slowly than the projected bullet, yet never pauses for an instant on the way, so little am I obliged to think of the mass of a body as consisting of atoms and the spaces between them, i.e., of absolute density and absolute vacuity; but I comprehend those two phenomena without difficulty as constant continua, one of which uniformly fills time and the other space. But just as the one motion may yet be quicker than another, i.e., in an equal time can pass through more space, so also one body may have a greater specific gravity than another, i.e., in equal space may contain more matter: in both cases the difference depends upon the intensity of the acting force; for Kant (following Priestley) has quite correctly reduced matter to forces. But even if the analogy here set up should not be admitted as valid, and it should be insisted upon that the difference of specific gravity can only have its ground in porosity, even this assumption would always lead, not to atoms, but only to a perfectly dense matter, unequally distributed among different bodies; a matter which would certainly be no longer compressible, when no pores ran through it, but yet, like the space which it fills, would always remain infinitely divisible. For the fact that it would have no pores by no means involves that no possible force could do away with the continuity of its spatial parts. For to say that everywhere this is only possible by extending the already existing intervals is a purely arbitrary assertion.
The assumption of atoms rests upon the two phenomena which have been touched upon, the difference of the specific gravity of bodies and that of their compressibility, for both are conveniently explained by the assumption of atoms. But then both must also always be present in like measure, which is by no means the case. For, for example, water has a far lower specific gravity than all metals properly so called. It must thus have fewer atoms and greater interstices between them, and consequently be very compressible: but it is almost entirely incompressible.
