The aspect of these curves so fascinated | of a polar force, and the ability of that force Faraday that the greater portion of his intel- to produce structural arrangement, your inlectual life was devoted to pondering over evitable answer will be, that those crystals them. He invested the space through which are built by the play of polar forces with they run with a kind of materiality; and the which their ultimate molecules are endowed. probability is, that the progress of science by In virtue of these forees, atom lays itself to connecting the phenomena of magnetism atom in a perfectly definite way, the final with the luminiferous ether, will prove these visible form of the crystal depending upon "lines of force," as Faraday loved to call this play of its molecules. the magnetic curves, to represent a condition of this mysterious substratum of all radiant action.

But it is not with the magnetic curves, as such, that I now wish to occupy your attention; it is their relationship to theoretic conceptions that we have now to consider. By the action of the bar magnet upon the needle we obtain the notion of a polar force; by the breaking of the strip of magnetized steel, we attain the notion that polarity can attach itself to the ultimate particles of matter. The experiment with the iron filings introduces a new idea into the mind; the idea, namely, of structural arrangement. Every pair of filings possesses four poles, two of which are attractive and two repulsive. The attractive poles approach, the repulsive poles retreat; the consequence being a certain definite arrangement of the particles with reference to each other.

Everywhere in Nature we observe this tendency to run into definite forms, and nothing is easier than to give scope to this tendency by artificial arrangements. Dissolve nitre in water, and allow the wate slowly to evaporate; the nitre remains, an the solution soon becomes so concentrate that the liquid form can no longer be pre served. The nitre-molecules approach eacl other, and come at length within the range of their polar forces. They arrange themselves in obedience to these forces, a minute crystal of nitre being at first produced. On this crystal the molecules continue to deposit themselves from the surrounding liquid. The crystal grows, and finally we have large prisms of nitre, each of a perfectly definite shape. Alum crystallizes with the utmost ease in this fash.on. The resultant crystal is, however, different in shape from that of nitre, because the poles of the molecules are differently disposed; and, if they be only nursed with proper care, crystals of these substances may be caused to grow to a great size.

Now, this idea of structure, as produced by polar force, opens a way for the intellect into an entirely new region, and the reason you are asked to accompany me into this region is, that our next inquiry relates to the action The condition of perfect crystallization is, of crystals upon light. Before I speak of that the crystallizing force shall act with dethis action, I wish you to realize the process liberation. There should be no hurry in its of crystalline architecture. Look then into a operation; but every molecule ought to be granite quarry, and spend a few minutes in permitted, without disturbance from its neighexamining the rock. It is not of perfectly bors, to exercise its own molecular rights. If uniform texture. It is rather an agglomera- the crystallization be too sudden, the regution of pieces, which, on examination, pre- larity disappears. Water may be saturated sent curiously-defined forms. You have there with sulphate of soda, dissolved when the what mineralogists call quartz, you have water is hot, and afterward permitted to cool. felspar, you have mica. In a mineralogical When cold, the solution is supersaturated; cabinet, where these substances are preserved that is to say, more solid matter is contained separately, you will obtain some notion of in it than corresponds to its temperature. their forms. You will see there, also, speci- Still the molecules show no signs of building mens of beryl, topaz, emerald, tourmaline, themselves together. This is a very remarkheavy spar, fluor-spar, Iceland spar-possibly able, though a very common fact. The a full-formed diamond, as it quitted the hand molecules in the centre of the liquid are so of Nature, not yet having got into the hands hampered by the action of their neighbors of the lapidary. These crystals, you will ob- that freedom to follow their own tendencies serve, are put together according to law; is denied to them. Fix your mind's eye upon they are not chance productions; and, if a molecule within the mass. It wishes to you care to examine them more minutely, unite with its neighbor to the right but it you will find their architecture capable of wishes equally to unite with its neighbor to being to some extent revealed. They split the left; the one tendency neutralizes the in certain directions before a knife-edge, ex- other, and it unites with neither. We have p sing smooth and shining surfaces, which here, in fact, translated into molecular action are called planes of cleavage; and by follow- the well-known suspension of animal volition ing these planes you sometimes reach an in-produced by two equally inviting bundles of ternal form, disguised beneath the external form of the crystal. Ponder these beautiful edifices of a hidden builder. You cannot help asking yourself how they were built; and familiar as you now are with the notion

