M 1C 



these effluvia, but it is generally admitted 

 that their base is hydrogen ; whence the 

 use of chlorine in fumigation. 



MI'CA, from mico, to glisten. A mineral 

 which appears to be always the result of 

 crystallisation, though rarely found in 

 regular well-defined crystals, but in their 

 flexible laminae, having a high polish 

 and a shining surface. " These laminae," 

 says Mr. Humble, "have sometimes an 

 extent of many square inches, and from 

 this gradually diminish till they become 

 mere spangles, discoverable indeed by 

 their lustre, but otherwise scarcely per- 

 ceptible to the naked eye. Its colours 

 vary from silver white to black. Sp. gr. 

 2'5 to 2'9. Fuses before the blowpipe into 

 a grey or black enamel. It is one of the 

 component parts of granite, gneiss, and 

 mica-slate ; it occurs also in syenite por- 

 phyry and other primitive rocks." The 

 large sheets of mica used instead of glass 

 in stoves, lanterns, &c., are brought from 

 Siberia, where this mineral serves all the 

 purposes of glass in windows, &c. Jameson 

 enumerates ten subspecies, viz. : mica, 

 pinite, lepidolite, chlorite, green-earth, 

 talc , nacrite, potstone, steatite, and figure- 

 stone. 



MICA'CEOUS IRON -ORE. A variety of iron- 

 ore which occurs in amorphous masses, 

 composed of six-sided laminae, like black 

 mica. Its lustre is metallic and opaque. 

 Feel, greasy. Sp. gr. 4'5 to 57. Yields 

 nearly 70 per cent, of iron. 



MI'CA SCHIST, I A metamorphic rock 



MI'CA SLATE. I composed of mica and 

 quartz. Its texture is slaty, and it passes 

 by insensible gradations into clay slate. 



MIC'HAEL, SAINT. A French order of 

 knighthood, instituted in 1469. 



MICROCOS'MIC SALT. A triple salt, a 

 phosphate of soda and ammonia, obtained 

 originally from urine, and so named be- 

 cause man was regarded by the alchemists 

 as a miniature of the world. It is now 



467 MID 



prepared by mixing equal parts of the 

 phosphate of soda and phosphate of am- 

 monia together in solution, and then crys- 

 tallising. It is much used in assays by 

 the blowpipe. 



MI'CROCOSM, the little world ; from 

 fuzzes, little, and xotrf^x;, the world. 

 The term is used of man, supposed to be 

 an epitome of the universe or great world. 



MICROM'ETER, from /MXfa?, small, and 

 jiter^flv, measure. 1. Any instrument fo' 

 measuring small spaces is a micrometer. 



2. An instrument, by the help of 



which, the apparent magnitude of objects 

 viewed through the telescope is measured 

 with great exactness. There are various 

 forms of the instrument, but the principle 

 of operation is the same. It moves a tine 

 wire parallel to itself in the plane of the 

 picture of the object, formed in the focus 

 of the telescope, and with such accuracy 

 as to measure with great precision its 

 perpendicular distance from a fixed wire- 

 in the same plane, by which means the 

 apparent diameters of the planets and 

 other small angles are e-xactly determined. 

 The micrometer (especially that of mi- 

 croscopes), is sometimes simply an ac- 

 curately and finely divided object-glass. 



MICROPRO'S ics, from fuxfos, small, and 

 tfcavri, sound. The science of magnifying 

 small sounds : whence microphone, an in- 

 strument for magnifying small sounds. 



MI'CROSCOPE, from /u,ix%os, small, and 

 ffxoxiu, to view. An optical instrument 

 contrived to give to the eye an enlarged 

 image of objects which are too mmute to 

 be examined without such aid. There 

 are several sorts of microscopes, as single, 

 compound, reflecting, and solar. The 

 single or simple microscope consists of a 

 single lens. Double or compound micro- 

 scopes have at least two, but generally 

 three or more lenses. The solar microscope 



consists of a common microscope con- 

 nected with a reflector AH, and condenser 

 CD, the first being used to throw the 

 sun's liaht on the latter, by which it is 

 condensed upon the object placed in its 

 focus. This object is also situated in the 

 fociu of a microscopic lens on the other 



side, which transmits a magnified image 

 upon a screen or wall, in a dark room. 

 The lucernal microscope differs from the 

 solar microscope only in this, that it is 

 illuminated by a lamp instead of the sun. 

 MIDDLE Aois. The period from the 8th 

 to the 15th century of the Christian ira. 



2 H 2 



