been wholly driven out; but not till a recent date has it formally and thoroughly entrenched itself there; and there must the conflict be waged to a final issue.

In place of its old weapons, unbelief has substituted the facts and especially the theories of science. But facts are notoriously "stubborn things," and would serve no purpose of the enemy were it not that the exigencies of human thought oblige the mind to reach truth through the process of generalization, a necessary road to truth, yet one that almost surely leads to error. The generalizations of science, its higher forms of truth, are expressed in theories, which, when fully matured and verified, become laws. Premature generalizations are a necessary evil. Without them the correct generalization would never be attained. As investigation in any department proceeds, facts must be provisionally grouped as they arise. Unless this is done, the mind is bewildered in the chaos itself creates. Facts unclassified are unmanageable, and therefore barren. It is not till they are generalized that they become fruitful. Any theory is said to be good when it groups facts in logical order; and hence it is true, as we often hear, that one man's theory is as good as another's, provided it be as consistent with facts. But theory, as the original of the word shows, is a "looking at" of facts, a mode of explaining phenomena. What explains them to-day may not to-morrow, when new ones are added; hence theories are in general but temporary, serving their purpose for a time, to give place to others equally short-lived, it may be. And yet, a theory that in its prime was of goodly and rotund proportions shall sometimes maintain its place upon the stage of science until in its decadence it comes to be but the phantom of its former self. Such a one was that of phlogiston, of which Liebig1 says it was an "idea so great and vast that even when entirely perforated, as it were, in all directions, it left enough matter to occupy the powers of thought of mankind for a century."

This lingering vitality of an effete theory is often due to the want of a better system; but as often, at least with individuals, it is assignable to quite another cause. Familiar as every intelligent person is with the difference between theory and fact so far as their definitions are concerned, no truth is more forcibly taught by the history of science than this, that an abstract theory has often taken concrete form and substance in the minds of its advocates, so as to be held at length by them as equivalent to or as practically being real fact.

On no other supposition can we explain the warfare in defence of

1 Letters on Chemistry, 3d London ed., p. 48.

phlogiston, waged against the consequences of their own discoveries by three of the four greatest chemical discoverers of the eighteenth century, Black,' Cavendish, and Priestly.

In no other way can we account for the fact that in our own century Berzelius held, with equal confidence in their validity, his own actual discoveries, and the celebrated electro-chemical theory that he originated. Supplying, as the latter did apparently, the firmest conceivable basis to the dualistic system of Lavoisier, "it gained the mastery over all minds. It reigned in books. It was supreme in education. It led to great discoveries. "The frequent repetition of an opinion often gives rise to a conviction of its truth.' Berzelius said so; and his words may be applied to his own opinions. These have reigned so long that we [too] have insensibly become accustomed to take for demonstrated truth that which is only hypothesis.' These last are the words of Wurtz,2 a distinguished promoter of chemical discovery, and historian of chemical theory. But accepted as was this double theory by all chemists in 1830, and completing mineral chemistry as it appeared to do, it was untrue, and was after a few years abandoned by all leading chemists except Berzelius. He maintained it because it had grown with him to be a fact. Long and powerfully did he defend it, but was compelled some years before his death, in 1848, to give it up. Yet, strange to say, it was in 1868, with slight modifications, still retained in every American text-book of the science, one only excepted, because teachers could find nothing comparable, as an instrument of elementary instruction, to this theory, defunct twenty-five years before. And now that at last they have laid it aside, it is with a sigh; for with it goes, we trust not forever, the beautiful simplicity of chemistry.

If an abstraction can become thus real to the masters of science, surely the same may happen to investigators of lower order, and to the ordinary teachers and students of science. Faraday has left us proof, in the notes of one of his lectures, that he saw need of pointing out to his hearers this danger. He says:3

I begin with a warning against speculation, so end by a warning against too much assurance. What is to us the experience of past ages? All sure in their days except the most wise. Yet how little remains. And are we wiser in our generation? Was earth, air, fire, and water right; then salt, sulphur, and mercury [alluding to the ancient doctrine of four elements and the alchemistic one of three]; then oxygen and oxy

1 Black at last became the supporter of the anti-phlogistic chemistry.

"History of Chemical Theory, Watts' trans, p 70.

2

Life and Letters, by Bence Jones, II, p. 179.

acids; now atoms? We may be sure of facts; but our interpretation of facts we should doubt. He is the wisest philosopher who holds his theory with some doubt, who is able to proportion his judgment and confidence to the value of the evidence set before him, taking a fact for a fact, and a supposition for a supposition, as much as possible keeping his mind free from all sources of prejudice, or where he cannot do this, as in the case of a theory, remembering that such a source is there.

Observe the closing caution of this great philosopher, that theory is necessarily a source of prejudice to the mind.

Elsewhere Faraday testifies that "all our theories are fixed upon uncertain data, and all want alteration and support from facts."1

The distinction between fact and theory, so obvious when stated, is particularly liable to be forgotten by us, because it is necessarily overlooked in scientific text-books, the source of the student's earliest and most lasting conceptions. One has said of them:2

It is the sole aim of these elementary treatises to teach the present state of knowledge; and they would fail in their object if they attempted by a critical analysis to separate the phenomena from the laws or systems by which alone the facts of nature are correlated and rendered intelligible.

