1. _Solids._ Solids retain whatever mechanical form is given to them; their parts are separated with difficulty, and cannot readily be made to unite after separation. They may be either elastic or non-elastic, and differ in hardness, in colour, in opacity, in density, in weight, and, if crystalline, in crystalline form.

2. _Fluids._ Fluids, when in small ma.s.ses, a.s.sume the spherical form; their parts possess freedom of motion; they differ in density and tenacity, in colour, and in opacity. They are usually regarded as incompressible; at least, a very great mechanical force is required to compress them.

3. _Gases._ Gases exist free in the atmosphere, but may be confined.

Their parts are highly movable; they are compressible and expansible, and their volumes are inversely as the weight compressing them. All known gases are transparent, and present only two or three varieties of colour; they differ materially in density.

4. _Ethereal Substances._ Ethereal substances are known to us only in their states of motion when acting upon our organs of sense, or upon other matter, and are not susceptible of being confined. It cannot be doubted that there is such matter in motion in s.p.a.ce. Ethereal matter differs either in its nature, or in its affections by motion, for it produces different effects; for instance, radiant heat, and different kinds of light.

All these forms of matter are under the influence of active forces, such as gravitation, cohesion, heat, chemical and electrical attraction, and these we must now consider.

1. _Gravitation._ When a stone is thrown into the atmosphere, it rapidly descends towards the earth. This is owing to gravitation. All the great bodies in the universe are urged towards each other by a similar force.

Bodies mutually gravitate towards each other, but the smaller body proportionately more than the larger one; hence the power of gravity is said to vary directly as the ma.s.s. Gravitation also varies with distance, and acts inversely as the square of the distance.

2. _Cohesion._ Cohesion is the force which preserves the forms of solids, and gives globularity to fluids. It is usually said to act only at the surface of bodies or by their immediate contact; but this does not seem to be the case. It certainly acts with much greater energy at small distances, but the spherical form of minute portions of fluid matter can be produced only by the attractions of all the parts of which they are composed, for each other; and most of these attractions must be exerted at sensible distances, so that gravitation and cohesion may be mere modifications of the same general power of attraction.

3. _Heat._ When a body which occasions the sensation of heat on our organs is brought into contact with another body which has no such effect, the hot body contracts and loses to a certain extent its power of communicating heat; and the other body expands. Different solids and fluids expand very differently when heated, and the expansive power of liquids, in general, is greater than that of solids.

It is evident that the density of bodies must be diminished by expansion; and in the case of fluids and gases, the parts of which are mobile, many important phenomena depend upon this circ.u.mstance. For instance, if heat be applied to fluids and gases, the heated parts change their places and rise, and the currents in the ocean and atmosphere are due princ.i.p.ally to this movement. There are very few exceptions to the law of the expansion of bodies at the time they become capable of communicating the sensation of heat, and these exceptions seem to depend upon some chemical change in the const.i.tution of bodies, or on their crystalline arrangements.

The power which bodies possess of communicating or receiving heat is known as _temperature_, and the temparature of a body is said to be high or low with respect to another in proportion as it occasions an expansion or contraction of its parts.

When equal volumes of different bodies of different temperatures are suffered to remain in contact till they acquire the same temperature, it is found that this temperature is not a mean one, as it would be in the case of equal volumes of the same body. Thus if a pint of quicksilver at 100 be mixed with a pint of water at 50, the resulting temperature is not 75, but 70; the mercury has lost thirty degrees, whereas the water has only gained twenty degrees. This difference is said to depend on the different _capacities_ of bodies for heat.

Not only do different bodies vary in their capacity for heat, but they likewise acquire heat with very different degrees of celerity. This last difference depends on the different power of bodies for _conducting_ heat, and it will be found that as a rule the densest bodies, with the least capacity for heat, are the best conductors.

