ii. 183-186

Hop, fungus constituting the mildew of the, i. 295

Hormosiphon arcticus, wide distribution of, i. 212

Hormotrichum, structure of the genus, i. 222

Hormotrichum collabens, structure and mode of reproduction of, i. 222

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,