« PreviousContinue »
heat, damp bed-clothes rapidly take it up, until the linen is actually dry, at the expense of that heat, which when the least generated is the most speedily carried off; and disease generally follows. Very many persons do not think as they should do, of the peril of passing hastily from heated places to cold air; of taking ices, or cold liquors, when hot and at rest; of not being sufficiently clothed ; continuing in drafts; standing in the dew; sitting or sleeping in rooms lately washed; remaining too long unclothed after bathing. By these, and similar indiscretions, many have become the subjects of disease, and sunk to an early grave.
Q. Be so kind as to state to me again, how it is that these things become dangerous.
A. The wet clothes, the damp linen, the cold air, the liquids, drafts, wet surfaces, &c., take up large portions of caloric from the body; the wet particles thus become vapour, and as such escape : as long as the humidity remains, the evaporation continues. This process cannot take place without carrying off heat and producing cold. This is well known by persons who live in hot countries: by artificial evaporation they cool their rooms and liquors, and in other ways add to their comfort. At home, the watering of streets and roads in a hot summer's day, not only lays the dust, but, as the water is evaporated, a great portion of the heat is carried off, and the air comfortably cooled.
Q. What is the cause of dew ?
A. The earth by radiation, as the sun declines, and when it is set, parts with its heat more rapidly than the heated air: the surface of the earth, speedily lowered in temperature, takes the caloric from the air immediately in contact with it; the watery particles become condensed, deposited, and are known as dew.
Q. Does this condensation continue during the night?
A. Yes: as the day has been hot, and the radiation of caloric from the earth ceases, dew is found more largely deposited, until the morning sun restores again the equilibrium, and the earth gives heat to the air, as by night it took it.
A. It is generally supposed that the water which falls as rain, is the moisture that is taken up by evaporation. The combination of hydrogen and oxygen may sometimes lead to rain, and very likely it often does, in what is termed thunder showers. These drops, repelled probably by their similarly electrical state by which they are both constituted and kept drops, if the temperature of the atmosphere is low, are frozen in their descent, and fall as hail. But how the vapour that is mixed with or dissolved in the atmosphere, forms clouds, mists, fogs, distils in gentle showers, or falls in torrents, is not fully known; though these changes are supposed to be greatly connected with electricity.
Q. Why are they supposed to be connected with electrical phenomena ?
A. During evaporation, large quantities of electrical matter are taken up from the earth. Such is the effect of evaporation, that a vessel in which it takes place will be changed from a positive to a negative state of electricity ; and be restored to its former state, by the condensation of vapour therein. Clouds and fogs are said to be highly electrified. The descent of rain restores electrical matter to the earth, which dry air could not convey. The state of the air is electrically changed by humidity; and it is restored to its previous state when dry. For these, and many other reasons, it has been thought that electricity is greatly employed in the dissolution of vapour, and perhaps iu its decomposition also. By repulsion these aqueous particles are probably kept asunder, when only mixed with air, until clouds in different states of electricity meet, come in contact with dry air, or the tops of mountains; then this repulsive operation may be so removed as to lead the particles to unite and become too heavy for the air to support: they fall; but, as they have become similarly electrified, they fall as drops, or rain.
Q. Do all persons attribute these atmospherical changes to electricity ?
A. They do not : in the Penny Magazine for June, 1832, it is said, there can be no doubt but the great agent, in the
influence, from various causes, is very unequally distributed over the different parts both of the earth and the air ; and in the same place, is much greater at one time than it is at another. Heat may affect a portion of the atmosphere by expanding its volume; or, in other words, by causing the same number of airy particles, that is, the same weight of air, to occupy more space. When this spreading takes place in the air over any particular part of the earth's surface, the diminished pressure on the exposed portion of the mercury in a barometer will, of course, be shown by the diminished elevation of that in the tube. But it will occasion other effects also in the air itself. The more condensed air from distant places will soon begin to rush with more or less violence towards the situation of diminished pressure. In other words, winds will occur. But the increase of heat has been in the mean time, also, occasioning a more rapid and plentiful evaporation of moisture from the earth, which rising into the air in the form of an invisible vapour, has of course produced more than the usual accumulation there, and is therefore ready to fall again to the earth as soon as the reduction of temperature takes place. Hence the fall of hail, snow, rain, mists, &c. The process, in short, is this: the heat operating on the air, dilutes it, diminishes its weight, and consequently lowers the mercury in the barometer; operating on the terrestrial moisture, raises it by evaporation into the air, and consequently soon becomes charged with more than its usual quantity of humidity. Then comes the wind, the fall of the temperature, and the rain. A fall in the barometer is found to be generally indicative of these changes.
