48 PHILOSOPHICAL SOCIETY OF WASHINGTON. 
The second force or manifestation of the atom, inertia,—or mass,— | 
unlike gravity, is not unlimited in range of agtion. As to this 
property matter is discrete. Mass has both a locus and a limit 
(being apparently dependent for dimension on multiplicity), and 
amounts to that incomprehensible property by which conservation 
of motion is maintained. Under gravity, quantity of motion varies 
according to relations of contiguity, but under inertia motion is 
conserved in direction and quantity, is modified in direction and 
quantity by interaction of mass with gravity, and is redistributed 
by interaction with repulsive force upon an indefinitely near ap- 
proach of particles, upon conservative principles. Its discreteness 
gives matter its numerical and finite character, and admits of that 
interplay which constitutes phenomena.* Its reality and primary 
written by Faraday to Dr. Playfair, in response to some inquiries of the 
latter about his atomic opinions: 
* * * “T believe in matter and its atoms as freely as most people—at 
least, I think so. As to the little solid particles which are by some supposed 
to exist independent of the forces of matter, and which in different sub- 
stances are imagined to have different amounts of these forces associated 
with or conferred upon them, * * * as I cannot form any idea of them 
apart from the forces, so I neither admit nor deny them. They do not 
afford me the least help in my endeavor to form an idea of a particle of 
matter. On the contrary, they greatly embarrass me; for, after taking an 
account of all the properties of matter, and allowing in my consideration for 
them, then these nuclei remain on the mind, and I cannot tell what to do 
with them. The notion of a solid nucleus without properties is a natural 
figure or stepping-stone to the mind at its first entrance on the consideration 
of natural phenomena; but when it has become instructed, the like notion 
of a solid nucleus apart from the repulsion, which gives our only notion of 
solidity, or the gravity, which gives our notion of weight, is to me too dif- 
ficult for comprehension; and so the notion becomes to me hypothetical, 
and, what is more, a very clumsy hypothesis.’’ (Playfair’s works, Vol. 4, 
p- 84.) 
Here we see a difficulty opposite to that usually encountered, for, while 
many people profess an infirmity of conception of the forces apart from the 
imaginary vehicle, Faraday finds the vehicle of no use as a carrier of the 
properties, but a positive impediment. 
* This property has a multiplicity of names in the Newtonian nomencla- 
ture, according to the varying aspect of its function. Thus, in the aspect 
of persistence of mass in state of rest or of motion uniform in direction 
