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from the Dialogue Pola, or Time, in the posthumous work of Sir H. Davy, already named. We have no where seen the philosophy of creation and decay more beautifully illustrated than in this treasurable little volume.

Why is marble used for sculpture and architecture in preference to harder substances?

Because their durability is not in proportion to their hardness; and marble, though much softer than granite, resists longer the attacks of air and moisture.

Why is the temperature of the sea more uniform than that of any inland water, exposed to the atmosphere, and not a hot spring?

Because it possesses in itself a peculiar source of caloric, owing to a variety of causes, the operation of which is unknown to us: The vast body of the water, and the perpetual agitation to which it is exposed, render it less liable to be affected by outward changes of temperature; and this is particularly the case at a considerable depth below the surface; at its upper part, however, it possesses an extensive range of temperature at different seasons of the year. On the shores of England, the surface of the sea is seldom, in the severest weather, lower in its temperature than 40o, or higher, in the hottest summer, than 65o; whereas the heat of rivers, especially when they are shallow and their currents slow, rises higher and sinks lower than either of these points. Booth.

Why does sea-water soon grow offensive by keeping? Because of the decomposition of the animal and vegetable matters which it contains in suspension; these, like all organic bodies, being peculiarly liable to change, and the salt in the water not being sufficient to preserve them from decay.

Why is the sea usually of a green colour ?

Because, probably, of vegetable matter, and perhaps, partially, of two elementary principles, iodine and

brome, which it certainly contains, though these are possibly the results of decayed marine vegetables. These give a yellow tint when dissolved in minute portions in water, and this mixed with the blue of pure water would occasion sea-green.-Sir H. Davy.

Why is it erroneous to suppose that the salt in the sea has been gradually augmented by saline particles brought into it by rivers?

Because this conclusion is totally inadequate to explain the immense quantity of salt existing in the whole mass of the ocean. If the average depth of the sea be ten miles, and it contain 24 per cent of salt, were the water entirely evaporated, the thickness of the saline residue would exceed one thousand feet.-Bakewell's Geology.

Connected with the depth of the sea, the following interesting question has been noticed in a German Journal. Whether in the sea there be depths where no creature is able to live, or whether a boundary be assigned to organic life within those depths, cannot be ascertained. It, however, clearly appears from the observations made by Biot, and other naturalists, that fishes, according to their different dispositions, live in different depths of the ocean.

Why is the Mediterranean of superior saltness to other seas P

Because the Mediterranean expends, by evaporation, three times more water than it receives; the fresh water being so carried off from the surface.

Why is the Dead Sea so called?

Because no living creature is to be found in it. Why is it said, that "nothing sinks in the Dead Sea"

Because of its extreme saltness. Mr. Madden, a recent traveller, bathed in this sea: he could lie, like a log of wood, on the surface, without stirring hand or foot, as long as he chose; but with much exertion he

could just dive sufficiently deep to cover all his body, when he was again thrown on the surface, in spite of his endeavours to descend lower. On coming out of the water, Mr. Madden found his body coated with it, and likewise with an incrustation of salt, about the thickness of a sixpence.

STEAM.

Why has steam such extraordinary power?

Because, in its constitution, two volumes of hydrogen, and one of oxygen, are condensed into two volumes. A cubic inch of water at 40° is expanded by heat into 1694 inches, or nearly a cubic foot of steam, at the temperature of 2120; at which point it is equal to the mean elasticity of the atmosphere, or thirty inches of mercury; when we see the phenomena of boiling.

Water is susceptible of compression, as was originally shown by Canton, and more lately by Mr. Perkins, who finds a pressure of 2000 atmospheres occasioned by a diminution of 1-12th its bulk. (Phil. Trans. 1820.) If submitted to very sudden compression, water becomes luminous, as has been shown by M. Desaignes.— Brande.

Why is steam considered universal in nature ?

Because it is not only formed from water at its boiling point, but rises slowly and quietly from it at all temperatures, even below the freezing point. It is always found mixed with the permanent gases of the atmosphere, even in the driest weather; as may be seen by the dew on a glass of water fresh drawn from a well in summer. Its elasticity at the freezing point is equal to 0.200-inch of mercury, and its force increases in a geometrical progression for equal increments of temperature.

We have elsewhere quoted and simplified the applications of steam,* and, accordingly, here only notice its constitution, which could not with propriety be

* See MECHANICS, pp. 62 to 66.

omitted: for, as Sir Humphry Davy observes, "the steam-engine in its rudest form was the result of a chemical experiment; in its refined state, it required the combinations of all the most recondite principles of chemistry and mechanics; and that excellent philosopher who has given this wonderful instrument of power to civil society, was led to the great improvements he made by the discoveries, of a kindred genius, on the heat absorbed when water becomes steam, and of the heat evolved when steam becomes water."

MURIATIC ACID.

Why is muriatic acid gas termed, in more modern nomenclature, hydrochloric acid gas?

Because it consists of equal volumes of hydrogen and chlorine, mixed and exposed to light. The best mode of showing its composition, is to introduce into a small but strong glass vessel a mixture of the two gases, and to inflame them by the electric spark; no change of volume ensues, and muriatic acid gas results.-Brande.

Oil of vitriol poured upon common salt is a common method of purifying the chambers of the sick; the chlorine, which is thus produced, being a powerful disinfectant. Common salt will remove fruit and wine stains from linen, from the salt being a compound of sodium and chlorine. Sir H. Davy observes upon the first of these processes, "When common salt is decomposed by oil of vitriol, it was usual to explain the phenomenon by saying, that the acid, by its superior affinity, aided by heat, expelled the gas, and united to the soda. But, as neither muriatic acid nor soda exists in common salt, we must now modify the explanation, by saying, that the water of the oil of vitriol is first decomposed; its oxygen unites to the sodium, to form soda, which is seized on by the sulphuric acid, while the chlorine combines with the hydrogen of the water, and exhales in the form of muriatic acid gas."

Why is muriatic acid, dissolved in water, called spirit of salt?

Because it is commonly procured by distilling a mixture of dilute sulphuric acid with common salt; as, 32 parts of salt, and 22 of sulphuric acid, diluted with one third its weight of water. The quantity of real acid in muriatic acid of different densities, is best ascertained by the quantity of pure carbonate of lime, (Carrara marble, for instance) which a given weight of the acid dissolves. Every 50 grains of the carbonate are equivalent to 37 of real acid.-Brande.

Muriatic acid is much employed in the arts. It is the best test for silver: if a single drop be poured into any solution containing this metal, a copious precipitate ensues, owing to the affinity of this acid for the silver, and the insolubility of the muriate of silver thus formed. Muriatic acid dissolves tin and lead.

Why is muriatic acid recommended for cleaning old books and prints?

Because, though it removes the stains of common ink, it does not affect printers' ink. For the latter use, add half an ounce of red lead to three ounces of common muriatic acid. Where writings have been effaced for fraudulent purposes with this acid, sulphuret of ammonia, and prussiate of potash, will revive the writing and discover the artifice. Very old writing may be revived in this way. If indigo and oxide of manganese be added to common ink, it will prevent its being effaced by chlorine.-Parkes.

With various bases, muriatic acid forms the salts called muriates; and muriates, when in a state of dryness, are actually chlorides.

NITROGEN.

Why is nitrogen also called azote?

Because an animal immersed in it is immediately suffocated (from a, privative, and wn, life): but, if that term be taken its strict sense, all gaseous bodies

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