Mzzsox.—On the recent Sun-glows, 885 
8. Again it may be asked, if dust were the cause of the sun-glows, why 
were they not seen in the daytime? Whatever foreign material in the 
upper air eaused the glows, this diffieulty would remain the same.  Prob- 
ably the greater perpendieularity of the sun and therefore the greater 
intensity of his light during the daytime aecount for the glows then being 
absent. Towards night and morning, also, his rays would traverse a greater 
length of the dust-laden upper air. 
9. Lastly, if dust caused the green and blue suns and the air is always 
dust-laden, why do we not have such phenomena more frequently? Ordin- 
arily the dust in the air is not very high, and the particles near the earth 
are large, and aqueous and gaseous vapour counteract the effect of dust by 
absorbing more light at the blue end of the visible spectrum than at the 
red. 
In conclusion, there is no reason, as far as I know, why we should con- 
fine ourselves to any one theory. Perhaps many causes have co-operated 
accidentally to produce one phenomenal result. If we must have only one 
theory, I incline to the Krakatoa dust theory, because I think the balance of 
evidence is in its favour. In any case, whatever the upper-air medium 
may have been, it certainly seems to have emanated from the Straits of 
Sunda during the memorable eruption of 27th August. However, when we 
come to speculate upon the nature of the substance thrown into the upper 
currents, difficulties and objections and doubts meet every suggestion or 
supposition. We shall, therefore, if we are wise, guard ourselves against 
all dogmatism on the subjeet, partieularly as fresh facts and fresh theories 
are coming forward daily. 
Even as I write there appears, in the notes on popular science by 
Dr. Taylor in the pages of the Australasian, a modification of the aqueous 
vapour theory which is well deserving attention. It appears that Dr. 
Prince, a well-known meteorologist, thinks the phenomenal sunsets are 
due to the crystallization of saline particles from masses of sea- 
water ejected in the form of vapour into the upper air by the Krakatoa 
eruption, and he argues that the greatest displays having occurred in the 
coldest weather, can only be accounted for on the theory that the crystal- 
lization of saline products is a great factor in their production. So long ago 
as last January, M. Beyerinck, of Wageningen, collected from rain which fell 
during a storm succeeding a fine after-glow a remarkable quantity of com- 
mon salt. Again, I find in “Nature” of 8rd July, that M. Gay, in a paper 
read before the Paris Academy of Sciences on the 23rd June, not only con- 
nects the recent sun-glows with the Krakatoa eruption, but thinks that the 
persistent rains of this year are owing to the same cause, directing attention 
to the fact that after the volcanic eruptions of 1788, 1881, 1856, and 1862, 
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