The action current (later called action potential) was discovered to be a kind of "electrical impulse wave" which propagated at a fixed and relatively slow speed along the nerve fiber. In 1852, Hermann von Helmholtz (1821-1894) was able to measure the speed of frog nerve impulses, and determined it to be about 27 meters/second. Du Bois-Reymond contributions, published in his book "Untersuchungen über thierische Elektricität". ("Researches on Animal Electricity") in 1848, created the field of scientific electrophysiology. The work of both men served to refute the view by their teacher, Johannes Müller, that the nerve impulse was an example of a vital function that could never be measured experimentally, and their collaboration with a remarkable group of physiologists, composed by Carl Ludwig and Ernst von Brücke was very important to reduce physiology to applied physics and chemistry, a trend that has dominated physiology and medicine ever since. They had "sworn to each other to validate the basic truth that in an organism no other forces have any effect than the common physiochemical ones. . . .