IMPONDERABLE AGENTS. Light, Heat, and the various forms of Electricity are called " imponderable agents," because they cannot be weighed. When the philosopher is called on to give a definition of matter, he replies, " Matter is whatever can be proved to possess weight, that is, to be subject to the law of gravitation." Light and Heat are probably the undulations of a medium so subtle that it has not yet been weighed ; and neither light, beat, nor electricity perceptibly affect the weight of bodies that are affected by them ; hence they are called imponderable or immaterial agents ; the terms " imponderable " and " immaterial " being strictly synonymous, according to the definition of matter.
It is probable that light, heat, and the various forms of electricity may all be undulations of an elastic medium pervading space, which, although its weight may not be appreciable by our senses aided by any form of apparatus, may still possess weight, and be material. This subtle but material ether may be supposed to fill the interstices between the atoms of bodies, and by its motions to disturb their re lative positions in such a way as to produce the phenomena due to what are called the " imponderable agents." The difference between
light, heat, and electricity would depend, according to this hypothesis, upon the kind and length of the undulation, and the velocity of its propagation ; so that these agents may actually glide one into the other, and become merely modified cases of one form of matter in mo tion. But these are speculations ; let them not be confounded with ascertained facts. Still, speculation is a necessary part of philosophic enquiry ; and as for the undulatory theory of light, that is now received by scientific men with the same faith as the law of universal gravita tion. Both hypotheses rest upon equally strong evidence, and there is the highest degree of moral certainty of their truth.