"Now in the calculus we write dx for a little bit of x. These things such as dx, and du, and dy, are called "differentials," the differential of x, or of u, or of y, as the case may be. [You read them as dee-eks or dee-you or dee-wy.] If dx be a small bit of x, and relatively small of itself, it does not follow that such quantities as x\cdot dx or x^2 dx or a^x dx are negligible. But dx\times dx would be negligible, being a small quantity of the second order."