Page:Popular Science Monthly Volume 14.djvu/283

Rh of time, vitality will follow the example of motion, and, after the necessary antecedent wrangling, take its place among the attributes of that "universal mother" who has been so often misdefined.

That "matter is not itself alive" Prof. Knight seems to regard as an axiomatic truth. Let us place in contrast with this the notion entertained by the philosopher Ueberweg, one of the subtilest heads that Germany has produced:

We may not be able to taste or smell alcohol in a tub of fermented cherries, but by distillation we obtain from them concentrated Kirschwasser. Hence Ueberweg's comparison of the brain to a still, which concentrates the sensation and feeling, preexisting, but diluted in the food.

"Definitions," says Mr. Holyoake, "grow as the horizon of experience expands. They are not inventions, but descriptions of the state of a question. No man sees all through, a discovery at once." Thus Descartes's notion of matter, and his explanation of motion, would be put aside as trivial by a physiologist or a crystallographer of the present day. They are not descriptions of the state of the question. And yet, it may be said in passing, a desire sometimes shows itself in distinguished quarters to bind us down to conceptions which passed muster in the infancy of knowledge, but which are wholly incompatible with our present enlightenment. Mr. Martineau, I think, errs when he seeks to hold me to views enunciated by "Democritus and the mathematicians." That definitions should change as knowledge advances is in accordance both with sound sense and scientific practice. When, for example, the undulatory theory was started, it was not imagined that the vibrations of light could be transverse to the direction of propagation. The example of sound was at hand, which was a case of longitudinal vibration. Now, the substitution of transverse for longitudinal vibrations in the case of light involved a radical change of conception as to the mechanical properties of the luminiferous medium. But, though this change went so far as to fill space with a substance possessing the properties of a solid, rather than those of a gas, the change was accepted, because the newly discovered facts imperatively demanded it. Following Mr. Martineau's example, the opponent of the