Heat, or the power of repulsion, may be considered as the _antagonist_ power to the attraction of cohesion. Thus solids by a certain increase of temperature become fluids, and fluids gases; and, _vice versa_, by a diminution of temperature, gases become fluids, and fluids solids.

Proofs of the conversion of solids, fluids, or gases into ethereal substances are not distinct. Heated bodies become luminous and give off radiant heat, which affects the bodies at a distance, and it may therefore be held that particles are thrown off from heated bodies with great velocity, which, by acting on our organs, produce the sensations of heat or light, and that their motion, communicated to the particles of other bodies, has the power of expanding them. It may, however, be said that the radiant matters emitted by bodies in ignition are specific substances, and that common matter is not susceptible of a.s.suming this form; or it may be contended that the phenomena of radiation do in fact, depend upon motions communicated to subtile matter everywhere existing in s.p.a.ce.

The temperatures at which bodies change their states from fluids to solids, though in general definite, are influenced by a few circ.u.mstances such as motion and pressure.

When solids are converted into fluids, or fluids into gases, there is always a loss of heat of temperature; and, _vice versa_, when gases are converted into fluids, or fluids into solids, there is an increase of heat of temperature, and in this case it is said that _latent_ heat is absorbed or given out.

The expansion due to heat has been accounted for by supposing a subtile fluid, or _caloric_, capable of combining with bodies and of separating their parts from each other, and the absorption and liberation of latent heat can be explained on this principle. But many other facts are incompatible with the theory. For instance, metal may be kept hot for any length of time by friction, so that if _caloric_ be pressed out it must exist in an inexhaustible quant.i.ty. Delicate experiments have shown that bodies, when heated, do not increase in weight.

It seems possible to account for all the phenomena of heat, if it be supposed that in solids the particles are in a constant state of vibratory motion, the particles of the hottest bodies moving with the greatest velocity and through the greatest s.p.a.ce; that in fluids and gases the particles have not only vibratory motion, but also a motion round their own axes with different velocities, and that in ethereal substances the particles move round their own axes and separate from each other, penetrating in right lines through s.p.a.ce. Temperature may be conceived to depend upon the velocity of the vibrations, increase of capacity on the motion being performed in greater s.p.a.ce; and the diminution of temperature during the conversion of solids into fluids or gases may be explained on the idea of the loss of vibratory motion in consequence of the revolution of particles round their axes at the moment when the body becomes fluid or aeriform, or from the loss of rapidity of vibration in consequence of the motion of particles through greater s.p.a.ce.

4. _Chemical Attraction._ Oil and water will not _combine_; they are said to have no chemical _attraction_ or _affinity_ for each other. But if oil and solution of pota.s.sa in water be mixed, the oil and the solution blend and form a soap; and they are said to attract each other chemically or to have a _chemical affinity_ for each other. It is a general character of chemical combination that it changes the qualities of the bodies. Thus, corrosive and pungent substances may become mild and tasteless; solids may become fluids, and solids and fluids gases.

No body will act chemically upon another body at any sensible distance; apparent contact is necessary for chemical action. A freedom of motion in the parts of the bodies or a want of cohesion greatly a.s.sists action, and it was formerly believed that bodies cannot act chemically upon each other unless one of them be fluid or gaseous.

Different bodies unite with different degrees of force, and hence one body is capable of separating others from certain of their combinations, and in consequence mutual decompositions of different compounds take place. This has been called _double affinity_, or _complex chemical affinity_.

As in all well-known compounds the proportions of the elements are in certain definite ratios to each other, it is evident that these ratios may be expressed by numbers; and if one number be employed to denote the smallest quant.i.ty in which a body combines, all other quant.i.ties of the same body will be multiples of this number, and the smallest proportions into which the undecomposed bodies enter into union being known, the const.i.tution of the compounds they form may be learnt, and the element which unites chemically in the smallest quant.i.ty being expressed by unity, all the other elements may be represented by the relations of their quant.i.ties to unity.

