VI PREFACE. 



abundance. Fuming phosphorus is a relatively weaker nucleator. Many gaseous 

 sulphides, on mixture with air, become good nucleators. Even dust -free coal 

 gas and dust-free air, on commingling, set free nuclei. 



Certain hygroscopic liquids like concentrated sulphuric acid are remarkable 

 nucleators. They probably make up a class by themselves. At least it is not 

 improbable that 1,000 or 1,000,000 molecules per cubic centimeter may escape 

 from such a body by ordinary evaporation, in spite of the low vapor pressure. 

 Since each such molecule is hygroscopic, stable nuclei may be formed in a 

 saturated atmosphere by condensation of the water vapor. Sulphide and 

 sulphur nuclei are in turn probably oxydized to sulphuric acid. 



In addition to mechanical, thermal, and chemical processes, high potential 

 is a fruitful source of nuclei. A metal highly charged with electricity, or even 

 a glass insulator, or the nodal points in the metallic pathway of a stationary 

 electric wave are a source of nuclei. There is probably always an electric glow 

 present in such cases, though there need be no spark. 



Finally (and here we reach debatable ground), certain kinds of radiation, 

 like ultra-violet light, or the X-rays, or radioactive bodies, generate nuclei in 

 the dust-free air through which the radiation passes. 



Air originally made quite dust-free by nitration or otherwise, if exposed to 

 any of these sources becomes more or less filled with a freight of nuclei, fleeting 

 or persistent, and we may for brevity introduce the term nucleation to denote 

 the number of nuclei per cubic centimeter, regardless of kind or origin or other 

 properties possessed, and considered solely with respect to their tendency to 

 promote the condensation of water vapor in supersaturated moist air. If the 

 supersaturation is sufficiently pronounced the air molecules in successively 

 greater numbers as the supersaturation increases must themselves become 

 nuclei, probably beginning with the more complex systems. This, for instance, 

 occurs in the blues, opaques, and the succeeding browns and yellows of the first 

 order of the axial colors of the steam jet. The importance of experiments in 

 the spontaneous condensation of dust-free moist air was pointed out in my 

 report to the Weather Bureau in 1893, p. 48 ei seq.; they were first carried out 

 in an independent manner and with exquisite finish by C. T. R. Wilson (1897). 



The nucleus as an inert excessively small body, just transcending the order 

 of molecular dimensions, and occurring in immense numbers, has an interest of 

 its own ; but this interest becomes much enhanced when it is found that by far 

 the greater number of nuclei are initially ionized, or at least carry electric charge. 

 The cases in which this does not occur are sufficiently exceptional to prove the 

 rule, though such nuclei need not for this reason be less efficient. They prob- 

 ably admit of a categorical classification, such as has been suggested above for 

 concentrated sulphuric acid and sulphides. Apart from these, all nuclei pro- 

 duced by ignition, by high potential, by the X-rays, or by radiation are power- 

 fully ionized. So marked is the quality that certain investigators (in particular 

 the younger von Helmholtz, 1887) have endeavored to find in the ionization a 

 sufficient cause for the condensation of supersaturated moist air, or at least an 



