ANTICIPATION IN SCIENCE.

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bearing on the subject we are considering. Almost all the great generalizations of science have been more or less fully anticipated, at least in so far that the general truth which they involve has been previously conceived. The Copernican theory was taught, substantially, by the disciples of Pythagoras. The law of gravitation was suggested, both by Hooke and Cassini, several years before Newton published his "Principia ;" and the same general fact has been recently very markedly illustrated in the discovery of the methods of spectrum analysis, every principle of which had been previously announced. The history of science shows that the age must be prepared before really new scientific truths can take root and grow. The barren premonitions of science have been barren because these seeds of truth fell upon unfruitful soil; and, as soon as the fullness of the time was come, the seed has taken root and the fruit has ripened. No one can doubt, for example, that the law of gravitation would have been discovered before the close of the seventeenth century if Newton had not lived; and it is equally true that, had Newton lived before Galileo and Kepler, he never could have mastered the difficult problems it was his privilege to solve. We justly honor with the greatest veneration the true men who, having been called to occupy these distinguished places in the history of science, have been equal to their position, and have acquitted themselves so nobly before the world; but every student is surprised to find how very little is the share of new truth which even the greatest genius has added to the previous stock. Science is a growth of time, and, though man's cultivation of the field is an essential condition of that growth, the development steadily progresses, independently of any in

dividual investigator, however great his mental power. The greatest philosophical generalizations, if premature, will fall on barren soil, and, when the age is ripe, they are never long delayed. The very discovery of law is regulated by law, or, as we rather believe, is directed by Providence; but, however we may prefer to represent the facts, this natural growth of knowledge gives us the strongest assurance that the growth is sound and the progress real. Although the foundations of science have been laid in such obscurity, its students have worked under the direction of the same guiding power which rules over the whole of Nature, and it cannot be that the structure they have reared with so much care is nothing but the phantom of a dream. Still it is true that, beyond the limits of direct observation, our science is not infallible, and our theories and systems, although they may all contain a kernel of truth, undergo frequent changes, and are often revolutionized.

Through such a revolution the theory of chemistry has recently passed, and the system which is now universally accepted by the principal students of the science is greatly different from that which has been taught in our schools and colleges until within a few years. I have, therefore, felt that the best service I could render in this course of lectures would be to explain, as clearly as I am able, the principles on which the new philosophy is based, and to show in what it differs from the old. I have felt that there were many who, having studied what we must now call the old chemistry, would be glad to bridge over the gulf which separates it from the new, and to become acquainted with the methods by which we now seek to group together and explain the old facts.

STARTING-POINT OF THE NEW CHEMISTRY.

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Those who studied the science of chemistry twenty years ago, as it was taught, for example, in the works of the late Dr. Turner, were greatly impressed with the simplicity of the system and the beauty of its nomenclature. Until within a few years the same could not be said of our modern chemistry. It has been passing through a process of reconstruction, and displayed the imperfections of any half-built edifice; but it has now reached a condition in which it can be presented with the unity of a philosophical system. Our starting-point in the exposition of the modern chemistry must be the great generalization which is now known as the law of Avogadro, or Ampère. This law was first stated by Amedeo Avogadro, an Italian physicist, in 1811, and was reproduced by Ampère, a French physicist, in 1814. But, although attained thus early in the history of our science, this grand conception remained barren for nearly half a century. Now, however, it holds the same place in chemistry that the law of gravitation does in astronomy, though, unlike the latter, it was announced half a century before the science was sufficiently mature to accept it. The law of Avogadro may be enunciated thus:

EQUAL VOLUMES OF ALL SUBSTANCES, WHEN IN THE STATE OF GAS, AND UNDER LIKE CONDITIONS, CONTAIN THE

SAME NUMBER OF MOLECULES (Avogadro, 1811—Ampère, 1814).

The enunciation of this law is very simple, but, before we can comprehend its meaning, we must understand what is meant by the term MOLECULE. This word is the one selected by Avogadro in the enunciation of his law. It is obviously of Latin origin, and means simply a little mass of matter. Ampère used in

its place the word particle, in precisely the same sense. Both words signify the smallest mass into which any substance is capable of being subdivided by physical processes; that is, by processes which do not change its chemical nature. In many of our text-books it is defined as the smallest mass of any substance which can exist by itself, but both definitions are in essence the same.

As this is a very important point, it must be fully illustrated. In the first place, we recognize in Nature a great variety of different substances. Indeed, on this fact the whole science of chemistry rests; for, if Nature were made out of a single substance, there could be no chemistry, even if there could be intelligences to study science at all. Chemistry deals exclusively with the relations of different substances. Now, these substances present themselves to us under three conditions: those of the solid, the liquid, and the gas. Some substances are only known in one of these conditions, others in only two, while very many may be made to assume all three. Charcoal, for example, is only known in the solid state; alcohol has never been frozen, but can easily be volatilized; while, as every one knows, water can most readily be changed both into solid ice and into aëriform steam. Let me begin with this most familiar of all substances to illustrate what I mean by the word molecule.

When, by boiling under the atmospheric pressure, water changes into steam, it expands 1,800 times; or, in other words, one cubic inch of water yields one cubic foot of steam, nearly. Now, two suppositions are possible as modes of explaining this change.

The first is, that, in expanding, the material of the water becomes diffused throughout the cubic foot, so as to fill the space completely with the substance we

PARTICLES SEPARATED IN STEAM.

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FIG. 1.

call water, the resulting mass of steam being absolutely homogeneous, so that there is no space within the cubic foot, however minute, which does not contain its proper proportion of water.

The second is, that the cubic inch of water consists of a certain number of definite particles, which, in the process of boiling, are not subdivided, so that the cubic foot of steam contains the same number of the same particles as the cubic inch of water, the conversion of the one into the other depending simply on the action of heat in separating these particles to a greater distance. Hence the steam is not absolutely homogeneous; for, if we consider spaces sufficiently minute, we can distinguish between such as contain a particle of water and those which lie between the particles. Now, the small masses of water, whose isolation we here assume, are what Avogadro calls molecules, and, follow

B