STRUCTURE PRODUCED BY MAGNETISM.

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stance enables us to illustrate the truth of the principle just stated, in a most striking manner: If we bring a bar-magnet near some iron filings sprinkled over a

plate of glass, these little bits of iron become at once polarized by induction; and, if then we gently tap the glass, the iron particles will swing round on its smooth surface, and arrange themselves in the most wonderful way. By means of my vertical lantern I can show you this effect most beautifully. I first sprinkle the filings on the glass stage of our lantern, and then, having protected them by a thin covering-glass, I bring near the glass one of the poles of a bar-magnet. . . . Notice how, on tapping the glass, the filings spring into position, arranging themselves on lines radiating from this pole (Fig. 12). Here, evidently, we have a definite structure produced. Let us now clear our stage, and arrange for a second experiment. This time, however, we will lay the bar-magnet on the covering-glass, so that the bits of iron shall be brought under the influence of both of its poles at the same time. . . . See what a beautiful set of curves results on tapping the glass (Fig. 13), and let me beg you to try to carry in your mind for a moment the general aspect of this structure, as well as of the first.

Now, we suppose that, in solid bodies, the structure depends on the polarity of the molecules, and that the molecules, like the bits of iron in our experiment, take up the relative position which the polar forces require. And, next, I will show you that a beam of polarized light develops in some solids an evidence of structure not very unlike that you have just seen.

. .

Returning, then, to our polariscope, I place in the beam of light a plate of Iceland-spar cut in a definite manner. . . . See those radiating lines, and those iriscolored circles (Fig. 14). Does not that remind you of the structure we developed around a single magnetic pole? Next, I will use a similar plate cut from a crystal of nitre; and, see, we have almost the reproduction of the curves about the double pole (Fig. 15). It is the form of the curves as indicating a certain structure, not the brilliant colors, to which I would direct your attention. The iris hues are caused simply by the breaking up of the white light we are using; for the crystal decomposes it to a greater or less extent, like a prism. If, by interposing a plate of red glass, we cut off all the rays except those of this one color, the varied tints disappear, but, in the black curves which now take their place, the analogy I am endeavoring to present becomes still more marked. Certainly, you could have no more striking analogy than this. I can add nothing by way of commentary to the experiments without entering into unsuitable details, and I will only say, further, that I am persuaded that the resemblances we have seen have a profound significance, and that the structure, which the polarized beam reveals in these solid bodies, is really analogous to that which the magnet produces from the iron filings,

LECTURE III.

1

HOW MOLECULES ARE WEIGHED.

In order that we may make sure of the ground we have thus far explored, let me recapitulate the characteristic qualities of the three conditions of matter which I sought to illustrate in the last lecture.

A gas always completely fills the vessel by which it is inclosed. It is in a state of permanent tension, and conforms to the three laws of Mariotte, of Charles, and of Avogadro. A liquid has a definite surface. It can be only very slightly compressed, and obeys neither of these three laws. A solid has a definite structure, and resists both longitudinal and shearing stresses to a limited extent.

Having now presented to you the molecular theory as fully as I can without entering into mathematical details, I come back again to the great law of Avogadro, which is at the foundation of our modern chemistry:

When in the condition of a perfect gas, all substances, under like conditions of temperature and pressure, contain in equal volumes the same number of molecules.

I have already shown you that, if we assume the general truth of the molecular theory (in other words,

E

if we assume that a mass of gas is an aggregate of isolated moving molecules), then the law of Avogadro follows as a necessary consequence from the known properties of aëriform matter, and may, therefore, in a certain limited sense, be said to be capable of proof. As yet, however, we have only considered the purely physical evidence in favor of the law. We come next to the chemical evidence which may be adduced in support of its validity, and this is equally strong.

It would be impossible at the present stage of our study to make the force of this evidence apparent, because, so far as chemistry is concerned, the law of Avogadro is a generalization from a large mass of facts, and the proof of its validity is to be found solely in the circumstance that it not only explains the known facts of chemistry, but that it is constantly leading to new discoveries. This law, as I have intimated, bears about the same relation to modern chemistry that the law of gravitation does to modern astronomy. Modern astronomy itself is the proof of the law of gravitation; modern optics the proof of the undulatory theory of light; and so the whole of modern chemistry, and nothing less, is the proof of the law of Avogadro. I do not say that this great law of chemistry stands as yet on as firm a basis as the law of gravitation; but I do say that it is based on as strong foundations as the undulatory theory of light, and is more fully established to-day than was the law of gravitation more than a century after it was announced by Newton. I have already briefly referred to the history of the law.

The original memoir was published by Amedeo Avogadro in the Journal de Physique, July, 1811. In this paper the Italian physicist "enunciated the opinion that gases are formed of material particles, sufficiently

PROGRESS OF THE INQUIRY.

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removed from one another to be free from all reciprocal attraction, and subject only to the repulsive action of heat;" and, from the facts, then already well established, that the same variations of temperature and pressure produce in all gases nearly the same changes of volume, he deduced the conclusion that equal volumes of all gases, compound as well as simple, contain, under like conditions, the same number of these molecules.

This conception, simple and exact as it now appears, was at the time a mere hypothesis, and was not advanced even with the semblance of proof. The discovery of Gay-Lussac, that gases combine in very simple proportions by volume, was made shortly after, and, had its important bearings been recognized at once, it would have been seen to be a most remarkable confirmation of Avogadro's doctrine. But the new ideas passed almost unnoticed, and were reproduced by Ampère in 1814, who based his theory on the experiments of Gay-Lussac, and defended it with far weightier evidence than his predecessor. Still, even after it was thus reaffirmed, the theory seems to have received but little attention either from the physicists or the chemists of the period. The reason appears to have been that the integrant molecules of Avogadro and the particles of Ampère were confused with the atoms of Dalton, and, in the sense which the chemists of the old school attached to the word atom, the proposition appeared to be true for only a very limited number even of the comparatively few aëriform substances which were then known. Moreover, the atomic theory itself was rejected by almost all the German chemists; and, in physics, the theory of a material caloric then prevailing was not enforced by the new doctrine. In a word, this beautiful conception of Avogadro and Am