A CONTINUOUS RECORD OF ATMOSPHERIC NUCLEATION. 45 



tures as a whole, notwithstanding their periodic character, than by direct meas- 

 urement. This is what I meant by stating that the two sets of observations 

 would probably sustain each other, for nobody would be justified in using the 

 apertures of abnormal coronas, unless such use was suggested and guided by 

 independent evidence. The subject will be resumed in Chapter VI, and treated 

 from a point of view different from the present, which is merely tentative. 



8. Summary. The result of this paper is then favorable to the use of the 

 apertures of coronas in place of the colors of the annuli, for estimating the 

 number of particles corresponding to a given degree of supersaturation at a given 

 temperature. Full allowance must, however, be made for the occurrence of 

 periodic variations of aperture in relation to the diameter of the fog particles; 

 in other words, a given aperture is only of value when qualified by the type of 

 corona (whether of the crimson or green order) to which the aperture belongs. 

 Thus it will not in any case be possible to dispense completely with the color 

 pattern. It was with the object of finding these corrections systematically that I 

 began a series of experiments (Chapter VI) with new forms of plate-glass appa- 

 ratus, and I shall there refer to other developments. Homogeneous light, 

 though in many respects desirable, gives effects so faint as to be useless in 

 practice. 



With the above data I am able to make an independent estimate of the 

 number of particles in the saturated phosphorus emanation. The number 

 found for the first fog of the series was (Phil. Mag. (6), iv, pp. 25-26, 1902) 

 n = 6X83,ooo; since n' = 2.zn, ^' = 6X183,000 particles per cub. cm. Now the 

 density ratio before and after exhaustion is y, so that i-y is the volume of 

 saturated emanation added. As this has passed directly and slowly over 

 excess of phosphorus, it must be very nearly saturated, becoming diluted on 

 mixture with the dust -free air of the receiver. Hence, if n particles per cub. 

 cm. correspond to saturation, n (i-y) = n = 6X 183,000; or w =io 6 X6. 

 There must therefore be at least 6 million nuclei ' per cub. cm. of the air in 

 contact with a surface of phosphorus. The value following from my electro- 

 meter work was w =2Xio. The two methods are absolutely distinct, but 

 lead to data of the same order. It is because of the general reasonableness of 

 the data which have followed from my simple hypothesis throughout a very 

 wide territory of observation that I have felt bound to adhere to it. 



PLATE-GLASS APPARATUS. 



9. Description. To test the results just adduced, the apparatus shown in 

 Chapter VI, figure i, and in Chapter VII, figure 10, was constructed. The 

 frame, 20 cm. deep, 35 cm. long, 27 cm. high, was of wood, nicely joined, and 

 covered within and without with a mixture of burgundy pitch and beeswax 

 while hot. The front and rear faces are of i/4-inch plate-glass, cemented on by 

 the same resinous mixture, and further held in place by the wooden clamps, 



1 The factor 6 is introduced in conformity with the work of Chapter VI. 



