Abbildungen der Seite
PDF
EPUB

rain had fallen that had covered the surface of the earth to the depth of one inch.

[Phil. Trans. 1766.

2. Mode of the Formation of Snow. The frequent changes of the weather that have taken place dur. ing the last winter *, having induced me to direct my attention to meteorology; I confess, that the manner in which philosophers ac. count for some of the phænomena that occur, is not, to me, altoge. ther satisfactory.

It is not surprising, that electricity (with the immediate agency of which we are so little acquainted) should be resorted to, as the grand agent in all meteorological phænomena. Accordingly we find, that snow, and indeed every variety of weather we experience, is considered to be more or less affected by the electric fluid.

Snow is generally supposed to be the vapours of the atmosphere, disengaged by the electric fluid, and frozen.

But it appears to me, that before we receive so vague an ex. planation, the following questions might be asked:

What are the vapours of the atmosphere composed of?

By what laws, and in what manner does the electric fluid act, either in the formation of snow, or as a component part of it?

I shall now offer a few remarks to strengthen a supposition that the electric fluid is not engaged in, or in the least essential to the production or existence of snow.

By an attentive observation of all the circumstances that have attended the fall of snow, during the last winter, I have, in almost every instance, found that it is accompanied with, or rather pre. ceded by a change of the wind; and that the wind, previous to the fall of snow, blew from some point between the south and the west; and afterward from some point between the east and the north-west +.

• If it is observed, that we have sometimes snow, without the wind changing to any of the points above-mentioned, or, even without a visible change to us ; yet it does not militate against the following remarks ; for it has been observed by aeronauts, that different strata of air blow froin opposite points at the same time. Therefore, notwithstanding a south wind may prevail at the surface of the earth, a superior stratem may blow from the north.

* 1804.

Such being the facts, is it not probable, that a change of the wind is the cause of snow?

Now let us examine, whether such a cause will produce such an effect.

The winds that blow from any of the points between the south and the west, by coming from warm climates, and passing over, perhaps, a very large tract of water, where there is a powerful evaporation going on, must possess a very great degree of humi. dity, and are most commonly of a temperature between 45o and 60° of Fahrenheit.

The winds which blow from any of the points between the east and the north-west, by coming mostly from such high latitudes, and passing over immense fields of ice, where evaporation is undoubt. edly greatly impeded, cannot be supposed to contain much water in solution, but must bring with them very great degrees of cold.

Now let us suppose that a north wind of any temperature be. tween 32° and 0° (which it generally is, in superior strata of the atmosphere) meets a south-west wind, as before-mentioned, the consequence will be, that the intense cold which accompanies the former will convert the water with which the latter is impregnated, into ice; and the instantaneous application of cold is pro. bably the reason why snow is produced in what we call flakes; for before the vapour can concentrate itself into large particles, or drops, it is arrested by the intense cold.

In this view, the formation of snow appears to be a beautiful chemical phænomenon; for the warmer air, having a greater affinity for the colder air than it has for the water which is held in solution, the water is disengaged, crystallized by the cold, and precipitated in the form of snow.

It is generally observed, that it is unusually cold for half an hour or an hour before the fall of snow, and warmer afterwards. Might not this be accounted for, by considering that the adverse wind must meet with consistence, in effecting either a union with, or a passage through a stratum of air surcharged with water, and con. sequently must be in a great degree reflected back again, 'not in the perpendicular, but as radii from a center, in an oblique direc. tion, part of which must descend to the earth. And it will un. doubtedly be warmer, after the stratum of north wind has either forced a passage through or effected a union with the south-west wind*.

Though I have not, in the preceding observations, considered the electric fluid as at all essential to the production of snow, yet I do not deny the presence of it. That snow contains the electric suid, cannot be doubted ; but it does not follow, that the latter is necessary to the existence of the former. We know of no substance in nature, that is impervious to that subtile fluid ; it seems to pervade all bodies with nearly the same facility as caloric. Therefore, though snow indicates electricity, it is probably no more than it has acquired in its passage through an electrified at. mosphere.

[Nicholson's Journal,

SECTION X.

Snow of a Red Colour.

1. Descent of Red Snow at Genoa. In a communication from Signor Sarotti, the Venetian Resident at Genoa, to the Honourable Mr. Boyle.

On St. Joseph's day, on the mountains called Le Langhe, there fell on the white snow, that lay there before, a great quantity of red, or if you please of bloody snow. From which, being squeezed, there came a water of the same colour.

