M E R M E R 31 As it takes a height of about 30 inches of mercury to balance the pressure of the atmosphere, a Geisler pump necessarily is a somewhat long-legged and unwieldy instrument. It can be con siderably shortened, the two vessels A and B brought more closely together, and the somewhat objectionable india-rubber tube be dis pensed with, if we connect the air-space in B with an ordinary air- pump, and by means of it do the greater part of the sucking and the whole of the lifting work. An instrument thus modified was constructed by Poggendorff (see his Annalcn, vol. cxxv. p. 151, 1865), and another, on somewhat different principles, by Prof. Dittmar (see the " Challenger" Reports). Even a Geisler s stop-cock requires to be lubricated to be abso lutely gas-tight, and this occasionally proves a nuisance. Hence a number of attempts have been made to do without stop-cocks alto gether. In Topler s pump J this is attained by using both for the inlet and the outlet vertical capillary glass tubes, soldered, the former to somewhere near the bottom, the latter to the top of the vessel. These tubes, being more than 30 inches high, obviously act as efficient mercury-traps ; but the already considerable height of the pump is thus multiplied by two. This consideration has led Alexander Mitscherlich (Pogg. Ann., cl. 420, 1873), and quite lately F. Neisen (Z. f. Instrumentenkunde, 1882, p. 285) to intro duce glass valves in lieu of stop-cocks. As glass floats on mercury, such valves do not necessarily detract from the exhaustive power of the pump. 2. The Dynamic Pump. This was invented in 1865 by H. Sprengel. The instrument, in its original (simplest) form (fig. 3), consists of a vertical capillary glass tube a of about 1 mm. bore, provided with a lateral branch b near its upper end, which latter, by an india- rubber joint governable by a screw- clamp, communicates with a funnel. The lower end is bent into the shape of a hook, and dips into a pneumatic trough. The vessel to be exhausted is attached to b, and, in order to extract its gas contents, a properly regulated stream of mer cury is allowed to fall through the vertical tube. Every drop of mer cury, as it enters from the funnel, entirely closes the narrow tube like a piston, and in going past the place where the side tube enters entraps a portion of air and carries it down to the trough, where it can be collected. If the vertical tube, measuring from the point where the branch comes in, is a few inches greater than the height of the barometer, and the glass and mer- FIG. 3. Sprengel s Air-Pump. cury are perfectly clean, the apparatus slowly but surely produces an almost absolute vacuum. The great advantages of Sprengel s pump lie in the simplicity of its construction and in the readiness with which it adapts itself to the collecting of the gas. It did excellent service in the hands of Graham for the extraction of gases occluded in metals, and since then has become very popular in gas-laboratories, especially in Britain. Many improvements upon the original construction have been proposed. One of these which deserves mention is to pass the mercury, before it enters the "falling" tube, through a bulb in which a good vacuum is maintained, by means of an ordinary air- pump or a second "Sprengel." (W. D. ) MERCURY was the Roman god who presided over barter, trade, and all commercial dealings. His nature is probably more intelligible and simple than that of any other Roman deity. His very name, which is connected with merx, mercator, &c., shows that he is the god of merchandise and the patron of merchants. In the native Italian states no merchants and no trade existed till the influence of the Greek colonies on the coast introduced Greek customs into the cities of the land. All the usages 1 See Dingier s Polytechn. Journal, 1862 ; an improved form by Bessel-Hageu is described in Wiedemann s Annalen, xii. 425, 1881. | and terminology of trade, and all the religious ceremonies connected with it, were borrowed by the Romans from the Greeks. It was no doubt under the rule of the Tarquins, when the prosperity of the state and its intercourse with the outer world were so much increased, that merchants began to ply their trade in Rome. Doubtless the merchants practised their religious ceremonies from the first, but their god Mercurius was not officially recognized by the state till the year 495 B.C. Rome frequently suffered from scarcity of corn during the unsettled times that followed the expulsion of the Tarquins. Various religious innovations were made to propitiate the gods ; in 496 the Greek worship of Demeter, Dionysus, and Persephone was established in the city (see LIBER), and in 495 the | Greek god HERMES (q.v.) was introduced into Rome under the Italian name of Mercurius (Livy, ii. 21, 27). Preller thinks that at the same time the trade in corn was regu lated by law, and a regular college of merchants was instituted. This collegium was under the protection of the god ; their annual festival was on the Ides of May, on which day the temple of the god had been dedicated at the southern end of the circus maximus, near the Aventine ; and the members were called mercwiales as well as mercatores. The Ides of May was chosen as the feast of Mercury, obviously because Maia was the mother of Hermes, i.e., of Mercury (see MAIA) ; and she was wor shipped along with her son by the mercuriales on this day. According to Preller, this religious foundation had a political object ; it established on a legitimate and sure basis the trade between Rome and the Greek colonies of the coast, whereas formerly this trade had been exposed to the capricious interference of the Government officials for the year. Like all borrowed religions in Rome, it must have retained the rites and the terminology of its Greek original (Festus, p. 257). Mercury became the god, not only of the mercatores and of the corn trade, but of buying and selling in general ; and it appears that, at least in the streets where shops were common, little chapels and images of the god were erected. There was a spring dedicated to Mercury between his temple and the Porta Capena ; every shopman drew water from this spring on the Ides of May, and sprinkled it with a laurel twig over his head and over his goods, at the same time entreating Mercury to remove from his head and his goods the guilt of all his deceits (Ovid, Fasti, v. 673 sq.). The art of the Roman tradesman was evidently like that of an Oriental tradesman in modern times, and the word mercurialis was popularly used as equivalent to " cheat." In the Latin poets Mercury is often gifted with some of the manifold characters of the Greek Hermes, but this finer conception seems to have had no real existence in Roman religion. Roman statuettes of bronze, in which Mercury is represented, like the Greek Hermes, standing holding the caduceus in the one hand and a purse in the other, are exceedingly common. The caduceus must have been introduced as a symbol of Mercury at a very early time, for it is found on Italian coins as early as the 4th century before Christ, and we learn that several were kept as sacred objects in the adytum of the sanctuary at Lavinium (Dion. Hal., i. 67). But its foreign origin is shown by the fact that, although it was a sign of peace, it was never borne by the fetiales, the old Italian heralds. The very name is derived from the Greek Kripvxttov. Preller s view (Rom. Myth.) that mercuriales and mercatores are the same guild is a tempting one, but its truth is very doubtful. Mommsen thinks that mercuriales were a purely local guild, viz., the pagani of the Circus valley. MERCURY, in chemistry, is a metal (symbol Hg) which is easily distinguished from all others by its being liquid at even the lowest temperatures naturally occurring in moderate climates. To this exceptional property it owes the synonyms of quicksilver in English (with the Germans quecksilber is the only recognized name) and of hydrar
gyrum (from vBwp, water, and apyvpos, silver) in Grseco-Latin.