MNIUM. 



[ 506 ] MOLECULAR MOTION. 



having a very prominent, thick dorsal nerve. 

 The base of the leaves composed of some- 

 what parallelogrammic cells, rounded-hexa- 

 gonal or with equal walls towards the apex, 

 very full of chlorophyll, or with the pri- 

 mordial utricle mostly very conspicuous, or 

 much thickened, firm, rarely papillose. This 

 family is divided into two tribes: MNIA- 

 CE^E and POLYTRICHACE.&:. 



MNI'UM, Dill. A genus of Mniaceous 

 Mosses, of acrocarpous and pleurocarpous 

 habit, including many Brya of the British 

 Flora. Among the commonest is M. hor- 

 num=ryum hornum, L. 



MOCHA STONES. The varieties of 

 chalcedony known under this name contain 

 a number of bodies (mineral dendrites) 

 which have been mistaken for plants. 



Compare AGATE, SILICA, and FLINT. 



BIBL. K. Miiller, Ann. N. H. 1843, xi. 

 415. 



MOH'RIA, Swartz. A genus of Schizae- 

 aceous Ferns. 



M. thurifraga (fig. 4GO) ; Cape. (Hook. 

 Syn. 436.) 



Fig. 460. 



Mohria thurifraga. 



A pinnule with sporanges. 



Magnified 25 diameters. 



MOI'NA, Baird. A genus of Entomos- 

 traca, order Cladocera, and family Daphni- 

 ada. 



Char. Head rounded and obtuse ; supe- 

 rior antennae of considerable length, of one 

 piece, and arising from the front of the head 

 near the middle; inferior antennae large, 

 fleshy at the base, and two-branched, one 

 branch three-jointed, the other four- jointed ; 

 legs five pairs. Freshwater. 



M. rectirostris (PI. 19. fig. 23). Cara- 

 pace almost straight or but slightly rounded 

 behind. 



M. brachiata or Iranchiata. Carapace 

 greatly rounded behind. 



BIBL. Baird, Br. Entomos. 100; Grobben, 

 Entw., Jn. Mic. Soc. iii. 77. 



MOIST CHAMBER. Introduced by 



Recklinghausen, improved by Schultze and 

 others ; it enables the object under micro- 

 scopic examination to be placed in a space 

 saturated with moisture, and to be examined 

 without or with the intervention of thin 

 glass. Also it enables an immersion-lens to 

 remain with its water in contact with thin 

 glass over an object in any liquid for a con- 

 siderable time. The simplest form is that 

 of a large glass ring cemented to a broad 

 glass plate : a thin and flexible caoutchouc 

 membrane is fixed to the ring and to the 

 object-glass by india-rubber rings. The 

 growing-slide mostly answers the same pur- 

 pose. See Frey, Mik. 63; Rutherford, 

 Hist. 150 ; Dallinger and Drysdale, M. M. 

 Jn. xi. 97. 



MOLECULAR MOTION. When ex- 

 tremely minute particles of any substance 

 immersed in water or other liquid are exa- 

 mined under the microscope, they are seen 

 to be in a state of vivid motion. A little 

 gamboge or Indian-ink mixed with water 

 will exhibit the phenomenon distinctly 

 enough. The minute particles or molecules 

 are seen to move irregularly, to the right 

 and left, backwards and forwards, as if re- 

 pelled by each other, until the attraction of 

 gravitation ultimately overcomes the force 

 upon which their motion depends, when 

 they sink to the surface of the slide. This 

 applies to the molecules of those substances 

 which are heavier than water. In the case 

 of those which are lighter than water, or 

 the liquid in which they are immersed, the 

 molecules ultimately become adherent to 

 the thin glass covering the slide. 



This motion is in no way connected with 

 evaporation, for it takes place equally when 

 this is completely prevented, just as when 

 it is not. Neither light, electricity, mag- 

 netism nor chemical reagents exert any 

 effect upon it. Heat is the only agent 

 which affects it ; this causes the motion to 

 become more rapid. Hence it might be 

 attributed to the various impulses which 

 each particle receives from the radiant heat 

 emitted by those adjacent. Or, as it takes 

 place when the temperature is uniform, may 

 it not arise from the physical repulsion of 

 the molecules, uninterfered with by gravi- 

 tation, hence free to move ? The effect of 

 heat would then be explicable, because this 

 increases the natural repulsion of the parti- 

 cles of matter, as in the conversion of water 

 into vapour. 

 Molecular motion plays a part in some 



