CHAPTER III. 



DIMENSIONS AND NUMBERS OF THE CLOUD PARTICLES PRODUCING CORONAS AND 



AXIAL COLORS. 



INTRODUCTORY EXPERI5IENT8 WITH BENZOL. 



1. Pwposes. — The remarks made above, that the coronas in benzol vapor 

 are all normal, and that fewer exhaustions would therefore be needed to make 

 definite measurements of the size and distribution of cloud particles, induced me 

 to attempt the following introductory expei-iments. These, however, were destined 

 to fail of a decisive issue, because such thermodynamic constants as I was able to 

 find in tlie standard tables are insufficiently correlated to admit of the severe test 

 which the computations, here in question, imply. In the second place, even if the 

 constants were forthcoming, the impossibility of maintaining regular coronas in 

 benzol, without shaking oi- some equivalent method of keeping the vapor uniformly 

 nucleated, is an almost insupei-able difficulty ; while shaking, as seen in Chapter IV 

 et seq., introduces a new supply of nuclei and removes others, for w'hich no adequate 

 allowance can be made. The data for benzol have certain peculiar features of 

 interest, and I shall therefore record them because of their bearing on the method 

 subsequently carried through for water vapor. 



The method has been suggested : the thei-modynamic conditions of expansion 

 suffice for computing vi, the liquid precipitated per cub. cm. The apeitures of the 

 coronas afford a means of measuring d, the diameter of the particles. Hence their 

 number per cub. cm. is n = Grn/W*. 



2. Apparatus. Goniometer. — The fleeting character of the coronas, their 

 small aperture, their faint and vague definition, made all attempts at using telescopic 

 instruments, or even the sextant, futile. I thei'efore constiucted the following 

 simple goniometer which, in one form or another, proved very efficient. On a 

 smooth board, AB, a fixed arm, nc, cari'ies at n a vertical needle about 3 cm. long. 

 Similarly, a movable arm, swivelled at c, carries a like vertical needle «.', so adjusted 

 that the points of the needles may be moved into contiguity by rotation. Finally, 

 a screen, s, with a small peep-hole at a in the axis of rotation and also about 3 cm. 

 above the table, completes the instrument. On use, the needles ii, n', are adjusted 

 so as to be tangent to a selected annulus of the corona when viewed by an eye 

 looking through a. These measurements are made very quickly, and under the 

 conditions given below the needles are rarely more than 2 cm. apart, usually less. 

 Let Ji be the length of the aims of the goniometer, « the angle between them. 

 Hence the distance nn' is pricked into a small [)iece of paper, which on measure- 

 ment gives at once nn' = « = 2li sin a. (1) 



