CHAPTER VIII. 



THE CORONAL METHOD OF ESTIMATING ATMOSPHERIC NUCLEATION. 



1. Introductory. -To produce coronas the nuclei must be very closely of 

 the same size, for in a large trough a rigorous uniformity of diameter of fog 

 particle and possibly of distribution is implied, if the corona is to be sharp and 

 brilliant. Particles of even slightly different sizes would give a blurred effect or a 

 mere fog. Therefore, as I understand it, the effect of ordinary dust to some 

 degree vanishes from the corona, and the nucleation observed is probably 

 something more definite. It is for this reason that in spite of very discouraging 

 drawbacks my interest in the subject has not waned, though I am well aware 

 that the effect of chemical products of combustion in winter, such as sulphuric 

 acid, or ionized matter in general, has not been eliminated. One may note, for 

 instance, that the distribution of atmospheric electrical potential is a maximum 

 in winter and falls off in its yearly period in a way similar to the observed nuclea- 

 tion; that there is frequent occurrence of day minima in both cases; that 

 maximum nucleation occurs, as shown in Chapter IX, during the winter months, 

 when one must certainly anticipate the maximum of dust contents during the 

 summer. 



The subject of atmospheric nucleation, as a whole, has received enhanced 

 interest in view of its bearing on Arrhenius's theory of the geophysical import- 

 ance of cosmical, and in particular of solar dust. Some limitation has been put 

 on the light -pressure theory of Schwarzschild, but this has rather stimulated 

 Arrhenius to give a sharper expression of his views, and the theory now appears 

 as the central feature of an admirable discussion of cosmical physics. 1 



I hoped therefore by aid of the present method to eventually add a contribu- 

 tion of my own. 



2. Apparatus. This consists of a rectangular box, AA, 50 cm. long, 15 

 cm. in thickness, and 10 cm., or preferably less, in height, made of some ma- 

 terial impervious to water. Wood covered while warm with a thick coating of 

 wax and burgundy pitch answers the purpose very well, and is much lighter 

 than rigid metallic vessels. The front and rear faces of the box are of thick 

 plate glass. This must be kept clean on the inside, and suitable scrapers with 

 a vertical straight edge of soft rubber movable to and fro along the glass by 

 aid of a long horziontal rod should be provided within the box. The rods pass 



1 Lehrbuch der Kosmischen Physik, Vols. i. and n. Leipzig, Hirzel, 1903. 



128 



