those of another molecule, or molecules, brought within its sphere of action? Here is another question which can only be answered after much experimental evidence has been accumulated. Now, these questions, I make bold to say, are the direct outcome of the dualism of Berzelius, modified by the unitary chemistry of Dumas and his followers.
If we glance back on the development of the two theories, the course of which I have endeavored to outline, we find that both began with a purely qualitative study of reactions, but that it was only when to this had been added the careful use of weights and measures that any solid advance became possible. Further, we find that the older theory was founded chiefly on a study of reactions, while that which was broached after the time of Lavoisier was founded most largely on a study of composition. With the phlogisteans function was of paramount importance; with the dualists composition was all. The modern theories, which have been developed from these, have attempted, with varying success, to combine both considerations. And if we examine the latest advances of theoretical chemistry we still find it at work on these two lines of advance. The composition of chemical compounds is studied by the majority of chemists; but the general laws of action of chemical force itself have of late received most important elucidation.
Again, if we look to the "lines of advance along which dynamical science is working its way to undermine, at least, the outworks of chemistry," we can distinguish two, essentially the same, lines as were used by the two classes, whose theories I have dealt with in this paper. "One is conducted by the help of the hypothesis that bodies consist of molecules in motion, and it seeks to determine the structure of the molecules and the nature of their motion from the phenomena of portions of matter of sensible size. The other line of advance, that of thermo-dynamics, makes no hypothesis about the ultimate structure of bodies, but deduces relations among observed phenomena by means of two general principles, the conservation of energy and its tendency toward diffusion." (Clerk Maxwell, "South Kensington Science Conferences," 1876, p. 145.)
I have thus sought to substantiate the claim of the new chemistry to be a development of the old. I believe that, if this claim is granted, the conclusion to be drawn must be, not that the old is better, but that to return to that which is admittedly an early stage of development would be to misread all the teachings even of the old chemistry itself.
In examining the progress of Science we see that she is not afraid to retrace her steps, and that she is able to retain and develop all that is probably true, while rejecting all that is proved to be false; and, when we learn that she does this, can we hesitate to find in her history the "promise and potency" of a mighty future?—Popular Science Review.