5. _Electrical Attraction._ A piece of dry silk briskly rubbed against a warm plate of polished flint gla.s.s acquires the property of adhering to the gla.s.s, and both the silk and the gla.s.s, if apart from each other, attract light substances. The bodies are said to be _electrically excited_. Probably, all bodies which differ from each other become electrically excited when rubbed and pressed together. The electrical excitement seems of two kinds. A pith-ball touched by gla.s.s excited by silk repels a pith-ball touched by silk excited by metals. Electrical excitement of the same nature as that in gla.s.s excited by silk is known as _vitreous_ or _positive_, and electrical excitement of the opposite nature is known as _resinous_ or _negative_.

A rod of gla.s.s touched by an electrified body is electrified only round the point of contact. A rod of metal, on the contrary, suspended on a rod of gla.s.s and brought into contact with an electrical surface, instantly becomes electrical throughout. The gla.s.s is said to be a _non-conductor_, or _insulating substance_; the metal a _conductor_.

When a non-conductor or imperfect conductor, provided it be a thin plate of matter placed upon a conductor, is brought in contact with an excited electrical body, the surface opposite to that of contact gains the opposite electricity from that of the excited body, and if the plate be removed it is found to possess two surfaces in opposite states. If a conductor be brought into the neighbourhood of an excited body--the air, which is a non-conductor, being between them--that extremity of the conductor which is opposite to the excited body gains the opposite electricity; and the other extremity, if opposite to a body connected with the ground, gains the same electricity, and the middle point is not electrical at all. This is known as _induced_ electricity.

The common exhibition of electrical effects is in attractions and repulsions; but electricity also produces chemical phenomena. If a piece of zinc and copper in contact with each other at one point be placed in contact at other points with the same portion of water, the zinc will corrode, and attract oxygen from the water much more rapidly than if it had not been in contact with the copper; and if sulphuric acid be added, globules of inflammable air are given off from the copper, though it is not dissolved or acted upon.

Chemical phenomena in connection with electrical effects can be shown even better by combinations in which the electrical effects are increased by alterations of different metals and fluids--the so-called _voltaic batteries_. Such are the decomposing powers of such batteries that not even insoluble compounds are capable of resisting their energy, for even gla.s.s, sulphate of baryta, fluorspar, etc., are slowly acted upon, and the alkaline, earthy, or acid matter carried to the poles in the common order.

The most powerful voltaic combinations are formed by substances that act chemically with most energy upon each other, and such substances as undergo no chemical changes in the combination exhibit no electrical powers. Hence it was supposed that the electrical powers of metals were entirely due to chemical changes; but this is not the case, for contact produces electricity even when no chemical change can be observed.

_II.--Radiant or Ethereal Matter_

When similar thermometers are placed in different parts of the solar beam, it is found that different effects are produced in the differently coloured rays. The greatest heat is exhibited in the red rays, the least in the violet rays; and in a s.p.a.ce beyond the red rays, where there is no visible light, the increase of temperature is greatest of all.

From these facts it is evident that matter set in motion by the sun has the power of producing heat without light, and that its rays are less refrangible than the visible rays. The invisible rays that produce heat are capable of reflection as well as refraction in the same manner as the visible rays.

Rays capable of producing heat with and without light proceed not only from the sun, but also from bodies at the surface of the globe under peculiar agencies or changes. If, for instance, a thermometer be held near an ignited body, it receives an impression connected with an elevation of temperature; this is partly produced by the conducting powers of the air, and partly by an impulse which is instantaneously communicated, even to a considerable distance. This effect is called the radiation of terrestrial heat.

The manner in which the temperatures of bodies are affected by rays producing heat is different for different substances, and is very much connected with their colours. The bodies that absorb most light, and reflect least, are most heated when exposed either to solar or terrestrial rays. Black bodies are, in general, more heated than red; red more than green; green more than yellow; and yellow more than white.

Metals are less heated than earthy or stony bodies, or than animal or vegetable matters. Polished surfaces are less heated than rough surfaces.

