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SCIENTIFIC.
SCIENTIFIC.
The splendid sunsets which have so vividly attracted the attention of men of science and of the general public were so remarkable and of so long a duration in the clear atmosphere of the Castilian tableland, where sunsets are usually dull, that they have not failed to impress observers with the notion that they were due to other causes than those of common atmospheric refraction and reflection.
When the phenomena had already lasted four or five days, I read Mr. Symons' letter, published in the Times of the 1st inst., and I thought that possibly evidence might be obtained towards the confirmation of this theory if the sediment of fresh fallen snow was thoroughly investigated; for if the dust of Krakatoa was really reflected in the higher regions of the atmosphere the sun's rays, some of it must necessarily be descending towards the earth.
Luckily, on the 7th of this month, and when the phenomenon was at its height, and had already lasted for about eight days, there was a fall of snow at Madrid, of which I naturally profited, submitting it to a thorough investigation, the result of which. I think, will throw some light on so remarkable a phenomenon.
The snow analyzed was obtained from what had fallen on some zinc plates before the exposed windows to the north of my house, which is situated at the extreme north end of the town, where there are no buildings facing it, and also from what my friend Dr. Francisco Quisoga gathered from the windows of his house, situated about a mile to the south-east of mine; and in both the same substances were found.
The snow yielded about a litre of water, which, when the sediment had collected, was decanted, and the solid part dried at a temperature below that of boiling water The dry powder was then tested for magnetism and it was found to be extremely magnetic. It was then incinerated on platinum foil to a bright red heat so as to destroy organic substances, and the remaining dust was then submitted to microscopical investigation. The greater part of it is made up of what probably is the natural dust of the atmosphere of Madrid; of particles of mica, generally brown, and similar to that of the Guadarrama range, and in various states of decomposition; splinters of quartz and felspar, the greater part of it orthoclase; some small fragments of tourmaline; magnetic iron, and fragments of diatoms. Besides these mineral substances, which may probably be traced to the rocks forming the vicinity of the capital, some others were found for the presence of which it is difficult to account. The most remarkable are small particles of a foliated mineral of a yellowish color, perceptibly dichroic, and which between crossed Nichols is extinguished when the cleavage traces are parallel to the principal section of the polarising Nichol; the interference colours being of bright blue, and red, and yellow colours. Treated by boiling hydrochloric acid for twenty minutes, not a trace of action was perceived. These characters are all referable to a rhombic pyroxene, and judging from its dichroism, this substance may be taken for a hypersthene, which has besides a most striking resemblance to volcanic hypersthene. In addition to this mineral,