Medusæ, pulmograde, form and structure of, ii. 91, 92 naked-eyed medusæ, ii. 91-96 mode of reproduction of, ii. 94 armed or stinging medusæ, ii. 99 luminosity of, in warm seas, ii. 99 development of medusa-buds, ii. 96 alternation of generation of hydræ and medusæ, ii. 96 covered-eyed medusæ, ii. 97 parasites of, ii. 99 abundance of species of, ii. 100 food of medusæ, ii. 101 Medusa-buds, development of, ii. 95 Medusiform zooids of hydrozoa, ii. 88, 89 Melanconiei, characters of the group of, i. 281, 282 Melanogaster, the red truffle of Bath, i. 268 Melanospermeæ, characters of, i. 181 marine forests of, i. 244 structure of the, i. 244, _et seq._ Mercury, conditions under which he may be habitable, i. 55 Mercury, its feeble affinity for iron, i. 98 Meridion circulare, structure and development of, i. 200, 201 Merulius lacrymans, the cause of dry rot in wood, i. 266 Mesocarpus, reproduction of, i. 217 Mesogloias, structure of, i. 245 Metals and their properties, i. 3 alkaline metals, i. 3 of alkaline earths, i. 3 from non-alkaline earths, i. 4 avidity of some of them for oxygen, i. 4 metals whose oxides are not reducible by heat, i. 4 diatomic metals, i. 4 triatomic metals, i. 5 conduction of heat and radiation, i. 5 vaporization, i. 5 spectra of volatilized metals, i. 64 coloured light obtained from the combustion of the salts of different metals, i. 132 spectrum of, not always the same, i. 145 number of the metals of which the spectra has been determined i. 147 metals shown to exist in the sun, i. 151-153 and not to exist in it, i. 154 metals contained in every plant, i. 414 Meteorites probably not of solar origin, i. 153 Mica, opacity of, to the invisible rays, i. 65 Mignonette, weight of the perfume of, i. 45 Mildew of the vine, hop, &c., i. 295 the black mildews of the Azores and Ceylon, i. 297 Miliola, structure and habitat of, ii. 30 abundance of, in the seas of the Eocene period, ii. 31 Miliolidæ, characters of the order, ii. 30 Milk sap of plants, i. 425 vessels of plants, i. 417 Milleporæ complanata or palmipora, structure of, ii. 141 Mimosa pudica, irritability of the tissues of, i. 432 Mind and matter connection between, ii. 5 Molecules of matter, i. 2 cohesion of, i. 25 unit of mechanical force, i. 26 causes of the ethereal undulating motions of, i. 34, 35 _See_ Atoms Mollusca, structure, habitat, and mode of reproduction of, ii. 229, _et seq._ shells of the, ii. 232 naked, ii. 240 winged, ii. 240 Monas corpusculus, extreme minuteness of, ii. 63, 67 and of its ova, ii. 67 Monocotyledonous, or endogenous plants, structure, growth, and reproduction of, i. 383 seeds of this class, i. 383 stems, or axes, i. 384 remarkable plants belonging to the monocotyledons, i. 387