water in the processes by which it is formed. This substance is obtained by various methods, which are detailed by writers on chemistry. It may be had by exposing to a strong heat mixtures of sulphur with vegetable matters, as sugar, oil, &c. The specific gravity is as 1.142 to 1. Its smell is extremely fœtid, approaching to that denominated putrid, the effluvia disengaged in the processes of putrefaction consisting partly of this gas. It extinguishes combustion, and is incapable of supporting animal life. If sulphuretted hy drogen gas be kindled in contact with the atmospheric air, it burns with a blue lambent flame. When mixed with atmospheric air, it does not detonate, and the combustion of its elements is not complete, part of the sulphur being deposited on the sides of the vessel. With oxygen gas it detonates. Sulphuretted hydrogen has a peculiar action on metals, by tarnishing them, and communicating to them shades of yellow, purple, &c. It is possessed of the properties of an acid, and enters into combination with the alkalies, and forms compounds, some of which are crystallizable. It changes vegetable blues to red. It decomposes soap; combines with the metallic oxides; and precipitates sulphur from its combinations with potash or lime.

SULPHURIC acid, in chemistry. The name of sulphuric acid is given to the combination of sulphur and of oxygen, with the greatest proportion of the latter. It was formerly called vitriolic acid, because it was obtained from vitriol by distillation, which is a compound of sulphuric acid and an oxide of iron. When it

is strongly concentrated, it is called oil of vitriol. If a quantity of flowers of sulphur be exposed to a degree of heat sufficient to inflame it, and if, when it is in a state of ignition, it be introduced into a jar filled with oxygen gas, it burns with great splendour, and emits a great quantity of white fumes. These fumes may be condensed, by pouring a small quantity of water into the jar, and when this is examined, it is found to possess acid properties. This is the sulphuric acid. It is procured, as appears by this experiment, by burning sulphur in oxygen gas.

The process for obtaining sulphuric acid, in the large way, is the following. A mixture of sulphur and nitre is burnt in leaden chambers. The use of the nitre is, to supply a quantity of oxygen for the combustion of the sulphur. There is a little water in the bottom of the vessel,

which serves to condense the vapours given out during combustion.

The acid which is obtained in this way is very weak, for it is diluted with the water in which it was condensed, which water may be separated by distillation. Even after this it is usually contaminated with a little lead from the vessels, some potash, and sometimes nitric and sulphurous acids. To obtain it perfectly pure, the sulphuric acid of commerce must be distilled. This process is conducted by putting a quantity of the acid into a retort, and exposing it to a degree of heat sufficient to make it boil. The beak of the retort is put into a receiver, in which the acid, as it come over, is condensed. The acid thus purified, is a transparent colourless liquid, of an oily consistence. It has no smell, but a strong acid taste. It destroys all animal and vegetable substances. It reddens all vegetable blues. It always contains water. When this is driven off by a moderate heat, the acid is said to be concentrated. When as much concentrated as possible, the specific gravity is 2, or double that of water; but it can rarely be obtained of greater density than 1.8. The sulphuric acid suffers no change from being exposed to the light. It boils at the temperature of 546°, or, according to Bergman, 540°. When this acid is deprived of its caloric, it is susceptible of congelation, and even of crystalliing in a six-sided pyramid. It crystallizes zation, in flat, six-sided prisms, terminatmost readily, when it is neither too much concentrated, nor diluted with water. Of the specific gravity of 1.6, it crystallizes at the temperature of a few degrees below the freezing point of water. Of the specific gravity of 1.8, it resists the greatest degree of cold. Chaptel observand Mr. Keir found that it froze at 45° of ed it crystallize at the temperature of 48°, the specific gravity of 1.78. Sulphuric acid has a strong attraction for water.

In some experiments that have been the atmosphere, attracted above six times made, sulphuric acid, when exposed to concentrated sulphuric acid, and one part its weight of water. When four parts of ed together, the moment they come in of ice at the temperature of 32°, are mixcontact the ice melts, and the temperature rises to 212°. A greater quantity of caloric is given out, when the two bodies are mixed together in the liquid state. If four parts of the acid and one of water are suddenly mixed together, the temperature of the mixture rises to about 300°. This extrication of caloric, it is

obvious, arises from the sudden condensation of the two liquids, the medium bulk of which is considerably less than the two taken together. So great is the attraction of this acid for water, that the strongest that can be prepared can scarcely be supposed to be entirely free from it. Attempts have been made to determine the proportion of oxygen and sulphur, which enter into the composition of sulphuric acid. According to the experiments of Lavoisier, in which he measur ed the quantity of oxygen absorbed by a given weight of sulphur during combustion, the proportions are,

