LESSONS IN CHEMISTRY.-I.
INTRODUCTION-ATTRACTION OF GRAVITY-FORCE OF
COHESION-FORCE OF AFFINITY.

THE object of Chemistry is to ascertain the nature and properties of the substances of which the crust of our planet is composed. Of late years, the curiosity of the chemist has penetrated beyond the tangible, and by the aid of the "spectrum analysis," of which in due time we shall treat, a new chapter has been added to the science on "Stellar Chemistry," which gives some insight into the composition of the great centre of our solar system, and even of the distant stars. In pursuing his investigations, the chemist submits the bodies under his consideration to experiment. he operates upon them with various forces-heat, electricity, etc.-brings them within the action of re-agents, watches their behaviour in all circumstances, and never predicts a result, but determines all by experiment; hence chemistry is purely an experimental science.

Seeing that we have to do with bodies, let us in this lesson dwell upon the forces which act upon "substance," and which oppose or assist the chemist in his research.

What is body? "That which has weight" is, perhaps, the least objectionable definition. Gases, although they are 80 intangible, and unlike anything solid, are yet bodies; they have weight. The weight of air on every square inch is 14.67 lbs., and when set in motion it becomes wind, which sways the trees, carries before it clouds of dust, or sweeping in the hurricane it devastates a country, which it could not do if the air were imponderable. There are, however, existences present in the world which have no weight. Caloric, which produces the phenomenon of heat; electricity; ether, whose waves cause the sensation of light, and the different forces of attraction-these, not being "bodies," do not strictly come within the range of Chemistry: they rather belong to the domain of the physicist; but it will be necessary to speak of them, seeing they take such a prominent part in the decomposition and combination of bodies.

The forces of attraction, by which the particles of bodies are bound together, are the attraction of gravity, the attraction of cohesion, the attraction of adhesion, and the force of affinity.

This

The attraction of gravity is that mysterious power by which the Creator has linked to each other the suns and worlds which occupy space; for he has ordained that all matter should exert an attractive force on all bodies in its neighbourhood. force varies with the mass of the bodies and their distances from each other. If a stone be dropped over the edge of a high perpendicular cliff into the sea beneath, it will strike the rock before it reaches the water, because the cliff attracts the stone and draws it towards it. If, however, the stone be carried away from the cliff, the attractive force decreases. The power which made the stone fall was "gravity," that is, the attraction which the earth has for the stone; the force of that attraction we call its "weight." That this force decreases with the distance the stone is taken above the earth, is proved by the fact that the stone would weigh less on the top of a high mountain than in the valley beneath. Of course, to test this fact a spring balance must be used.

The force of cohesion, which has more claim upon our attention, differs from "gravity" chiefly in this, that "gravity" acts upon bodies at a distance, whereas the force of cohesion only begins to operate when the particles of matter are brought into the closest contact.

It is due to this force that the bodies possess solidity, and it would seem that in liquids "cohesion" was very weak, and had

no existence at all in gases.

If I file a piece of iron, the teeth of the file separate small pieces of the metal from the rest; that is, I have applied a force

VOL. II.

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 the 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 of bodies are under two forces -one attracting, the other repelling them, for there must be some force which keeps the particles apart; 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. Molecular repulsion is so closely allied to caloric, the one is so intimately dependent upon the other that they have been thought to be one and 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 temperature, 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