Hyacinth, structure and mode of reproduction of the, i. 388 Hyalæa, structure of the, ii. 242 Hydra fusca, structure and mode of propagation of, ii. 84, 85 Hydra viridis, structure and mode of propagation of, ii. 85 Hydra vulgaris, structure and mode of propagation of, ii. 85 Hydræ, structure of, ii. 81, 84 mode of propagation of, ii. 84 compound fresh-water Hydræ, ii. 85 development of medusa-buds, ii. 95 alternation of generation of hydræ and medusæ, ii. 96 Hydraulic machine, bog moss acts as a, i. 333 Hydridæ, characters of the group, ii. 81 Hydrobromic acid, amount of absorption of radiant heat by, i. 42 Hydrochloric acid, absorption of radiant heat by, i. 41, 42 Hydrodictyon utriculatum, structure and habitat of, i. 211 reproduction and development of, i. 211 Hydrofluoric acid, i. 18 Hydrogen gas, i. 11 affinity of chlorine for hydrogen, i. 19 absorptive power of, i. 39, 41 proportion of hydrogen to oxygen in the composition of water, i. 94 will not combine with carbon, i. 97 weights of the atoms of, as compared with those of oxygen, i. 99 probable cause of its greater cooling power than that of oxygen, i. 115 one of the illuminants in coal gas, i. 118 spectrum analysis of, i. 139, 144 and of hydrogen, carburetted and attenuated, during and after decomposition, i. 141 carburetted, one of the illuminants in coal gas, i. 118 marsh-gas and fire-damp, i. 118 peroxide of, i. 9 combination forming, i. 104 combines with other substances and simple atoms as if it were a simple element, i. 106 sulphuretted, amount of absorption of radiant heat by, i. 41 opacity of, to the invisible rays, i. 65 one of the illuminants in coal gas, i. 118 how freed from coal gas, i. 118 Hydrozoa, campanograde, characters of, ii. 103 ciliograde, characters of the, ii. 101 mode of reproduction of, ii. 103 oceanic, structure of, ii. 81, 86 compound oceanic, structure of, ii. 87 medusiform zooids, ii. 88, 89 mode of reproduction of, ii. 89, 90 Hymenomycetes, family of, i. 261 Hymenophyllum, stem of, i. 339 Hymenophyllum tunbridgense, or film fern, structure and habitat of, i. 362 Hymenophyllum unilaterale, habitat and structure of, i. 362 Hyphomycetes, characters of the family, i. 282 Hypogæi, structure and fructification of the subterranean sub-order of, i. 267, 268 Hyponitrous acid, combination forming, i. 95 Hypopterygium Smithianum, leaves of, i. 330 I Ice, absorption of invisible rays by, i. 65 believed to be a colloid body, i. 111 Iceland moss, characters of, i. 304 Iceland spar, polarization of light and heat in, by refraction, i. 69 change of position of the optical axes of the crystals of, by heat, i. 73 Ileodictyon, eaten as food, i. 268 Impetus, i. 26, 27 heat generated by, i. 27 combustion a case of, i. 30