hay. But, if a crystal of sulphate of soda be dropped into the solution, the molecular indecision ceases. On the crystal the adjacent molecules will immediately precipitate themselves; on these again others will be precipi

tated, and this act of precipitation will con- | metal, we liberate this metal by the electrotinue from the top of the flask to the bottom, lysis. This small cell contains a solution of until the solution has, as far as possible, as- acetate of lead, and this substance is chosen sumed the solid form. The crystals here because lead lends itself freely to this crysformed are small, and confusedly arranged. tallizing power. Into the cell dip two very The process has been too hasty to admit of thin plati. um wires, and these are connected the pure and orderly action of the crystalliz- by other wires with a small voltaic battery.. ing force. It typifies the state of a nation in On sending the voltaic current through the which natural and healthy change is resisted, solution, the 1 ad will be slwly severed from until society becomes, as it were, supersatu- the atoms with which it is now combined; it rated with the desire for change, the change will be liberated upon one of the wires, and being effected through confusion and revolu- at the moment of its liberation it will obey tion, which a wise foresight might have the polar forces of its atoms, and produce avoided. crystalline forms of exquisite beauty. They Let me illustrate the action of crystallizing are now before you, sprouting like ferns force by two examples of it: Nitre might be from the wire, appearing indeed like vegetaemployed, but another well-known substance ble growths rendered so rapid as to be plainenables me to make the experiment in a bet-ly visible to the naked eye. On reversing the ter form. The substance is common sal- current, these wonderful lead-fror ds will disammoniac, or chloride of ammonium, dis-solve, while from the other wire filaments of solved in water. Cleansing perfectly a glass lead dart through the liquid. In a moment' plate, the solution of the chloride is poured or two the growth of the lead-trees recomover the glass, to which, when the plate is set mences, but they now cover the other wire. on edge, a thin film of the liquid adheres. In the process of crystallization, Nature first Warming the glass slightly, evaporation is reveals herself as a builder. Where do her promoted; the plate is then placed in a solar operations stop? Does she continue, by the microscope, and an image of the film is thrown play of the same forces, to form the vegetaupon a white screen. The warmth of the il- ble, and afterwards the animal? Whatever luminating beam adds itself to that already the answer to these questions may be, trust imparted to the glass plate, so that after a me that the notions of the coming generamoment or two the film can no longer exist in tions regarding this mysterious thing, which the liquid condition. Molecule then closes some have called "brute matter,' will be with molecule, and you have a most impres- very different from those of the generations sive display of crystallizing energy overspread- past. ing the whole screen. You may produce something similar if you breathe upon the frost ferns which overspread your windowpanes in winter, and then observe through a lens the subsequent recongelation of the film. Here the crystallizing force is hampered by the adhesion of the film to the glass; nevertheless, the play of power is strikingly beautiful, Sometimes the crystals start from the edge of the film and run through it from that edge, for, the crystallization being once started, the molecules throw themselves by preference on the crystals already formed. Sometimes the crystals start from definite nuclei in the centre of the film; every small crystalline particle which rests in the film furnishes a starting-point. Throughout the proess you notice one feature which is perfectly unalterable, and that is, angular magnitude. The spiculæ branch from the trunk, and from these branches others shoot; but the angles enclosed by the spiculæ are unalterable. In like manner you may find alum-crystals, quartz-crystals, and all other crystals, distorted in shape. They are thus far at the mercy of the accidents of crystallization; but in one particular they assert their superiority over all such accidents—angular magnitude is always rigidly preserved.

My second example of the action of crystallizing force is this: y sending a voltaic current through a liquid, you know that we decompose the liquid, and if it contains a

There is hardly a more beautiful and instructive example of this play of molecular force than that furnished by the case of water. You have seen the exquisite fern-like forms produced by the crystallization of a film of water on a cold window pane. You have also probably noticed the beautiful rosettes tied together by the crystallizing force during the descent of a snow-shower on a very calm day. The slopes and summits of the Alps are loaded in winter with these blossoms of

the frost. They vary infinitely in detail of beauty, but the same angular magnitude is preserved throughout. An inflexible power binds spears and spicule to the angle of 60 degrees. The common ice of our lakes is also ruled in its deposition by the same angle, You may sometimes see in freezing water small crystals of stellar shapes, each star consisting of six rays, with this angle of 60° between every two of them. This structure may be revealed in ordinary ice. In a sue beam, or, failing that, in our electric beam, we have an instrument delicate enough to unlock the frozen molecules without disturbing the order of their architecture. Cutting from clear, sound, regularly-frozen ice a slab parallel to the planes of freezing, and sending a sunbeam through such a slab, it liquefies internally at special points, round each point a six-petalled liquid flower of exquisite beauty being formed. Crowds of such flowers are thus produced.