Sulphuric acid does not combine with oxygen, nor has it any action with azotic gas. It appears that hydrogen has a greater affinity for oxygen than the sulphur has, and therefore the sulphuric acid is decomposed by means of hydrogen gas. In the cold there is no action between hydrogen gas and sulphuric acid; but if they are made to pass through a red hot porcelain tube, the acid is decomposed, water is formed, and sulphur is precipitated. When hydrogen gas is employed in a greater proportion than the half of the acid, the superabundant gas dissolves the sulphur, and is disengaged in the form of sulphurated hydrogen gas. Charcoal has no action on sulphuric acid in the cold; but at the boiling temperature it decomposes it, and converts it into sulphurous acid. If a piece of red hot charcoal be immersed in a quantity of concentrated sulphuric acid, part of the acid is suddenly disengaged under the form of thick white fumes, accompanied with sulphurous acid gas. The sulphuric acid is decomposed; part of its oxygen is attracted by the charcoal, forming carbonic acid, and thus it is reduced to the lowest proportion of oxygen, in the state of sulphurous acid. A similar effect is produced by phosphorus. Phosphorus, with the

assistance of heat, partially decomposes the sulphuric acid, by abstracting part of its oxygen. Phosphoric acid is formed, and sulphurous acid driven off. In the cold, sulphur has no action on sulphuric acid; but when they are boiled together, the sulphur is partly dissolved in the acid, and converts it into sulphurous acid. The sulphur which has been added combines with the oxygen, which is necessary for the constitution of sulphuric acid, and thus the whole is converted into sulphurous acid. Sulphuric acid combines with alkalies, the earths, and the metals forming salts; which in the present language of chemistry are denominated sulphates. This acid is employed in great quantity in many arts and manufactures. It is em ployed also in medicine and pharmacy; the preparation of it, therefore, has long been an object of considerable importance.

SULPHUROUS acid, was formerly called spirit of sulphur, and volatile sulphurthat its nature and composition were disous acid. It was not till the year 1774 covered by the labours of Priestley and Lavoisier. Berthollet afterwards investigated the formation, decomposition, combinations, and uses, of this acid. Fourcroy and Vauquelin also have examined many of its properties, especially the saline compounds which it forms, so that now its properties are well known. The sulphurous acid exists in nature in great abundance, and particularly in the neighbourhood of volcanoes. It is disengaged from some lavas in a state of fusion, and from the soil which is impreg nated with sulphur, when a sufficient degree of heat is applied. It was by the vapours of sulphurous acid that Pliny the naturalist was suffocated in the eruption of Mount Vesuvius, which destroyed Herculaneum, in the 79th year before the Christian æra. When sulphur is burnt in the open air, the fumes that are generated by this slow combustion are sulphurous acid. It was in this way that this acid was formerly obtained. The method of procuring it, which is now followed, is, to decompose the sulphuric acid by means of any substance which deprives it of part of its oxygen. If one part of mercury and two parts of concentrated sulphuric acid be exposed to heat in a glass retort, the mixture effervesces, and a gas is disengaged, which may be collected in jars over mercury In this process the mercury attracts part of the oxygen of the sulphuric acid, and leaves behind that portion which constitutes the sulphurous acid. Sulphurous acid thus obtained is in the state of gas, and it is an

elastic, invisible, and colourless fluid, like common air. It is rather more than double the weight of atmospheric air. It reddens vegetable blues, and then destroys the greater number of them. It is on account of this property that the fumes of sulphur are employed to remove the stains of fruit from linen, and that the sulphurous acid is often used in bleaching When the sulphurous acid is in the form of gas, it does not readily combine with oxygen. In its fluid form it unites more readily, and is converted into sulphuric acid. In making a mixture of sulphurous acid gas and oxygen gas pass through a red hot tube, they combine together, and are converted into sulphuric acid. There seems to be no action between sulphurous acid and azotic gas. Water has a strong attraction for sulphurous acid gas. A piece of ice brought in contact with it is immediately melted, without any perceptible change of temperature. When water is saturated with this gas, it is known by the name of sulphurous acid, or liquid sulphurous acid. The specific gravity is 1.04. At the temperature of 43° water combines with onethird of its weight of sulphurous acid gas; but as the temperature increases, it absorbs it in smaller proportion. It freezes at a temperature a few degrees below 32°, and it passes into the solid state without parting with any of its acid. The liquid sulphurous acid has the smell, taste, and other properties of the gas, and particularly that of destroying vegetable colours. When exposed to the atmosphere, it gradually absorbs oxygen, and passes into the state of sulphuric acid. This change goes on more rapidly when it is diluted with water, and agitated in contact with the air. The sulphuric acid separates the sulphurous acid in the gaseous form from its combinations, and even from water. Concentrated sulphuric acid absorbs this gas, which imparts to it a yellowish brown colour, and renders it pungent and fuming. The two acids strongly attract each other, so that when they are exposed to the action of heat, the first vapour which rises crystallizes in long, white, needle-shaped prisms. This is a compound of the two acids. It smokes in the air, dissolves with effervescence in it, and when thrown into water produces a hissing noise, like a redhot iron. Sulphurous acid is very much employed in the arts, and sometimes in medicine. In the state of gas it is used for the bleaching of silk and wool, by extracting the colouring matter. It re

The compound salts formed by this acid are denominated sulphites.

