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The main currents of nineteenth-century science have produced more and higher specialization than ever before. Descartes was philosopher, scientist, and mathematician; some of the great men of the eighteenth century were hardly less so. Even through a large part of the nineteenth century many of the greater men ranged widely over the field of science and mathematics. To-day the force of circumstances has largely changed all that. The chemist is likely to look upon the physicist, or even the physical chemist, with suspicion on account of his mathematical interests. On the other hand, the mathematician, unlike Newton, Euler, and Gauss, is commonly no longer a physicist at all. There are to-day very few men who possess even a superficial acquaintance with all the principal departments of science, and between the work of the astronomer, on the one hand, and that of the anatomist, on the other, there is perhaps no closer relationship than the fact that both employ optical instruments in their researches.

The nineteenth century will ever be known in history for at least two of its scientific achievements—the unification of our knowledge of matter, energy, and life, and the final organization of the army of scientific workers, whereby discovery ceased to be dependent solely upon the individual and became a part of the business of humanity at large, at length and for the first time systematically undertaken.

THE UNIFICATIONS OF SCIENCE

1. Conservation of Energy

The middle of the nineteenth century witnessed the discovery of all three of the great unifications of science. These are the unification of energy by the discovery of the principle of the conservation of energy, the unification of matter by the discovery of the periodic system, and the unification of life by the work of Charles Darwin.

Not for decades after Bolton and Watt, as the result of commercial necessity, introduced the idea of measuring energy in horsepower,