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Rh process. Moreover, being exactly alike, they cannot have existed forever, and therefore they must have been made. As Sir John Herschel said, "they bear the stamp of the manufactured article."

Now, into these further deductions I do not propose to enter at all. I confine myself strictly to the first of the deductions which Prof. Clerk Maxwell made upon this theory. He said that because these molecules are exactly alike, and because they have not been in the least altered since the beginning of time, therefore they cannot have been produced by any process of evolution. It is just that question which I want to discuss. I want to consider whether the evidence that we have to prove that these molecules are exactly alike is sufficient to make it impossible that they can have been produced by any process of evolution. The position, that this evidence is not sufficient, is evidently by far the easier to defend, because the negative is proverbially hard to prove; and, if any one should prove that a process of evolution was impossible, it would be an entirely unique thing in science and philosophy. In fact, we may see from this example precisely how great is the influence of authority in matters of science.

If there is any name among contemporary natural philosophers to whom is due the reverence of all true students of science, it is that of Prof. Clerk Maxwell. But if any one, not possessing his great authority, had put forward an argument founded apparently upon a scientific basis, in which there occurred assumptions about what things can and what things cannot have existed from eternity, and about the exact similarity of few or more things established by experiment, we should say, "Past eternity; absolute exactness;" and we should pass on to another book. The experience of all scientific culture, for all ages during which it has been a light to men, has shown us that we never do get at any conclusions of that sort. We do not get at conclusions about infinite time or infinite exactness. We get at conclusions which are as nearly true as experiment can show, and sometimes which are a great deal more correct than direct experiment can be, so that we are able actually to correct one experiment by deductions from another; but we never get at conclusions which we have a right to say are absolutely exact; so that, even if we find a man of the highest powers saying that he had reason to believe a certain statement to be exactly true, or that he believed a certain thing to have existed from the beginning exactly as it is now, we must say, "It is quite possible that a man of so great eminence may have found out something which is entirely different from the whole of our previous knowledge, and the thing must be inquired into. But, notwithstanding that, it remains a fact that this piece of knowledge will be absolutely of a different kind from any thing that we knew before."

Now, let us examine the evidence by which we know that the molecules of the same gas are as near as may be alike in weight and in rates of vibration. There were experiments made by Dr. Graham,