[Phil. Tran. 1678.

Although no mention is here made of any volcanic eruption in the neighbourhood, it is probable that the cause of the colour was owing to some colorific material thrown forth in the course of such a phænomenon. In article 4, of section yii. we have already noticed a descent of coloured rain, evidently deriving its peculiar tinctnre from such a cause.

In the following article, M. Saussure offers another opinion, or rather several other opinions, for he seems by no means to have satisfied himself upon the subject.

[EDITOR.

• The water gives out heat in congelation. Vide Irwine, Black, Craw. ford, &c.

It

2. Snow of a Red Colour found in the Alps. When M. de Saussure explored mount Breven, for the first time, in the year 1760, he found in several places on a declivity spow still remaining, and was not a little surprised to see the surface of it, in various parts, tinged with a very lively red colour. This colour was brightest in the middle of such spots as had their centres more depressed than the edges, or where different planes co. vered with snow seemed to be joined to each other. When he examined this snow more closely, he remarked that its redness proceeded from a very fine powder mixed with it, and which bad penetrated to the depth of two or three inches, but no farther. did not appear that this powder had come from the higher parts of the mountain, because some of it was found in places at a consi. derable distance from the rocks and much lower down; and it appeared also that it had not been conveyed thither by the winds, because it was not disposed in stripes or in the form of radii. The most probable conjecture therefore was, that it was a production of the snow itself, or the remains of its partial melting suspended at its surface as in a filtre when the water passed through it. What seemed to favour this conjecture still more, was, that the colour at the edges of the hollow places where little water had sunk down was extremely faint; and, on the other hand, shewed itself stronger in those parts where the greatest quantity of water seemed to have penetrated.

M. de Saussure took a tumbler full of this snow, as he had no other vessel with him, and held it in his hand till the snow melted, when he soon saw the red dust deposit itself at the bottom. Its colour then did not appear so dazzling as before, and when dry it lost it entirely: it decreased also in quantity, so as almost to appear nothing.

Next year M. de Saussure ascended the Breven, and found on it a quantity of the same kind of red soow, some of which he squeezed closely together and put into a large handkerchief, but before he got home it was entirely dissolved by the heat of the sun. It was not, however, on the Breven alone that he discovered snow of this kind; for he found it on all the high mountains of the Alps, about the same season of the year, and in similar situa. tions ; so that he was much surprised that authors who had written

respecting the Alps, such as Scheuchzer, had made no mention of it. It is, indeed, true that it is found only in hollows, where the snow lies deep, and at the season of the year when the melting of it has proceeded to a certain degree; for, when none of the snow or when very little of it has been melted, the dust is then in too small quantity to attract the eye; and if the melting has proceeded too far, the whole of the powder has passed through with the water, and it hecomes equally invisible. Besides, towards the end of the melting, a great many foreign particles and impurities, conveyed thither by the wind, are mixed with it, so that its colour is no longer distinguishable.

In the year 1778, when M. de Saussure was on mount St. Ber. nard, he found a great deal of the same kind of snow. He collected as much of it as he possibly could; and Mr. Murrith, an experi. enced naturalist, collected some of it also; so that they were enabled to make someexperiments. On account of its great specific gravity, M. de Saussure treated this red powder as an earth, first with dis. tilled vinegar, but he employed so little that he had no result. He then boiled it in the moriatic acid, and obtained a solution, which, when carefully distilled and filtered, had so brown á colour that he was quite at a loss respecting the nature of this substance. He therefore applied it to the blow-pipe, and observed that it inflamed with a smell like that of burnt vegetables.

This experiment induced M. de Saussure to digest forty grains of the powder in spirit of wine; and having filtered the solution, he found that the residue weighed seven grains less: the spirit of wine had become of a golden yellow colour. He then distilled it in a balneum mariæ, and the spirit of wine came off perfectly pure. An oily transparent matter of a golden brown colour, which by the warmth of the balneum mariæ had not become dry, remained at the bottom of the retort. This oily matter had a smell like that of wax, which it emitted also when burning. The deposit, which the spirit of wine had not desolved, was, in regard to its extractive part, also inflammable; and the ashes which remained after it was burnt, though they did not seem alkaline, were refused by the the blow-pipe into a porous kind of greenish glass.

These experiments seem to prove that this powder was a vegeta. ble substance, and probably the farina of some flower. M. de Saussure was acquainted with no plant in Swisserland that pro.

« ZurückWeiter »