that all Matter is Heavy. 269 



" To this I reply, that, practically speaking, quantity of matter is 

 always measured by weight, both by mechanicians and chemists: and 

 as we have proved that this procedure is utterly insecure in all cases, 

 on the hypothesis of weightless matteii^ the practice rests upon a con- 

 viction that the hypothesis is false. And yet the practice is universal. 

 Every experimenter measures quantity of matter by the balance. No 

 one has ever thought of measuring quantity of matter by its inertia 

 practically ; no one has constructed a measure of quantity of matter in 

 which the matter produces its indications of quantity by its motion. 

 When we have to take into account the inertia of a body, we inquire 

 what its weight is, and assume this as the measure of the inertia ; but 

 we never take the contrary course, and ascertain the inertia first in 

 order to determine by that means the weight. 



" But it may be asked. Is it not then true, and an important scientific 

 truth, that the quantity of matter is measured by the inertia ? Is it not 

 true, and proved by experiment, that the weight is proportional to the 

 inertia ? If this be not the result of Newton's experiments mentioned 

 above, what, it may be demanded, do they prove .? 



" To these questions I reply : It is true that quantity of matter is 

 measured by the inertia, for it is true that inertia is as the quantity of 

 matter. This truth is indeed one of the laws of motion. That weight 

 is proportional to inertia is proved by experiment, as far as the laws of 

 motion are so proved : and Newton's experiments prove one of the 

 laws of motion, so far as any experiments can prove them, or are 

 needed to prove them. 



" That inertia is proportional to weight, is a law equivalent to that 

 law which asserts, that when pressure produces motion in a given body, 

 the velocity produced in a given time is as the pressure. For if the 

 velocity be as the pressure, when the body is given, the velocity will 

 be constant if the inertia also be as the pressure. For the inertia is 

 understood to be that property of bodies to which, ceteris parihis, the 

 velocity impressed is inversely proportional. 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. 



" In Newton's pendulum experiments, the pressure producing motion 

 was a certain resolved part of the weight, and was proportional to the 

 weight. It appeared by the experiments, that whatever were the mate- 

 rial of which the pendulum was formed, the rate of oscillation was the 

 same ; that is, the velocity acquired was the same. Hence the inertia 

 of the different bodies must have been in each case as the weight : and 

 thus this assertion is true of all different kinds of bodies. 



