We have observed that mercury unites with many of the metals: from this pro perty it is used to separate gold and sil ver from the substances with which they are mixed. It is thus capable of extract ing the hundred-thousandth part of its weight of gold. In gilding and silvering, it is, from the same property, the medium of union between the gold or silver, and the metal on which the operation is per formed. Hence mercury is of extensive use in the arts : its amalgam with tin is used in silvering mirrors, and in electri cal experiments. Its importance in the structure of the common barometer is well known : and the uniformity of its ex pansion, at various degrees of heat, has shewn it to be the best fluid for thermo meters.
Miscue:, in astronomy, is a small star that emits a very bright white light : though, by reason of his always keeping near the sun, he is seldom to be seen; and when he does make his appearance, his motion towards the sun is so swift,that he can only be discerned for a short time. He appears a little after sunset, and again a little before sunrise. Mercury never goes to a greater distance from the sun than about 27° 5'; so that he is never longer in setting after the sun than an hour and fifty minutes; nor does he ever rise sooner than an hour and fifty mi nutes before that luminary. Very fre quently, he goes so near the sun as to be lost altogether in his rays. When he be gins to make his appearance in the even mg after sunset, he can scarcely at first be distinguished in the rays of the twi light. But the planet disengages itself more and more, and is seen at a greater distance from the sun every successive evening; and having got to the distance of about 22° 5', it begins to return again. During this interval, the motion of Mer cury, referred to the stars, is direct; but when it approaches within 18° of the sun, it appears for some time stationary ; and then its motion begins to be retrograde. The planet continues to approach the sun, and at last plunges into his rays in the evening, and disappears. Soon after it may be perceived in the morning, before sunrise, separating further and further from the sun, his motion being retrograde as before he disappeared. At the distance of 18° it becomes stationary, and assumes a direct motion, continuing, however, to separate, till it comes to 22° 5' of distance. then it returns again to the sun, plunges into his rays, and appears soon after in the evening, after sunset, to repeat the same career. The angular distance from the sun, which the planet reaches on both sides of that luminary, varies from 16° to nearly 28°. The duration of a complete oscillation, or the interval of time that elapse* before the planet returns again to the point from which it set out, varies al so from 100 to 130 days. The mean arc of
his retrogradation is about 131,°; its mean duration twenty-three days ; but the quantity differs greatly in different re trogradations. In general, the laws of the movements of Mercury are very com plicated; he does not move exactly in the plane of the ecliptic ; sometimes he deviates from it more than 5°. Some considerable time must have elapsed, be fore astronomers suspected that the stars which were seen approaching the sun in the evening and in the morning were one and the same. The circumstance, how ever, of the one never being seen at the same time with the other, would gradual ly lead them to the right conclusion. The apparent diameter of Mercury varies as well as that of the sun and moon, and this variation is obyiously connected with his position relatively to the sun, and with the direction of his movement. The diameter is as its minimum when the planet plun ges into the solar rays in the moaning, or when it disengages itself from them : it is at its maximum when the planet plunges into the solar rays in the evening, or when it disengages itself from them in the eveninii that is to say, when the planet passes the sun in its retrograde motion, its diameter is the greatest possible ; when it passes the sun in its direct motion, it is the smallest possible ; and the mean length of the apparent diameter of Mercury is 11'. Sometimes when the planet disappears during its retrograde motion, that is to say, when it plunges into the sun's rays in the evening, it may be seen crossing the sun under the form of a black spot, which describes a chord along the disk ofthe sun. This black spot is recognized to be the planet, by its position, its apparent dia meter, and its retrograde motion. These transits of Mercury, as they are termed, are real annular eclipses of the sun : they demonstrate that the planet is an opaque body, and that it,horrows its light from the sun. When examined by means of telescopes, magnifying about 200 or 300 times, he appears equally luminous throughout his whole surface, without the least dark spot. But he exhibits the same difference of phases with the moon, being sometimes horned, sometimes gib bous, and sometimes shining almost with a round face, though not entirely full, be cause his enlightened side is never turned directly towards us ; but at all times per fectly well defined, without any ragged edge, and perfectly bright. Like the moon, the crescent is always turned to wards the sun. These different phases throw considerable light on the orbit of Mercury. See Vitzus.