[For most of the velocities with which we are familiar $E/C^2$ is, like the difference between $K$ and unity, such an extremely small quantity that the most delicate measurements fail to detect it. But the electrons in a highly evacuated tube and the particles shot out from radioactive materials attain in some cases velocities as high as eight-tenths that of light. When we measure the mass of such particles at different velocities we find that it actually increases with the velocity, and in accordance with the foregoing law.]194 [This observation, in fact, antedates Einstein's explanation, which is far more satisfactory than the earlier differentiation between "normal mass" and "electrical mass" which was called upon to account for the increase.]* [But if the quantity $E/C^2$ is to be considered as an actual increase in mass, may it not be possible that all mass is energy? This would lead to the conclusion that the energy stored up in any mass is $mC^2$. The value is very great, since $C$ is so large; but it is in good agreement with the internal energy of the atom as calculated from other considerations. It is obvious that conservation of mass and of momentum cannot both hold good under a theory that translates the one into the other. Mass is then not considered by Einstein as conservative in the ordinary sense, but it is the total quantity of mass plus energy in any closed system that remains constant. Small amounts of energy may be transformed into mass, and vice versa.]194 [Other features of the theory which are often displayed as consequences are really more in the nature of assumptions. It will be recalled that when we had agreed upon the necessity of employing signals of some sort, we selected as the means of signalling the speediest messenger with which we happened to be acquainted. Our subsequent difficulties were largely due to the impossibility of making a proper allowance for this messenger's speed, even though we knew its numerical value; and as a consequence, this speed enters into our formulae. Now we have not said in so many words that $C$ is the greatest speed attainable, but we have tacitly assumed that it is. We need not, therefore, be surprised if our formulae give us absurd results for speeds higher than $C$, and indicate the impossibility of ever attaining these. Whatever we put into a problem the algebra is bound to give us back. If we look at our formula for $K$, we see that in the event of $v$ equalling $C$, lengths become zero and times infinite. The light messenger itself, then, has no dimension; and for it time stands still. If we suppose $v$ to be greater than $C$, we get even more bizarre results, for then the factor $K$ is the square root of a negative number, or as the mathematician calls it an "imaginary" quantity; and with it, lengths and times become imaginary too.