MEN 



MER 



Argenteuil, near Paris, imbedded in a 

 clayey marl. 



MENI'SCUS iiJirjvn, the moon). A lens 

 which is concave on one side and convex 

 on the other, its section resembling the 

 appearance of the new moon. 



MENISPERMA'CEiE {/jirjvn, the moon, 

 (nrepfxa, seed ; so named from the cres- 

 cent-jike form of the fruit of the typical 

 genus menispermum). The Cocculus tribe 

 of Dicotyledonous plants. Leaves alter- 

 nate ; flowers polypetalous, unisexual ; 

 stamens hypogynous; fruit a 1-seeded 

 drupe. 



MENISPE'RMIC ACID. An acid 

 obtained from the berries of the meni- 

 spermum cocculus, where it exists in 

 combination with the alkaloid picro- 



MENKAR, or a CETUS. A star of 

 the second magnitude in the head of the 

 southern constellation Cetus. 



ME'NSTRUAL EQUATION. The 

 name given to an apparent monthly dis- 

 placement of the sun in longitude, of a 

 parallactic kind, owing to the real nature 

 of the curve described by the earth's 

 centre, which is not an exact ellipse, but 

 an undulated curve. The actual devia- 

 tion or excursion of the earth from the 

 ellipse is, however, very small, the great- 

 est amount of the menstrual equation 

 being less than the sun's horizontal 

 parallax, or than 8 6". 



ME'NSTRUUM. A chemical term 

 synonymous with solvent, and denoting 

 a liquid which does not change the na- 

 ture of the substance to be dissolved. 

 Thus, pure water is employed to dissolve 

 gum, alcohol to dissolve resins, acids to 

 dissolve the bases of colchicum and 

 squill. 



MENSURATION. The operation of 

 measuring; a term generally denoting 

 the application of arithmetic to geometry, 

 and the methods of finding the dimen- 

 sions and areas of figures, the contents 

 of solid^. &c. 



MEPHI'TIC ACID. A designation 

 of carbonic acid gas, derived from Me- 

 phitis, the fabled Roman goddess of im- 

 pure exhalations. Mephitic air is another 

 name for nitrogen gas. 



MERCA'PTAN. A compound of hy- 

 drogen, carboiji, and sulphur, named from 

 its energetic action on binoxide of mer- 

 cury — quasi mercurium captans. It is 

 alcohol, of which the oxygen is replaced 

 by sulphur. 



MERCA'TOR'S CHART. An artifi- 

 cial mode of representing a sphere upon 

 215 



a plane, adopted for nautical charts. See 

 Projection. 



ME'RCURY. The nearest planet to 

 the sun, from which it appears to be se- 

 parated only from sixteen to twenty-nine 

 degrees. Its direct distance from the 

 earth is 37,143,000 miles; its apparent 

 diameter is about seven minutes, nearly 

 two-fifths that of the earth. It turns on 

 its axis in 24^, 5' 3", and completes its 

 orbit in 87^, 23^, 25' 44", with a velocity 

 of 111,000 miles an hour. It sometimes 

 crosses the disc of the sun, so as to ap- 

 pear like a small dark spot passing over 

 the sun's face ; this is called the transit 

 of Mercury. 



ME'RCURY (in Chemistry). A metal 

 which is always fluid at a temperature 

 above —39°. From its mobility and its 

 resemblance to silver, it is commonly 

 called quicksilver. 



1. Native or Virgin Mercury. The 

 pure metal, found in the form of globules, 

 in cavities of the ores of this metal. 



2. Native Amalgam. An ore consisting 

 of mercury combined with silver. 



3. Native Cinnabar. Native vermilion, 

 or the bisulphuret of mercury ; the ore 

 which yields the mercury of commerce. 



4. Corneous Mercury. Mercurial horn 

 ore, or the proto-chloride of mercury, 



MERE'NCHYMA (/nepos, a part, ?>- 

 xvfxa, an infusion). Sphcsrenchyma. A 

 term applied by Morren to the spherical 

 variety of the parenchyma of plants. 



MERGANl'NiE. Mergina. A sub- 

 family of the Anatidae, consisting of the 

 genus mergus, the goosander or mer- 

 ganser of the British. 



ME'RICARP {fxepo?, a part, Kapirop, 

 fruit). The botanical designation of a 

 half of the fruit of Umbelliferous plants. 

 What are called seeds in these plants are, 

 in fact, fruits, each consisting of two 

 achenia, or mericarps, placed face to face, 

 and separating from a central axis. The 

 two together are called cremocarp {Kpe- 

 fjidco, to suspend), from their being 

 suspended from the common central 

 axis. 



MERI'DIAN (meridies, noon or mid- 

 day). A geographical term, denoting a 

 great circle of the sphere passing through 

 the zenith and the poles of the earth. 

 The line which marks the longitude of a 

 place is called its meridian, because, 

 when the sun passes this circle, it if? 

 noon in all the places situated under it. 

 Modern nations generally adopt the 

 capitals, or observatories, of their own 

 countries as First Meridians; thus the 