A moment's further devotion to the crys- the definite temperature of 39" Fahr. Crys tallization of water will be well repaid; for tallization has virtually here commenced, the the sum of qualities which renders this sub-molecules preparing themselves for the subse. stance fitted to play its part in Nature may well excite wonder and stimulate thought. Like almost all other substances, water is expanded by heat and contracted by cold. Let this expansion and contraction be first illustrated:

A small flack is filled with cofored water, and stopped with a cork. Through the cork passes a glass tube water-tight, the liquid standing at a certain height (ť, Fig. 7) in the tube. The flask and its tube resemble the bulb and stem of a thermometer. Applying the heat of a spirit-lamp, the water rises in the tube, and finally trickles over the top (t). Expansion by heat is thus illustrated.

quent act of solidification which occurs at 32°, and in which the expansion suddenly culminates. In virtue of this expansion, ice, as you know, is lighter than water in the proportion of 8 to 9.*

It is my desire, in these lectures, to lead you as closely as possible to the limits hitherto attained by scientific thought, and, in pursuance of this desire, I have now to invite your attention to a molecular problem of great interest, but of great complexity. I wish you to obtain such an insight of the molecular world as shall give the intellect satisfaction when reflecting on the deportment of water before and during the act of

FIG. 7.

Projection of experiment: E is the nozzle of the lamp, L a converging lens, and ii the image of the liquid column.

Removing the lamp and piling a freezing mixture in the vessel (B) round the flask, the Hiquid column falls, thus showing the contraction of the water by the cold. But let the freezing mixture continue to act: the falling of the column continues to a certain point; it then ceases. The top of the column remains stationary for son e seconds, and afterwards begins to rise. The contraction has ceased, and expansion by cold sets in. Let the expansion continue till the liquid trickles a second time over the top of the tube. The freezing mixture has here produced to all appearance the same effect as the fame. In the case of water, contraction by cold ceases and expansion by cold sets in at

crystallization. Consider, then, the ideal case of a number of magnets deprived of

In a little volume entitled "Forms of Water," have mentioned that cold iron floats upon molten iron. In company with my friend Sir William Armstrong, I had repeated opportunities of witnessing this fact in his works at Elswick, in 1863. Faraday, I remember, spoke to me subsequently of the completeness of iron castings as probably due to the swelling of the metal on solidification. Beyond this, have given the subject no special attention, and I fact of expansion. It is quite possible that the solid know that many intelligent iron-founders doubt the floats because it is not wetted by the molten iron, its volume being virtually augmented by capillary repulsion. Certain flies walk freely upon water in virever, it is easy to burst iron bottles by the force of tue of an action of this kind. With bismuth, how solidification.

I

weight, but retaining their polar forces. If we had a liquid of the specific gravity of steel, we might, by making the magnets float in it, realize this state of things, for in such a liquid the magnets would neither sink nor swim. Now, the principle of gravitation is that every particle of matter attracts every other particle with a force varying as the ininverse square of the distance. In virtue of the attraction of gravity, then, the magnets, if perfectly free to move, would slowly approach each other.

with the force of contraction until the freezing temperature is attained. Here the polar forces suddenly and finally gain the victory. The molecules close up and form solid crystals, a considerable augmentation of volume being the immediate consequence.

We can still further satisfy the intellect by showing that these conceptions can be realized by a model. The molecule of water is composed of two atoms of hydrogen, united to one of oxygen. We may assume the molecule built up of these atoms to be pyramidal. Suppose the triangles in Fig. 8 to be drawn touching the sides of the molecule, and the disposition of the polar forces to be that indicated by the letters; the points marked A being attractive, and those marked R repellent. In virtue of the general attraction of the molecules, let them be drewn towards the E

But besides the unpolar force of gravity, which belongs to matter in general, the magnets are endowed with the polar force of magnetism. For a time, however, the polar forces do not sensibly come into play. In this condition the magnets resemble our water molecules at the temperature say of 50°.

