Acetic acid, chemical combination forming, i. 97 Acetic fermentation, fungus producing, i. 288 Acetylene, formation of, i. 97 Acetylene, chemical composition of, i. 128 Achlya prolifera, structure and reproduction of, i. 220 Acids, their affinity for alkalies, i. 96 Acineta, structure and mode of reproduction of, ii. 77, 78 food of, and mode of seizing it, ii. 78 Acorn-shell, structure of the, ii. 213 Acrocarpi, characters of the group, i. 328 Acrocladia mamillata, spines of the, ii. 180, 181 Acrosticheæ, characters of the, i. 359, 360 Actinia sulcata, structure of, ii. 132 Actinian polype, structure and mode of reproduction of, ii. 130, 131 Actinian zoophytes, characters of, ii. 130, 131 thread-cells, ii. 132 structure of, ii. 134-136 Actinism of star-light, i. 55 of moon-light, i. 55 of sun-light, i. 56 Actinocyclus undulatus, structure of, i. 199, 200 Actinomma drymodes, structure of, ii. 21 Actinophrys sol, structure and reproduction of, ii. 16-18 certain degree of instinct exhibited by, ii. 18 its enemy, the Amœba, ii. 18 Adder’s tongue fern, i. 365 Adiantieæ, characters of the groups, i. 358 Adiantum Capillus-Veneris, or maiden’s-hair fern, structure and habitat of, i. 359 Adriatic sea, Foraminifera in the ooze of the bed of the, ii. 51 Adularia, or moonstone, fluorescent property of, i. 66 Æcidium, characters of, i. 280 Ægean sea, zones of Algæ in the, i. 259 Æthalium septicum, structure and habitat of, i. 270-272 Amœba-like motions of, i. 270 mode of reproduction of, i. 271 Affinity, chemical, of kind and of degree, i. 95 relation between chemical affinity and mechanical force, i. 98 Agaricini, the order, i. 261 Agaricus, the genus, i. 261, 263 structure of the, i. 263 Agaricus arvensis, spawn of, i. 262 Agaricus campestris, or common mushroom, i. 261 structure and mode of reproduction of, i. 261 Agaricus gardneri, luminosity of, i. 264 Agaricus olearius, luminosity of, i. 264 Air, a non-conductor of electricity, i. 32 amount of force exerted by the sun’s light within the limits of the terrestrial atmosphere, i. 34 absorption of radiant heat by, i. 41 effect of the rays of the sun falling on the earth through pure dry air, i. 53 spectrum analysis of the component parts of the atmosphere, i. 139 Air-vessel of Nereocystis Lutkeana, i. 249, 250 Alaria esculenta, fruit of, i. 249 Alaria Pylaii, fruit of, i. 249 Albumen, formation and structure of, in vegetable organisms, i. 423 Alcohol, chemical combination forming, i. 97 Alcyon zoophytes, characters of, ii. 119, 120 structure and mode of reproduction of, ii. 120-123 Alcyonidium elegans, structure of, ii. 120 Alcyonium digitatum, or dead man’s fingers, structure of, ii. 121, 122 Aldebaran, spectrum of, i. 155