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into the cumulostratus is effected in the following manner: The cumulus, losing its hemispherical figure, increases irregularly upward, grows more dense, and overhangs its base in uneven or rugged folds : a pre-existing cirrus, cirrocumulus, or cirrostratus, or one perhaps immediately formed for the occasion, alights on its summit, and inosculates. The cumulostratus varies in appearance: sometimes it overhangs a perpendicular stem, and looks like a great mushroom; frequently a long range of cumulostrati appear together, which have the appearance of a chain of mountains with silvery tops. Before thunder-storms it seems frequently reddish, which some have imagined to arise from its being highly charged with the electric fluid.

7. Nimbus.-Clouds of any one of the abovementioned modifications, at the same degree of elevation, may increase so much as completely to obscure the sky: two or more different modifications may also do the same thing in different elevations, and the effect of this obscuration may be such as would induce an inattentive observer to expect the speedy fall of rain. It appears, however, from attentive observation, that no cloud effuses rain until it has previously undergone a change sufficiently remarkable to constitute it a distinct modification, to which the term nimbus has not inaptly been applied.

The best time for viewing the progress of nimbification is by stormy weather ; cumuli may then be seen rising into mountains and becoming cumulostrati, while long strata of cirrostratus permeate their summits; and the whole phenomenon has the appearance of a range of mountains, transfixed by the mighty shafts of giants. After having existed some while in this form, they become large and irregular, and they get darker by intensity, till all seem concentrated in a dense black mass, with a cirrose crown extending from the top, and ragged cumuli entering from below, and eventually the whole resolves itself into rain".




At nusquam emicuit pia cura et mira potestas
Splendidior, quam quod tanta ille eflinxerit arte
Aëra, qui semper vitæ spiramina nutrit. C******
But not alone in yon stupendous spheres
His power immense, his majesty appears :
We view his kind, his life-preserving care,
In all the wond'rous properties of air. LOBB.

AIR, in a general sense, is that invisible fluid surrounding our globe, on which depends not only animal but vegetable life; and which seems, in short, to be one of the great agents employed by Nature, in carrying on her operations throughout the world.

Though the attention of philosophers, in all ages, has been employed, in some measure, by inquiries concerning the nature of the atmosphere, yet, till the year 1750, little more than the mere mechanical action of this fluid was discovered, with the existence of some anomalous and permanently

* See Forster's Researches about Atmospheric Phenomena, a second edition of which has lately been published, much enlarged.

elastic vapours, whose properties and relations to the air we breathe were almost entirely unknown. Since that time, however, the discoveries concerning the constituent parts of the atmosphere itself, as well as the nature of the different permanently elastic fluids that go under the general name of air, have been so numerous and rapid, that they have, at once, raised the subject to the dignity of a science, and now form, under the name of aërology, a very important part of the modern system of natural philosophy.

These discoveries, indeed, have not been more interesting to philosophers, than useful to science and beneficial to society. Many perplexing processes in chemistry have been explained in consequence of them ; several have been facilitated; and a number of new and useful ones have been introduced. The phenomena attending metallic calcinations and reductions have been greatly elucidated. The knowledge of the use of the air in respiration; the method of ascertaining its purity and fitness for that function; the investigation of dephlogisticated air; the method of impregnating water with fixed air; are all calculated to answer purposes of the highest utility. The medicinal properties of fixed air have been, in a great measure, ascertained; and its antiseptic qualities, in other respects, promise to be of considerable advantage. The method of ascertaining the purity of the air of a place, and the manner of ventilating an apartment, are of great use to those concerned in public buildings. In short, there is, perhaps, no station in life where some knowledge of this subject may not be of use; and yet, interesting and important as are these discoveries, they were unknown, not to the antients only, but to the Bacons, the Newtons, the Boyles, and other illustrious philosophers of the 17th century':--A very minute and particular inquiry into this subject cannot be expected in the limits of a short essay. Volumes have been written upon it, the invaluable acquisitions of the most patient and laborious investigation. To these, therefore, and particularly to the Observations and Experiments of Dr. Priestley, I must refer my more curious and inquisitive readers.

For many ages the air was supposed to be a simple and homogeneous fluid; its common operations to depend on its heat, cold, moisture, or dryness; and any effects which could not be explained by these (such as the appearance of pestilential diseases) were thought to be entirely supernatural, and the immediate effects of the divine Power. It was reserved for Dr. Priestley to make the great discovery concerning the nature of our atmosphere; and to inform the world, that it is composed of two fluids; the one absolutely noxious, and incapable of supporting animal life for a moment; the other extremely salutary, and capable of preserving animals alive and healthy for a much longer time than the purest air we can meet with. The first of these he called phlogisticated air; that is, the combination of common air with phlogiston, which, in chemistry, is defined to be the most pure and inflammable principle; a principle existing in different bodies in various degrees and modes of combination ?. The other he called dephlogisticated air,

* Even the late Dr. Goldsmith, whose History of the Earth and Animated Nature was published in 1774, appears to have had no knowledge of any difference in the kinds of air, except what was known in the time of Mr. Boyle; although the important discoveries of Hales, Priestley, Brownrigg, Black, Cavendish, &c. were then known to most philosophers.

2 The combination of phlogiston with different kinds of air, and also with water, has been illustrated and evinced

by which he meant no other than exceedingly pure atmospherical air, entirely free from those heterogeneous vapours which contaminate the air we commonly breathe. It has been found by experience, that animals will live much longer in this kind of air, than in an equal quantity of common air; whence it is supposed, that the breathing of it must be much more healthy, and contribute to longevity much more than the common atmosphere. Nor are there wanting some who attribute the longevity of the antediluvians to the great purity of the atmosphere at that time; the whole mass being af. terward tainted by the deluge, in such a manner, that it could never regain its former purity and salubrity. But all this is mere conjecture: and excepting the single fact, that animals live much longer in a quantity of dephlogisticated than of common air, there is no evidence that the former contributes more to longevity than the latter. Dr. Priestley even throws out a conjecture, that the by a variety of experiments by Dr. Priestley. All the kinds of air, he says, that appear to be essentially distinct from each other, are fixed air, acid, and alkaline ; because these, and another principle, called phlogiston, which he has not been able to exhibit in the form of air, and which has never yet been exhibited by itself in any form, seem to constitute all the kinds of air with which he is acquainted. He has also shown how common air is de-, praved and rendered unfit for the purposes of respiration, combustion, &c. by mixing with the phlogiston discharged from the lungs of animals, from burning bodies, from cal. cined metals, from putrefying substances, &c. and in some cases being saturated with it, so as thereby to become phlogisticated. And as a variety of causes contributes to the release and discharge of phlog'iston from bodies in which it is combined, and from which, by certain processes, it is disengaged, which, by uniting with the air, diminishes precipitates its fixed air, and renders it noxious, he has discovered that vegetating plants purify the air by imbibing its phlogiston, and that the agitation of water is also con ducive to the same beneficial purpose.

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