SUM, in mathematics, signifies the quantity that arises from the addition of two or more magnitudes, numbers, or quantities together. The sum of an equation is, when the absolute number being brought over to the other side of the equation, with a contrary sign, the whole becomes equal to 0: thus, the sum of the equation x3 — 12 x2 + 41 x = 42, is x3 — 12x2+41 x — 42 = 0.

SUMMER, in architecture, is a large stone, the first that is laid over columns and pilasters, in beginning to make a cross vault.

SUMMER, in carpentry, is a large piece of timber, which, being supported on two stone piers, or posts, serves as a lintel to a door, window, &c.

SUN, in astronomy, the most conspicuous of the heavenly bodies, which occupies the centre of the system which comprehends the earth, the primary and secondary planets, and comets. The sun is the magnificent luminary which enlightens these worlds, and by its presence constitutes day. We have referred to this article from the fixed STARS, because the sun agrees with them in several particulars, as in the property of emitting light continually, and in retaining constantly its relative situation, with but little variation: they may have probably many other properties in common. The sun is, therefore, justly considered as a fixed star comparatively near us; and the

stars as suns at immense distances from our earth; and we reasonably infer, from the same analogy, that the stars are possessed of gravitation, and of the other general properties of matter; they are

way between the pole star and Lyra, the plane of the rotation being inclined a Little more than 7° to that in which the earth revolves. The direction of this motion is from west to east. All the rotations of the different bodies which compose the solar system, as far as they have been ascertained, are in the same direction, and likewise all their revolutions, excepting those of some of the comets, and those of some of the satellites of the Herschel planet.

The time and the direction of the sun's rotation are ascertained by the change of the situation of the spots, which are usually visible on his disc, and which some astronomers supposed to be elevations, and others to be excavations in the luminous matter covering the sun's surface. These spots are frequently observed to appear and disappear, and they are in the mean time liable to great varia. tions, though they are generally found about the same points of the sun's surface. M. Lalande supposes them to be parts of the solid body of the sun, which by some agitations of the luminous ocean with which he conceives the sun to be surrounded, are left nearly or entirely bare. Dr. Wilson and Dr. Herschel are disposed to consider this ocean as consisting rather of a flame than of a liquid substance; and Dr. Herschel, in an ingenious paper, attributes the spots to the emission of an æriform fluid, not yet in combustion, which displaces the general luminous atmosphere, and which is afterwards to serve for fuel for supporting the process; hence he supposes the appearance of copious spots to be indicative of the approach of warm seasons on the surface of the earth, a theory which he has attempted to maintain by historical evidence. The exterior luminous atmosphere has an appearance somewhat mottled; some parts of it, appearing brighter than others, have been called facule; but Dr. Herschel distinguishes them by the names of ridges and nodules. The spots are usually surrounded by margins less dark than themselves, which are called shallows, and which are considered as parts of an inferior stratum, consisting of opaque clouds, capable of protecting the immediate surface of the sun from the excessive heat produced by combustion in the superior stratum, and perhaps rendering it habitable to animated beings.

To which Dr. Young replies, if we inquire into the intensity of the heat which must necessarily exist wherever this com

bustion is performed, we shall soon be convinced that no clouds, however dense, could impede its rapid transmission to the parts below. Besides, the diameter of the sun is 111 times as great as that of the earth; and at its surface, a heavy body would fall through no less than 450 feet in a single second; so that if every other circumstance permitted human beings to reside on it, their own weight would present an insuperable difficulty, since it would become thirty times as great as upon the surface of the earth, and a man of moderate size would weigh above two tons.

Dr. Herschel, in another paper, supposes that the spots in the sun are mountains on its surface, which, considering the great attraction exerted by the sun upon bodies placed at its surface, and the slow revolution it has about its axis, he thinks may be more than 300 miles high, and yet stand very firmly. He says, that in August 1792, he examined the sun with several powers, from 90 to 500. And it evidently appeared that the black spots are the opaque ground or body of the sun; and that the luminous part is an atmosphere, which, being intercepted or broken, gives us a glimpse of the sun itself. Hence he concludes that the sun has a very extensive atmosphere, which consists of elastic fluids that are more or less lucid and transparent; and of which the lucid ones furnish us with light. This atmosphere he thinks is not less than 1843 nor more than 2765 miles in height; and he supposes that the density of the luminous solar clouds need not be exceedingly more than that of our aurora borealis, in order to produce the effects with which we are acquainted.

The sun, then, appears to be a very eminent, large, and lucid planet, evidently the first and only primary one belonging to our system. Its similarity to the other globes of the solar system, with regard to its solidity; its atmosphere; its surface diversified with mountains and vallies; its rotation on its exis; and the fall of heavy bodies on its surface; leads us to suppose that it is most probably inhabited, like the rest of the planets, by beings whose organs are adapted to the peculiar circumstances of that vast globe. If it be objected that, from the effects produced at the distance of 95,000,000 miles, we may infer that every thing must be scorched up at its surface; we reply, that there are many facts in natural philosophy, which show that heat is produced by the sun's rays only when they