7 o8 THE POPULAR SCIENCE MONTHLY. 



once of this pound in the carbonic acid, which is the product of com- 

 bustion, and from which the original weight of carbon may be re- 

 covered. The quantity of matter is measured by its weight, and this 

 weight is unchangeable. 



Such is the fact, familiar to every one, and its interpretation, equally 

 familiar. To test the correctness of this interpretation, we may be 

 permitted slightly to vary the method of verifying it. Instead of 

 burning the pound of carbon, let us simply carry it to the summit of a 

 mountain, or remove it to a lower latitude ; is its weight still the same ? 

 Relatively it is ; it will still balance the original counterpoise. But 

 the absolute weight is no longer the same. This appears at once, if 

 we give to the balance another form, taking a pendulum instead of a 

 pair of scales. The pendulum on the mountain or near the equator 

 vibrates more slowly than at the foot of the mountain or near the 

 pole, for the reason that it has become specifically lighter by being 

 farther removed from the centre of the earth's attraction, in conformity 

 to the law that the attractions of bodies vary inversely as the squares 

 of their distances. 



It is thus evident that the constancy, upon the observation of which 

 the assertion of the indestructibility of matter is based, is simply the 

 constancy of a relation, and that the ordinary statement of the fact is 

 crude and inadequate. Indeed, while it is true that the weight of a 

 body is a measure of its mass, this is but a single case of the more 

 general fact that the masses of bodies are inversely as the velocities 

 imparted to them by the action of the same force, or, more generally 

 still, inversely as the accelerations produced in them by the same force. 

 In the case of gravity, the forces of attraction are directly propor- 

 tional to the masses, so that the action of the forces {weight) is the 

 simplest measure of the relation between any two masses as such ; 

 but, in any inquiry relating to the validity of the atomic theory, it is 

 necessary to bear in mind that this weight is not the equivalent, or 

 rather presentation, of an absolute substantive entity in one of the 

 bodies (the body weighed), but the mere expression of a relation be- 

 tween two bodies mutually attracting each other. And it is further 

 necessary to remember that this weight may be indefinitely reduced, 

 without any diminution in the mass of the body weighed, by a mere 

 change of its position in reference to the body between which and the 

 body weighed the relation subsists. 1 



1 The thoughtlessness with which it is assumed by some of the most eminent mathe- 

 maticians and physicists that matter is composed of particles which have an absolute 

 primordial weight persisting in all positions, and under all circumstances, is one of the 

 most remarkable facts in the history of science. To cite but one instance : Prof. Ret- 

 tenbacher, one of the ablest analysts of his day, in his " Dynamidensystem " (Mannheim, 

 Bassermann, 1857), p. 14, says, " The absolute weight of atoms is unknown " his 

 meaning being, as is evident from the context and from the whole tenor of his discus- 

 sion, that our ignorance of this absolute weight is due solely to the practical impossi- 

 bility of insulating an atom, and of contriving instruments delicate enough to weigh it. 



