PREFACE. 



"What is a nucleus?" asks my friend, smiling incredulously. The convic- 

 tion has become prevalent that only the ions induce condensation, and my own 

 consistent adhesion to the nucleus since my report to the U. S. Weather Bureau 

 (Bull. 12) in 1893 is often looked upon as heretical obstinacy. But this is quite 

 unjust, unjust even to those who have with such brilliancy maintained the 

 occurrence of condensation in ions. The nucleus has not left the field in dis- 

 comfiture. It has merely been forced reluctantly and under conditions of ex- 

 treme supersaturation to share its functions in this respect with the ubiquitous 

 and irrepressible ion. 



But to reply: The nucleus is at the outset simply a dust particle, small 

 enough to float in the air, but much larger than the order of molecular size. 

 Such a particle precipitates condensation in an atmosphere supersaturated with 

 water vapor in its immediate vicinity, for the reasons long ago (1880) pointed 

 out by Lord Kelvin in his brief but epoch-making paper. The support of this 

 explanation was established experimentally by Coulier (1875), Kiessling (1884 

 et seq.), von Helmholtz (1886, 1887), and others, and, with particular ingenuity 

 and breadth of view, by John Aitken (1880, particularly 1888 et seq.) 



A single nucleus, however, would be of but little interest. It is when the 

 nuclei occur approximately of uniform size in thousands and millions that they 

 give rise to condensational phenomena of transcendent beauty and importance. 

 To produce these legions of nuclei is not impossible by irechanical means, just 

 as we can, for instance, triturate a solid to a remarkable degree of fineness; but 

 the impalpable powders are perhaps best produced by chemical or at least by 

 very refined physical processes. Similarly, though a class of interesting nuclei 

 may be produced by vigorously shaking liquids, or, better, by mutually im- 

 pinging jets or by jets impinging on a solid obstacle, nuclei are more abun- 

 dantly produced by ignition or combustion. Such ignition, moreover, should 

 be unaccompanied by any kind of smoke, as the gross particles in this case are 

 an efficient means of absorbing nuclei. A clear non -luminous bunsen flame, a 

 red-hot metal or any other solid, like glass, for instance, is a powerful nucleator. 

 It is not even necessary that the solid be red-hot. Phosphorus is subject to a 

 peculiar kind of chemical reaction, whereby nuclei are produced at 13 or 

 a little below, and are then produced from the smokeless body in maximum 



