have taken these nuclei as the type of nuclei 

 in general, as thei'e is reason to believe that they may be either electrically 

 charged or not, as the critical density by which the stability of the nuclei is con- 

 ditioned, varie.s. To this impoitant question I shall at another opportunity lecui'. 

 The final chapter (VI) is an endeavor to prove, again by the simplest possible 

 method of direct observation, that the nuclei diffuse or are endowed with definite 

 velocities. Whoever looks upon nuclei as ions will have no difticulty with this 

 proposition ; but for me the nucleus has dimensions which are lai'ger than the 

 molecule, and for such nuclei as are produced by shaking or those sluggish nuclei 



' Lenard: JViei/. Arm., xlvi, 584-636, 1892. 