Q. Do all bodies evaporate at the same temperature ?
A. No: some bodies contain so great a portion of caloric, and their particles are held by each other so slenderly, that at the temperature of the atmosphere they will fly off rapidly; as ether, for example.
Q. Has atmospheric pressure great influence in evaporation?
A. Yes, very great. Water does not usually boil until it
180°; and when by an air-pump the atmospheric pressure is removed, it will boil at a very low temperature.
Q. Be so kind as to explain this more fully.
A. Ere liquids can boil, two forces must be met: the first of these is, the attraction of aggregation, by which the particles are more or less firmly held together, and which is generally connected with the caloric they contain. The second is the weight that rests on the liquid, by which the elastic force of the vapour is counteracted. The atmosphere which presses on the surface, is the latter force. As this is lessened by ascent on mountains, by which the length of the superincumbent column is diminished, or by artificial means, the less heat is required to overcome the lessened force, and the elasticity of the vapour is just equal to the actual atmospheric pressure.
Q. What would be the effect of the entire removal of this pressure ?
A. In all probability every liquid would evaporate, and be dissipated : even quicksilver, it is supposed would in this case become vapour.
Q. If the atmosphere were more dense or more rare than it is, what would be the consequences?
A. If more dense, it would be unfit to take up and hold vapour ; if of a high temperature, all the vapour taken up would be a permanently elastic fluid ; and in either case there would be no rain, &c. The atmosphere would also be unsuitable to the wants of man and animals. But in its present state it is adapted to its ends, as by weight and mea
“O Lord, how manifold are thy works ! in wisdom hast thou made them all.”
INVENTION OF LETTERS.
Concerning the learning of the Egyptians, much has been said, and little is known. The testimony, however, of
ments of architecture, and traces of literature in the shape of hieroglyphics and symbols, however unintelligible, prove that they were a wonderful people for gigantic enterprise and indefatigable industry, in achieving what were then the highest feats of manual, intellectual, and mechanic power. On these we shall not expatiate here, as another opportunity will be afforded in the next paper of this series, of considering by whom, and by what means, such marvellous works were executed. At present we shall only allude to them generally, in connexion with the discovery of alphabetical writing. When, where, and by whom, letters were invented, it is now vain to imagine. Notwithstanding the pretensions of Hermes Trismegistos, Memnos, Cadmus, and others, the true history, nay, even the personal existence of these supposed claimants, must be ascertained before the unappropriated honour can be conceded to any one of them. It may, meanwhile, be affirmed, as one of those circumstances humbling to human pride that occasionally occur in history, and which, while they strangely stir the imagination, awaken sublime but melancholy reflection in minds given to muse upon the vanity and mortality of all the things that are done under the sun; it may be affirmed, as one of these humbling circumstances, that the man who conquered the greatest trophy ever won from fate and oblivion, lost his own name, after divulging the secret by which others might immortalize theirs. As a figure of speech, one may be allowed to wish that the first letters in which he wrote that name, whether with a pen of iron on granite, or with his finger on sand, had remained indelible. But his own invention is his monument, which, like the undated and uninscribed pyramid, will remain a wonder and a riddle to the end of the world.
It is allowed, I believe, on all hands, that the Egyptians, from time whereof the memory of man knoweth not to the contrary, possessed three kinds of writing,-hieroglyphical, alphabetical, and, probably, as a link between, logographic, of which latter the Chinese is the only surviving example at this day. Indeed, in all countries where society has emerged