To Professor Whewell:
1797. Dear Sir: I thank you for your kind attention in sending me a copy of your pamphlet, entitled a “Demonstration that all Matter is heavy,” comprising a communication made to the Cambridge Philosophical Society.
1798. I conceive that to demonstrate that all matter is heavy, is, in other words, to prove that all matter is endowed with attraction of gravitation, or that general property which, when it causes bodies to tend toward the centre of the earth, is called weight. Hence to assert that all matter is heavy, is no more than to say, that attraction of gravitation exists between all or any masses of matter.
1799. You say, “it may be urged that we have no difficulty in conceiving of matter which is not heavy.” I have no hesitation in asserting that there should be no difficulty in entertaining such a conception; since I cannot understand why any two masses may not be as readily conceived to repel, as to attract each other, or neither to attract nor to repel. Is it not easier to imagine two remote masses indifferent to each other, than that they act upon each other? Is any thing more difficult to understand than that a body can act where it is not?
1800. It is also mentioned by you, that it may be urged “that inertia and weight are two separate properties of matter.” Now I will not only urge, but also, with all due deference, will undertake to show, that the existence of inertia may as well be proven, and its quantity estimated, by means of repulsion as by means of attraction.
1801. Suppose two bodies, A and B, to be endowed with reciprocal attraction, or, in other words, to gravitate toward each other. Being placed at a distance, and then allowed to approach, if, after any given time, it were found that they had moved severally any ascertained distances, evidently their relative inertias would be considered as inversely as those distances.
1802. In the next place, let us suppose two bodies, X and Y, endowed with the opposite force of reciprocal repulsion, to be placed in proximity, and then allowed to fly apart. The distances run through by them severally, being, at any given time, determined, might not their respective inertias be taken to be inversely as those distances; so that the question would be as well ascertained in this case as in that above stated, in which gravitation should be resorted to as the test?
1803. It seems to me that this question is sufficiently answered in the affirmative, in your second paragraph, page 7, (p. 269,) in which you allege, that “one body has twice as much inertia as another, if, when the same force acts upon it for the same time, it acquires but half the velocity. This is the fundamental conception of inertia.”
1804. In the third paragraph, fourth page, (p. 261,) you say, “that the quantity of matter is measured by those sensible properties of matter which undergo quantitative addition, subtraction, and division, as the matter is added, subtracted, or divided, the quantity of matter cannot be known in any other way; but this mode of measuring the quantity of matter, in order to be true at all, must be true universally.”
1805. Also your fourth paragraph, fifth page, (p. 268,) concludes with this allegation: “And thus we have proved, that if there be any kind of matter which is not heavy, the weight can no longer avail us, in any case, to any extent, as the measure of the quantity of matter.”