M I C 



[ 166 ] 



M I E 



Previously to that period, the want of 

 proper instruments precluded the culti- 

 vation of this science ; but the discovery 

 of the barometer and thermometer in 

 the 17th, and the invention of accurate 

 hygrometers in the 18th century, sup- 

 plied the pre-existing defects, and ena- 

 bled philosophers to enter on meteorologi- 

 cal observations with accuracy and facility. 



MI'CA. (from mico, to glisten.) This 

 mineral appears to be always the result 

 of crystallization, but is rarely found in 

 regular, well-defined crystals. Most com- 

 monly it appears in thin, flexible, elastic 

 laminae, which exhibit a high polish and 

 strong lustre. These laminae have some- 

 times an extent of many square inches, 

 and, from this, gradually diminish till 

 they become mere spangles, discoverable, 

 indeed, by their lustre, but whose area is 

 scarcely perceptible by the naked eye. 

 Mica is said to contain forty-four per 

 cent, of oxygen. The laminae of mica 

 are easily separated, and may be reduced 

 to a thickness not much exceeding the 

 millionth part of an inch. Mica is easily 

 scratched with a knife, and, commonly, 

 even by the finger-nail. Its surface is 

 smooth to the touch ; its powder is dull, 

 usually grayish, and feels soft. Its co- 

 lours are silver-white, gray, green, brown, 

 reddish, and black, or nearly black. Spe- 

 cific gravity from 2'50 to 2'90. When 

 rubbed on sealing-wax, it communicates 

 to the wax negative electricity. Before 

 the blow-pipe, it fuses into a grey or 

 black enamel. Its constituent parts are, 

 according to Klaproth, silex 48'0, alu- 

 mine 34'20, potash 8 75, oxide of iron 

 4'5, oxide of manganese 0'5. According 

 to others, it is a compound of silicium, 

 potassium, magnesium, calcium, &c., 

 combined with oxygen. Mica is one of 

 the component parts of granite, gneiss, 

 and mica-slate ; it occurs also in syenite, 

 porphyry, and other primary rocks. To 

 quartz and limestone it frequently com- 

 municates a slaty texture. It may always 

 be distinguished from talc by the elas- 

 ticity of its plates, in its want of unctuo- 

 sity, and by its communicating negative 

 electricity to sealing-wax. There are 

 several varieties, or sub-species ; Jame- 

 son enumerates ten. Mica has been em- 

 ployed, instead of glass, in the windows 

 of dwelling-houses. In lanterns it is su- 

 perior to horn, being more transparent, 

 and not so easily injured by the flame. 

 Mica is a doubly refracting substance, 

 with two optic axes ; along which, light is 

 refracted in one pencil. 



MICA'CEOUS. Containing mica ; resem- 

 bling mica. 



MICA'CEOUS IRON ORE. A variety of 

 oxide of iron. This occurs generally in 



amorphous masses, composed of thin six. 

 sided laminae. Colour iron-black, or 

 steel-grey. Lustre metallic. Opaque. 

 Feel greasy. Hardness 5 to 7. Specific 

 gravity from 4'5 to 5'7. It is said to 

 yield nearly 70 per cent, of iron. 



MI'CA SCHIST. > A metamorphic rock, com- 



MI'CA SLATE. $ posed of mica and quartz ; 

 it passes by insensible gradations into 

 clay-slate, and its texture is slaty. Some- 

 times the mica and quartz alternate, 

 though commonly they are more or less 

 intimately mingled ; the mica usually 

 predominating. 



MI'CARELLE. The Finite of Kirwan. See 

 Finite. 



MI'CROPYLE. (from /ziKpoc, small, and 

 TruXoc, gate, Gr. ) A term for the fora- 

 men in the perfect seed ; this foramen is 

 often visible, as in the pea and bean. 



MI'CROSCOPE. (from /tuKpoe, small, and 

 OTKOTTHO, to behold, Gr. microscope, Fr. 

 microscopio, It.) A microscope is an 

 optical instrument for examining and 

 magnifying minute objects. Jansen and 

 Drebell are supposed to have separately 

 invented the single microscope, and Fon- 

 tana and Galileo seem to have been the 

 first who constructed the instrument in 

 its compound form. The single micro- 

 scope is nothing more than a lens or 

 sphere of any transparent substance, in 

 the focus of which minute objects are 

 placed. The best single microscopes are 

 minute lenses ground and polished on a 

 concave tool ; but as the perfect execu- 

 tion of these requires considerable skill, 

 small spheres have often been constructed 

 as a substitute. The most perfect single 

 microscopes ever executed, of solid sub- 

 stances, are those made of the gems, such 

 as garnet, ruby, diamond, &c. Garnet is 

 the best material, as it has no double re- 

 fraction, and may be procured perfectly 

 pure and homogenous. When a single 

 microscope is used for opaque objects, 

 the lens is placed within a concave silver 

 speculum, which concentrates parallel or 

 converging rays upon the face of the ob- 

 ject next the eye. 



When a microscope consists of two or 

 more lenses or specula, one of which 

 forms an enlarged image of objects, while 

 the rest magnify that image, it is called a 

 compound microscope. The ingenuity of 

 philosophers and of artists p has been 

 nearly exhausted in devising the best 

 forms of object-glasses and of eye-glasses 

 for the compound microscope. Dr. 

 Brewster. 



MICROSCO'PIC. That may be seen only by 

 the aid of a microscope. 



MI'EMITE. A mineral, thus named from 

 having been found at Miemo, in Tuscany. 

 A green variety of Dolomite, occurring in 