The same able chemist, and most philosophical of American writers on the science, goes on to say:3

But although while studying science itself we may for a time waive the distinction between fact and theory, the moment we come to compare the results of science with the eternal truths of religion, the distinction here enforced becomes of paramount importance; and it must be ever our chief aim to separate that which is absolute and eternal truth from that which even in its highest development is the result of human thought, and, like all things human, subject to limitations and liable. to change.

Having thus noticed the existence of a source of danger, both to science and religion, that is very commonly overlooked, let us pass in brief review the theoretical foundations of two of the natural sciences, for the sake of the suggestions which they yield in respect to the proper attitude of the Christian teacher.

But though we turn from the bright and attractive obverse to the darker reverse of science, let us not be mistaken for its impugner. At the proper time and place we would earnestly claim for it a larger share in the scheme of higher education than has been accorded to it

1 Life and Letters, by Bence Jones, I, p. 310.

2 Prof. J. P. Cooke, Religion and Chemistry, p. 24.

Ib., pp. 24 and 25.

in the past, and would assert as emphatically as any the fitness of the study of nature to promote both the culture and discipline demanded by modern life, at the same time yielding to none in our estimate of the ancient classics as a precious element in the course of liberal studies. And first, where stands chemistry, the best developed and maturest of the natural sciences? It meets us at the threshold of the study of nature; and we can hardly over-estimate what it has contributed to our knowledge of the universe and to our power of dealing with its elements. With Briarean arms it reaches to the farthest known regions of space, and applying its analysis to the sun and the remotest stars, tells us of their components with as much certainty as it reveals those of the stones of the wayside. Yet are the feet of this giant not even clay, but only cloud.

The foundation of the present science of chemistry is the atomic theory of Dalton. Reviving an old idea of the Greek philosophers, he supposed all bodies to be made up of particles so small as to be practically indivisible, hence called atoms. Beginning to gain ground in 1811, this theory soon became the trunk to which all the other theories of the science have been and are still subordinated as branches. The blasts of fierce controversy have swept away the original boughs. These were the dualistic system of Lavoisier, and the related and confirmatory electro-chemical theory of Berzelius. In their stead have appeared the two theories of Dumas, those of substitution and of compound radicals, which, generalized by Gerhardt, yielded the unitary system, and since 1849 have been amalgamated into the now accepted theory of types. The sturdy trunk, defying the tornadoes that have snapped its arms, has so far proved flexible enough to bend to all facts. And to-day the atomic theory, with its distinction between molecules and atoms and its latest refinement of atomicities, together with the accompanying changes of nomenclature and notation, interprets consistently at least all known cases of chemical change and composition. The doctrine of atomicity, dating no farther back than 1858, has for the first time unveiled the constitution of organic compounds, supplied an approach to a philosophical classification of elementary substances, in place of the confessedly unphilosophical ones before tolerated, and bridged the gulf between organic and inorganic chemistry. It has even furnished a plausible explanation of, — or perhaps it may be more justly said, lessened the degree in which we are confounded by,- those chemical paradoxes known as isomerism and allotropism.

Not stopping with the limits of chemistry, this far-reaching theory has spread its roots into physics, and supplanting the old doctrine

that heat, light, and electricity are imponderable fluids, explains them in a way that is surprisingly ingenious. In this day of popular articles and lectures on science, the subject is no longer novel, and our purpose requires us to touch but lightly upon its outlines.

The balance of Lavoisier had proved that matter is indestructible. When the material theory of the imponderable agents began to wane, they came to be more cautiously spoken of, first seemingly by Berthollet, as forces; that is, causes of phenomena, a term that avoids all attempt to explain the thing named. The doctrine of indestructibility having been established for matter, it was natural to ask can force be destroyed? But if force cannot be lost, what becomes of it when it is, in popular phrase, spent? This question led to experiments, in the course of which it was observed that when motion was arrested, as in the case of friction, heat, light, or electricity was developed. And when heat disappeared as such, it reappeared as motion in some cases, in others as electricity. So, too, when electricity vanished, heat, light, magnetism or motion followed. But what can produce motion but motion itself? Into what can motion be converted but into motion? And as what is ordinarily recognized as motion is the motion of masses, so heat is a motion of the atoms of the hot body; light is another kind of motion of atoms, and electricity, magnetism, and the chemical force are severally other modes of atomic motion.

Here in a nutshell is the dynamic theory of force, first enunciated in full proportions by Grove, in 1842. Yet, when the records of old experiments were looked up, it was found, curiously enough, that an eminent son of New England, peer of the great Franklin, Benjamin Thompson, Count Rumford, had distinctly proved in 1798, and put the proof on record in the clearest terms, that heat "cannot possibly be a material substance," and cannot be understood as aught else than "motion."

And as of the forces we have considered, every one is probably capable of producing all the others, or, in other words, since they are so directly related and mutually dependent as to be convertible one into another, we arrive at the great fact of the correlation of forces, which has been pronounced the most important discovery of the present century, towards which all investigation for fifty years had been constantly tending.

Nor did the march of speculation stop here. Gravity and the vital forces have been put in turn to the test of strict experiment,

1 E. L. Youmans, Correlation and Conservation of Forces, p. xxiii.