M ID 



MIN 



MIDDLE EPOCH. A geological epoch 

 characterized by the presence of the new 

 red sandstone, a formation consisting of 

 sand and marl, with rare local interpola- 

 tions of limestone. See Geology. 



MIDDLE LATITUDE SAILING. A 

 nautical term employed in estimating the 

 difference of longitude by means of the 

 differences. of latitude and the interme- 

 diate departure; this departure is sup- 

 posed to be an arc of a parallel of 

 longitude at the intermediate or middle 

 latitude. 



MIDDLE TERM. In a logical syllo- 

 gism, the middle term (called by the older 

 logicians " argumenturn") is that with 

 which each of the other terms, the major 

 and the minor, is separately compared, in 

 order to judge of their agreement or dis- 

 agreement with each other. 



MI'DRIB. Costa. The midrib of a 

 leaf is the principal vein, which forms a 

 continuation of the petiole and the axis 

 of the leaf; from this all the other veins 

 diverge, either from its sides or its 

 base. 



MI'EMITE. A variety of Werner's 

 rhomb-spar, or magnesian limestone, 

 found at Miemo in Tuscany, imbedded 

 in gypsum. There are two kinds of 

 miemite, the granular and the prismatic. 



MILDEW. A disease in plants, caused 

 by the ravages of parasitic fungi. 



MILE, GEOGRAPHICAL. Every 

 degree of latitude contains 60 geographi- 

 cal miles, and these are always under- 

 stood to be used, unless other miles are 

 specified. They must not be confounded 

 with the British statute mile, which is 

 the common itinerary measure of our 

 country, and 69| of which are contained 

 in a degree of latitude. Hence a great 

 circle contains only 21,600 geographical 

 or nautical miles, whilst it contains 

 24,869 common or statute miles ; or, a 

 common mile is to a geographical mile 

 as 21,600 to 24,869. To express this 

 proportion in feet, we must say, 

 21,600 : 24,869 ; : 5280ft. I 6079089ft. 

 Hence a geographical or nautical mile is 

 about 6079 English feet. 



MILK aUARTZ. Rose quartz. A 

 sub-species of rhomboidal quartz, with a 

 milk-white or rose-red colour, found in 

 Bavaria in beds of quartz in granite. 

 It is probably silica, coloured by man- 

 ganese. 



MILK VESSELS. Laticiferous tissue. 



A peculiar tissue, consisting of branched 



anastomizing tubes, lying in the bark or 



near the surface of plants, and containing 



220 



a milky juice. They are also called vital 

 vessels, vessels of the latex, &c. See 

 Cinenchyma. 



MILKY WAY. An astronomical term 

 applied to that great luminous band 

 which stretches across the sky, from 

 horizon to horizon, and which, when 

 examined through powerful telescopes, 

 is found to consist entirely of stars scat- 

 tered by millions, like glittering dust, on 

 the black ground of the general heavens. 



MILLEPO'RID^ {mille, a thousand, 

 porus. a pore). A tribe of lithophytous 

 polyps, comprising those in which the 

 calcareous axis is perforated by numerous 

 conical pores. » 



MILLSTONE GRIT. A coarse sandy 

 grit, unlike the old red sandstone, fre- 

 quently replacing or covering up the 

 coral limestones in many parts of Eng- 

 land and Ireland. 



MI'NERAL. A simple mineral is a 

 homogeneous substance, whether simple 

 in an elementary point of view or not, 

 which presents itself in certain definite 

 forms, and has a texture, mode of break- 

 ing, and other physical properties, as 

 hardness and colour, by which it may be 

 distinguished from all other substances. 

 The science which treats of these sub- 

 stances is called mineralogy, by some 

 oryctognosy ; while to geology belongs, 

 among other subjects, the consideration 

 of their aggregation into rocks, which, in 

 a geological sense, are masses of mineral 

 matter, of sufficient extent to constitute 

 an essential portion of the solid part of 

 the globe. 



1. Minerals are termed crystalline, 

 when they assume particular symmetri- 

 cal forms, as that of a cube or a prism ; 

 amorphous, when they occur in no defi- 

 nite form. 



2. The hardness of minerals is the 

 resistance which they oppose to the sepa- 

 ration of their parts. Under this head 

 (p. 159) the mode of testing their hard- 

 ness is explained. Their fragility denotes 

 the comparative ease with which they 

 may be fractured; their friability, an 

 imperfect state of aggregation, rendering 

 them easily divisible into grains. 



3. The fracture of minerals, or the 

 surface exposed when they are broken 

 by a hammer, varies considerably : it 

 may be even, splintery, scaly, or con- 

 choidal— that is, concave like a shell. 



4. Minerals are transparent, when they 

 allow sufficient light to pass through 

 them to enable a body to be distinguished 

 when placed behind them; translucent^ 



