that all Matter is Heavy. 261 
 §. Suppose two bodies, A and B, to be endowed with reciprocal 
attraction; or, in other words, to gravitate towards 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. 
6. In the next place, let us suppose two bodies, X and Y, en- 
dowed 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? 
7. 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 il for the same time, 
it acquires but half the velocity. This is the fundamental con- 
ception of inertia.” 
8. 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 di- 
vision, as the matter is added, subtracted or divided, the quan- 
tity 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.” 
9. 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 quan- 
tity of matter.” 
10. In reply to these allegations let me inquire, cannot a matter 
exist of which the sensible properties do not admit of being 
measured by human means? Because some kinds of matter 
can be measured by ‘those sensible qualities which undergo 
quantitative addition, subtraction and division,” does it follow 
Hat there may not be matter which is incapable of being thus 
measured»? And wherefore would the method of obtaining phi- 
