PAGE XXVIII. Passifloracea, or the Passion-flower Tribe 56 XXIX. Cucurbitaceæ, or 87 the Cucumber Tribe XXX., XXXI. Solanaceae, or the Nightshade Tribe 88, 120 XXXII., XXXIII. Leguminose, or the Leguminous Tribe 121, 152 XXXIV. Boraginaceae, or the Borage Tribe. XXXV. Labiatæ, or the Lip-flower Tribe XXXVI. Scrophulariacea, or the Foxglove Tribe 177 XXXVII., XXXVIII., XXXIX. Compositæ, or . 289 335 362 Lower Oxides of Nitrogen and Compounds of this Gas with Hydrogen Carbon and its Oxides Hydro-carbons, Coal-gas, Flame, etc. Compounds of Carbon with Nitrogen and SulphurThe Halogens-Chlorine 399 "POPULAR CLASSES, - Molluscoida Coelenterata-Hydrozoa 113 145 Actinozoa (Rayed Animals) 183 Echinodermata (Hedgehog Sketch of Haddock, showing its external form, and also the arrangement of its internal organs Transverse section of Haddock - Sketch of Lobster Transverse Vermicularis. Nereis-Transverse Section of Eunice-Proboscis of a nearly allied speciesLeech Circulation in Leech-Section of Earthworm-Transverse Seetion of Ditto- Circulation in Lob-worm Scolopendra Morsitans Glomeris Julus-Antenna and Eyes of Ditto-Under side of double segment of Ditto-Notomata Centrura-Scheme showing nature of Rotary Illusion 313 Privet Hawk Moth-Caterpillar: Pupa, ImagoCommon Wasp: Larva, Pupa, Imago-Under side of Head of Bed-bug Beetle with Dorsal Integuments removed to show Viscera-Bee, Parasitical Insect, seen by Transmitted Light, and highly magnified to show Tracheal System DRAWING, LESSONS IN: 411 17 81 7,39 Foregrounds-High Lights -Setting Drawings, etc. 72 Treatment of Trees and Rule of Three, Single and Double 294, 326 Profit and Per-centage Simple Interest BOOKKEEPING, LESSONS IN: Principles of Double Entry Forms of Drafts, Promis tina-Sectional Diagram showing circulation in a Sponge-Group of Vorticella-Noctiluca Miliaris 113 Eudendrian Ramosum Hydrozoon encrusting a -- verse Section of Pleurobrachia. 1 145 218 . 348 Metalloids-Metals-Salt -Acid- Alkali- BaseChemical Nomenclature Measures of Length, Capacity, and Weight-Thermometry To make a Mercurial Thermometer Caryophyllia Smithii-Dry Coral of Caryophyllia 36 76 Smithii - - Diagrammatic Section, showing how Red Coral is secretedCestum Veneris-One of the Polypes of Alcyonaria 184 An Atoll Fringing Reefs -Formation of Atoll Plates and Holes on Echinus Shell - Ambulacral Plates-Echinus divided to show Alimentary Canal -Spine-Jaws and Teeth -Side View of Single Jaw-Tooth-Inside of Purple-tipped Sea-Urchin 217 'ULAR EDUCATOR. 19 IN CHEMISTRY.—I. ITRACTION OF GRAVITY-FORCE OF ry is to ascertain the nature and proper- ve to do with bodies, let us in this lesson That which has weight" is, perhaps, the action, by which the particles of bodies are the attraction of gravity, the attraction of ion of adhesion, and the force of affinity. gravity is that mysterious power by which ced to each other the suns and worlds which le has ordained that all matter should exert on all bodies in its neighbourhood. This he mass of the bodies and their distances a stone be dropped over the edge of a perpensea beneath, it will strike the rock before it because the cliff attracts the stone and draws however, the stone be carried away from the force decreases. The power which made the avity," that is, the attraction which the earth ; the force of that attraction we call its this force decreases with the distance the ve the earth, is proved by the fact that the less on the top of a high mountain than in Of course, to test this fact a spring balance sion, which has more claim upon our attention, ity" chiefly in this, that "gravity" acts upon pieces of the metal from the rest; that is, I have applied a force which has overcome the power of cohesion, and therefore certain particles have been wrenched from their neighbours. Now I may collect the "filings," and submit them to the greatest pressure I can exert, but I cannot bring them back into their solid state; no pressure which we at present possess seems to be capable of bringing the particles sufficiently near to each other to allow the force of cohesion to come into play. But although particles of bodies are bound thus closely together, yet in no body do they seem to be in actual contact, for all solids are porous. Two hundred years ago this was proved in the case of gold by the "Florentine Experiment;" and if gold, which is almost the densest of metals, can be shown to be porous, we may well believe it of the rest. The "Florentine Experiment" is so celebrated that it demands recital. The question was raised concerning the compressibility of water, and it was determined to try the experiment in the following manner :-A hollow sphere of gold was filled with that liquid; and seeing that a sphere is that solid which possesses the maximum capacity, any alteration in its shape would therefore lessen the quantity of water it could contain. The gold globe was accordingly slightly flattened, and the water oozed through the gold, appearing as dew on the outside. The Florentines, therefore, declared that water was not compressible-a conclusion they had no right to draw unless they could have collected the dew, and found that it exactly filled the space by which the pressure had diminished the capacity of the hollow sphere. Water has been proved to be slightly compressible, and the only use of the Florentine Experiment is to assert that gold is porous. This truth, that the particles of bodies, in spite of the great force of cohesion, are not in actual contact, may be inferred from the fact, that all bodies contract when cooled, which they could not do if their particles were already in contact. Thus it would appear that the particles or molecules of bodies are under two forces-one attracting, the other repelling them; and that the state of the substance, whether it be solid, liquid, or gaseous, will depend upon the ratio which these two forces bear to each other. In the solid state the molecular attraction, or cohesion, is by far the stronger. In the liquid condition the repelling power almost balances the attractive; in a gas it entirely supersedes it, and the atoms are solely under the influence of "molecular repulsion." When the temperature of a body is raised, this molecular repulsion is always increased, each atom being repelled from its neighbour. The body expands, and at last the cohesion is so nearly overcome that the solid becomes a liquid. If the temperature still increase, the atoms are still further repelled, until they cease to have any attraction for each other, and the body becomes a gas. The molecular repulsion is so closely allied to caloric, the one is so intimately dependent upon the other, that they have been thought to be the same thing. That the physical condition of a body entirely depends upon heat may be shown in almost all bodies. Ice becomes, when heated, water-then steam. Put a small piece of zinc in the flame of a blow-pipe: it first becomes red-hot, then melts, and finally goes away in vapour, which burns with a bright white flame, into the oxide of zinc. There is the strongest evidence that all bodies are capable of assuming these three states. and gases, by sufficiently reducing their temperatures, may become, first liquids, then solids. In future lessons we shall find many examples of this interesting fact. rate, whereas the force of cohesion only begins Solids may be gases under certain circumstances; the particles of matter are brought into the ais force that bodies possess solidity, and it of iron, the teeth of the file separate small Adhesion is a force which binds two bodies together by means of some adhesive substance, such as gum, glue, etc. The force of affinity. This is eminently a chemical force. 27 |