Heat generated by impetus, i. 27 quantity of heat which would be generated if it were arrested in its orbit, i. 27 probable cause of the heat of the sun, i. 28 effect of the absorption of heat on a body in expansion and contraction, i. 28 specific heat, i. 28 mechanical equivalent of heat, i. 29 causes of the heat which is the motive force of the steam-engine, i. 29 reciprocal action of heat and electricity, i. 31 intensity of the heat of the electric spark, i. 32 proof of the correlation of heat and electricity, i. 33 constancy in the amount and refrangibility of the light and heat absorbed and radiated, i. 34 property of some substances in the transmission of heat, i. 36 substances which transmit radiant heat freely but radiate badly, and vice versâ, i. 37 Tyndall’s experiments on the radiation and absorption of radiant heat by gases and vapours, i. 38 relation between the density of the gas and the quantity of heat extinguished or absorbed, i. 40 absorption of radiant heat by the vapours of volatile liquids, i. 40 Prof. Tyndall’s experiments showing the radiation to be equal to the absorption of radiant heat, i. 46 action of different thicknesses of the same gas or vapour on radiant heat, i. 47 dynamic absorption and radiation, i. 49 dynamical evolution of heat, i. 52 experiment illustrating the change of heat into light, i. 62 polarization of heat and light, i. 68, 69 by reflection and refraction, i. 69 effect of heat on the magnetism of iron, nickel, and cobalt, i. 77 extreme heat and light of electric discharges, i. 84 instances of the correlation of electricity and heat, i. 91 capacity of atoms for heat, i. 100 Mr. Croll’s experiments, i. 101 effects of heat on vegetation, i. 169 large amount of heat evolved by vegetables, i. 265 cause of animal heat, ii. 63 Helix, or snail, tentacles of, ii. 236 Helix aspersa, structure of the tongue of, ii. 237 Helix pomatia, teeth of, ii. 237 Hellebore, white, poisonous alkaloid of, i. 427 Helmet shell, structure of, ii. 234 Helminthosporium Hoffmanni, spores of, i. 285, 286 Helminthosporium nodosum, spores of, i. 286. Helvella, structure and habitat of, i. 291 Hepaticæ, or liverworts, characters of the, i. 316 Hepialus virescens of New Zealand, Cordyceps Robertsii of the, i. 293 Herschel, Sir W., his discovery of invisible rays of light of high heating power, i. 36 Hewardia, structure of, i. 359 Himanthalia lorea, structure, fructification and habitat of, i. 256 Holly fern, structure of, i. 347 Holothuridæ, or sea-cucumbers, structure and mode of reproduction of, 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