School of Mines at Santa-Fé de Bogota, whilst climbing the hills of Caràcas, witnessed in the middle of the day, during an excursion to the summit of Mount Silla, a phenomenon which appeared to prove in a striking manner the existence of ascending currents of air. He, and his companion, Don Mariano de Rivero, saw numbers of white, shining bodies rise from the Valley of Caracas, and reach the summit of Mount Silla, 5400 feet high, whence they fell on the opposite slope. This phenomenon lasted for an entire hour, without any interruption. At first, M. Boussingault imagined it was a flight of birds, but he was soon able to convince himself that these bodies floating in the air were small, round bundles of grass, chiefly composed of the species Aira tenacissima, which is plentifully mixed with Agrostis grass of the valleys of Caracas and Cumana.1 The account of the phenomenon witnessed by me was published in the Comptes-rendus of the Paris Academy 2 before I knew of Boussingault's observation.

The presence of salt (chloride of sodium) in the air of the sea-coast, especially when a brisk breeze plays upon the spray of the waves, is a matter of common experience. But it is not so generally known that another sodium salt, namely, sulphate of soda, in the solid state, appears to be constantly present in the atmosphere, at least in the lower regions.

A supersaturated solution of sulphate of soda

1 Humboldt, Tableaux de la Nature, vol. ii. p. 37.

2 Phipson, "Sur une pluie de foin," in the Comptes-rendus, Paris,

crystallises immediately the bottle containing it is opened, in whatever place the observation may happen to be made, whilst solutions, equally saturated, of other salts do not present the same phenomenon.

This fact, which has been brought to light by F. Parmentier and M. Margueritte, appears to prove most conclusively that sulphate of soda must be constantly present in the lower regions of the atmosphere, and this salt being universally suspended in the air, supplies the necessary microscopic nucleus-crystal, which causes the whole solution to solidify the moment the bottle is opened.

It has long been known that butterflies and other insects are occasionally transported, by aërial currents, from the valleys to the summits of the highest mountains, and far out to sea; and the spiders' webs which are found floating in the air (generally, with us, in October, though I have seen them also in March, both at St. Cloud, near Paris, and in the South of England), are alluded to by many of our poets as "gossamer." In this respect I shall never forget a journey I once made, in October, from Paris to Brussels, by the line then recently constructed through Erquelinnes. On alighting at the station of Erquelinnes, I noticed that the funnel of the engine was covered with these spiders' webs, and appeared just as if it had been wrapped in a silk shawl several inches thick. The description of the particular species of spider that migrates in this manner by the currents of the atmosphere is given by Kirby 1 Comptes-rendus of the Paris Academy, 1889.

E

and Spence in their great work on Entomology. I have since seen it rise in the air, like a fly, from the summit of plants in my garden at Putney, in September, by means of its invisible thread.

Doubtless, the migration of birds is aided in a similar manner by currents of air. Our swallows usually appear in great numbers after a stiff gale has been blowing for many hours. It is difficult, however, to account for the manner in which the minute red spiders, known as social mites, are transported through the air. Suddenly, some fine morning, they will be found clustering by thousands, on nails, or other prominent objects, at the top of gates, or fencing, disappearing, in a few days, as mysteriously as they came. This singular occurrence has been sometimes witnessed in Putney and other westerly quarters of London.

In another place, I have given numerous instances of what is supposed to be Cosmic dust, as distinguished from the better known dust, or ashes, due to volcanic eruptions, such as those of Krakatoa which, not very long ago, diffused into the atmosphere immense quantities of volcanic ash that remained suspended in the atmosphere for several years, producing the most gorgeous effects of sunset for two or three years in succession. It is very probable that fine sunsets of a less remarkable kind are often due to a similar

cause.

The whole subject of Cosmic dust, and other sub

1 Phipson, Meteors, Aërolites, and Falling Stars, 1 vol., London,

stances observed to fall from the atmosphere, has been discussed in Chapters XIII. and XVII. of my work just alluded to. The late learned Baron von

Reichenbach long ago collected virgin soil from the summits of mountains in Germany, and found it contained traces of nickel, a metal invariably present in aerolites. I did the same, in Waldeck, in 1865, taking the earth from places where human industry had not yet penetrated, and with similar results, which shows that meteoric dust, the produce of meteors, finds its way through the air to the surface of the Earth.

Professor Nordenskiold asserts that he has, also, collected meteoric dust in the snows and ice of the

Arctic regions. But all these observations require more careful discussion than they have yet received, and so does that one published by Mr. Baumhauer, who found iron pyrites enclosed in hailstones which fell in Holland.1

The cause of the explosion of a meteor in the Earth's atmosphere is due to the sudden rise of temperature on the surface (which is thereby fused, whilst the interior is most intensely cold), by the friction with the air through which it rushes. It has been estimated that if the speed of a meteor is only slackened by th, during its passage through the air, its temperature is raised on the surface to a much higher degree than that at which iron burns; and I have proved by actual experiment, that the sudden heating of the surface of a 1 Comptes-rendus, 1872.

body intensely cold in its interior, as a meteor coming from cosmic space must be, is quite sufficient to explain the explosion and rupture of the aërolite.1

""

The great Italian naturalist, Spallanzani, has declared that he was never able to discover that curious plant, Nostoc commune, though he had often sought for it along garden walks, after showers of rain; and he thought botanists were mistaken in giving to it the specific name "commune." Now, I have noticed that the algae of the Nostoc group appear abundantly in certain years and in certain places, without any apparent cause; and then disappear, apparently for ever, from these localities. The same singular phenomenon is found to occur with more highly organized plants; for instance, with the rare "bloody-finger grass known to botanists as Digitaria sanguinalis, which appeared one year in my garden near London, though it is only to be found, now and then, as a great rarity, in the county of Norfolk. I could cite many other instances, not only of plants, but of animals, that pay us periodic visits in this manner. In 1893, the summer of which was unusually hot and dry, I found in my garden a species of wasp which inhabits the South of France, and is never seen in England; and I have found a dead African locust on the wild, deserted sandhills beyond Ostend, on the coast of Flanders.

All these facts are due, in the first instance, to accidental transportation by gales of wind, which carry

1 Phipson, "Sur l'explosion et la chute des météores" in the Comptesrendus, 1869.