The velocities observable up to now, are only small, because the celestial bodies, although much faster than automobiles, hardly make 60 or 100 kilometers per second; yet, light is ca. 3000 times faster, thus we aren't dealing with moving matter, but with a disturbance of equilibrium which propagates in a relatively unmovable substance, like a wave at the surface of the sea. All experiments conducted at these small velocities, have always confirmed the constancy of mass, and nobody has asked himself the question, whether this law is also valid at higher velocities.
The speed record of mercury (the fastest planet) was broken by those infinitely small bodies: I'm talking about the corpuscles by whose motions the cathode rays and the radium rays arise. It's known that these emanations were caused by a veritable bombardment of molecules. The projectiles ejected at this occasion, are charged with negative electricity; one can convince oneself of this fact by collecting this electricity with a suitable apparatus. In consequence of this charge, they will be deflected by a magnetic or an electric field, and by measuring these deflections one can determine their velocity and the ratio of their charge to their mass.
On one hand such measurement have taught us that their velocity is enormously great, by achieving approximately a tenth or a third of the speed of light, and thus being a thousand times greater than the velocity of the planets; on the other hand they have taught us that their charge