The bodies that have their temperatures most easily raised by heat rays are likewise those that are most easily cooled by their own radiation, or that at the same temperature emit most heat-making rays. Metals radiate less heat than gla.s.s, gla.s.s less than vegetable substances, and charcoal has the highest radiating powers of any body as yet made the subject of experiment.

Radiant matter has the power of producing chemical changes partly through its heating power, and partly through some other specific and peculiar influence. Thus chlorine and hydrogen detonate when a mixture of them is exposed to the solar beams, even though the heat is inadequate to produce detonation.

If moistened silver be exposed to the different rays of the solar spectrum, it will be found that no effect is produced upon it by the least refrangible rays which occasion heat without light; that a slight discoloration only will be produced by the red rays; that the effect of blackening will be greater towards the violet end of the spectrum; and that in a s.p.a.ce beyond the violet, where there is no sensible heat or light, the chemical effect will be very distinct. There seem to be rays, therefore, more refrangible than the rays producing light and heat.

The general facts of the refraction and effects of the solar beam offer an a.n.a.logy to the agencies of electricity.

In general, in Nature the effects of the solar rays are very compounded.

Healthy vegetation depends upon the presence of the solar beams or of light, and while the heat gives fluidity and mobility to the vegetable juices, chemical effects are likewise occasioned, oxygen is separated from them, and inflammable compounds are formed. Plants deprived of light become white and contain an excess of saccharine and aqueous particles; and flowers owe the variety of their hues to the influence of the solar beams. Even animals require the presence of the rays of the sun, and their colours seem to depend upon the chemical influence of these rays.

Two hypotheses have been invented to account for the princ.i.p.al operations of radiant matter. In the first it is supposed that the universe contains a highly rare elastic substance, which, when put into a state of undulation, produces those effects on our organs of sight which const.i.tute the sensations of vision and other phenomena caused by solar and terrestrial rays. In the second it is conceived that particles are emitted from luminous or heat-making bodies with great velocity, and that they produce their effects by communicating their motions to substances, or by entering into them and changing their composition.

Newton has attempted to explain the different refrangibility of the rays of light by supposing them composed of particles differing in size. The same great man has put the query whether light and common matter are not convertible into each other; and, adopting the idea that the phenomena of sensible heat depend upon vibrations of the particles of bodies, supposes that a certain intensity of vibrations may send off particles into free s.p.a.ce, and that particles in rapid motion in right lines, in losing their own motion, may communicate a vibratory motion to the particles of terrestrial bodies.

MICHAEL FARADAY

Experimental Researches in Electricity

Michael Faraday was the son of a Yorkshire blacksmith, and was born in London on September 22, 1791. At the age of twenty he became a.s.sistant to Sir Humphry Davy, whose lectures he had attended at the Royal Inst.i.tution. Here he worked for the rest of his laborious life, which closed on August 25, 1867. The fame of Faraday, among those whose studies qualify them for a verdict, has risen steadily since his death, great though it then was. His researches were of truly epoch-making character, and he was the undisputed founder of the modern science of electricity, which is rapidly coming to dominate chemistry itself. Faraday excelled as a lecturer, and could stand even the supreme test of lecturing to children.

Faraday's "Experimental Researches in Electricity" is a record of some of the most brilliant experiments in the history of science.

In the course of his investigations he made discoveries which have had momentous consequences. His discovery of the mutual relation of magnets and of wires conducting electric currents was the beginning of the modern dynamo and all that it involves; while his discoveries of electric induction and of electrolysis were of equal significance. Most of the researches are too technical for epitomisation; but those given are representative of his manner and methods.

_I.--Atmospheric Magnetism_

It is to me an impossible thing to perceive that two-ninths of the atmosphere by weight is a highly magnetic body, subject to great changes in its magnetic character, by variations in its temperature and condensation or rarefaction, without being persuaded that it has much to do with the variable disposition of the magnetic forces upon the surface of the earth.