But the magnets come at length sufficiently positions marked by the full lines, and then near each other to enable their poles to nter- suppose the polar attractions and repulsions act. From this point the action ceases to be to act. A will turn towards A', and R will a general attraction of the masses. An at- retreat from R'. The molecules will be caused traction of special points of the masses and a to rotate, their final positions being that shown repulsion of other points now come into play; by the dotted lines. But the circle surround and it is easy to see that the rearrangement ing the latter is larger than that surrounding of the magnets consequent upon the intro- the full lines, which shows that the molecules duction of these new forces may be such as in their new positions require more room. In to require a greater amount of room. This, this way we obtain an image of the molecular I take it, is the case with our water-mole-mechanism active in the case of water. cules. Like the magnets, they approach each other as wholes, until the temperature 39° is reached. Previous to this temperature, doubtless, the polar forces had begun to act, and at this temperature their action exactly balances the contraction due to cold. At lower temperatures the polar forces predominate. But they carry on a gradual struggle

The

demand for more room is made with an energy sufficient to overcome all ordinary resistances. Your lead pipes yield readily to this power; but iron does the same, and bomb-shells, as you know, can be burst by the freezing of water. Thick iron bottles filled with water and placed in a freezing mixture are shivered into fragments by the resistless vigor of molecular force.

We have now to exhibit the bearings of | crystallization upon optical phenomena. According to the undulatory theory, the velocity of light in water and glass is less than in air. Consider, then, a small portion of a wave issuing from a point of light so distant that the portion may be regarded as practically straight. Moving vertically downwards, and impinging on an horizontal surface of glass, the wave would go through the glass without change of direction. But, as the velocity in glass is less than the velocity in air, the wave would be retarded on passing into the denser medium.

But suppose the wave, before reaching the glass, to be oblique to the surface; that end of the wave which first reaches the glass will be the first retarded, the other portions as they enter the glass being retarded in succession. This retardation of the one end of the wave causes it to swing round and change its front, so that when the wave has fully entered the glass its course is oblique to its original direction. According to the undulatory theory, light is thus refracted.

In water, for example, there is nothing in the grouping of the molecules to interfere with the perfect homogeneity of the ether; but, when water crystallizes to ice, the case is different. In a plate of ice the elasticity of the ether in a direction perpendicular to the surface of freezing is different from what it is parallel to the surface of freezing; ice is. therefore, a double refracting substance. Double refraction is displayed in a particularly impressive manner by Iceland spar, which is crystallized carbonate of lime. The difference of ethereal density in two directions in this crystal is very great, the separation of the beam into the two halves being, therefore, particularly striking.

Before you is now projected an image of our carbon-points. Introducing the spar, the beam which builds the image is permitted to pass through it; instantly you have the single image divided into two. Projecting an image of the aperature through which the light issues from the electric lamp, and introducing the spar, two luminous disks, instead of one, appear 'immediately upon the screen. (See Fig. 9.)

The two elements of rapidity of propagation, both of sound and light, in any substance whatever, are elasticity and density, and the enormous velocity of light is attainable because the ether is at the same time of infinitesimal density and of enormous elasticity. It surrounds the atoms of all bodies, but seems to be so acted upon by them that its density is increased without a proportionate increase of elasticity; this would account for the diminished velocity of light in refracting bodies. In virtue of the crystalline architecture that we have been considering, the ether in many crystals possesses different densities in different directions; and the consequence is, that some of these media transmit light with two different-velocities. Now, refraction depends wholly upon the change of velocity on entering the refracting medium; and is greatest where the change of volicity is greatest. Hence, as, in many crystals, we have two different velocities, we have also two different refractions, a beam of light being divided by such crystals into two. This effect is called double refraction.

The two beams into which the spar divides the single incident-beam do not behave alike. One of them obeys the ordinary law of refraction discovered by Snell, and this is called the ordinary ray. The other does not obey the ordinary law. Its index of refraction, for example, is not constant, nor do the incident and refracted rays always lie in the same plane. It is, therefore, called the extraordinary ray. Pour water and bisulphide of carbon into two cups of the same depth; looked at through the liquid, the cup that contains the more strongly-refracting liquid will appear shallower than the other. Place a piece of Iceland spar over a dot of ink; two dots are seen, but one appears nearer than the other. The nearest dot belongs to the most. strongly-refracted ray, which in this case is ths ordinary ray. Turn the spar round, and the extraordinary image of the spot rotates round the ordinary one.

The double refraction of Iceland spar was first treated in a work published by Erasmus Bartholimus, in 1669. The celebrated Huyghens sought to account for the phenomenon