Head and forepart of Tania Solium-Joint of same,

Oxyuris

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near tail end-Head of Larva of Tænia Solium -Distoma ConjunctumLiver Fluke 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

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IN CHEMISTRY.-I.

TRACTION OF GRAVITY-FORCE OF
N-FORCE OF AFFINITY.

y is to ascertain the nature and properof which our world is composed. Of late the chemist has penetrated beyond the 1 of the "spectrum analysis," of which in treat, a new chapter has been added to r Chemistry," which gives some insight of the great centre of our solar system, t stars. In pursuing his investigations, e bodies under his consideration to expeupon them with various forces-heat, s them within the action of re-agents, r in all circumstances, and never predicts es all by experiment; hence chemistry is science.

e to do with bodies, let us in this lesson which act upon "substance," and which emist in his research.

That which has weight" is, perhaps, the efinition. Gases, although they are so anything solid, are yet bodies; they have of air on every square inch is 14.67 lbs., and ecomes wind, which sways the trees, carries t, or in the hurricane devastates a country, lo if the air were imponderable. There ces present in the world which have no ich produces the phenomenon of heat; elecwaves cause the sensation of light, and the traction-these, not being "bodies," do not the range of Chemistry: they rather belong physicist; but it will be necessary to speak take such a prominent part in the decomtion of bodies.

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 ed to each other the suns and worlds which e has ordained that all matter should exert on all bodies in its neighbourhood. This e 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 vity," that is, the attraction which the earth ; the force of that attraction we call its this force decreases with the distance the e 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 ion, which has more claim upon our attention, ty" chiefly in this, that "gravity" acts upon e, whereas the force of cohesion only begins he particles of matter are brought into the

is force that bodies possess solidity, and it liquids "cohesion" was very weak, and had no gases.

of iron, the teeth of the file separate small

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. Solids may be gases under certain circumstances; 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.

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