It is a record of the exact determination and approximative analysis of the electric phenomena presented by living beings. Du Bois-Reymond, beginning with the imperfect observations of Matteucci, built up this branch of science. He did so by inventing or improving methods, by devising new instruments of observation or by adapting old ones.
On the other hand, the volumes in question contain an exposition of a theory. In them Du Bois-Reymond put forward a general conception by the help of which he strove to explain the phenomena which he had observed. He developed the view that a living tissue, such as muscle, might be regarded as composed of a number of "electric molecules", of molecules having certain electric properties, and that the electric behaviour of the muscle as a whole in varying circumstances was the outcome of the behaviour of these native electric molecules. We now know that these are the sodium, potassium and other ions which are responsible for electric membrane phenomena in excitable cells.
His theory was soon attacked by several contemporary physiologists, such as Ludimar Hermann, who maintained that a living untouched tissue, such as a muscle, is not the subject of electric currents so long as it is at rest, it is isoelectric in substance, and therefore need not be supposed to be made up of electric molecules, all the electric phenomena which it manifests being due to internal molecular changes associated with activity or injury. Du Bois-Reymond's theory was of great value if only as a working hypothesis, and that as such it greatly helped in the advance of science. Thus, Du Bois-Reymond's work lay chiefly in the direction of animal electricity, yet he carried his inquiries—such as could be studied by physical methods—into other parts of physiology, more especially into the phenomena of diffusion, though he published little or nothing concerning the results at which he arrived.
For many years, too, Du Bois-Reymond exerted a great influence as a teacher. In 1858, upon the death of Johannes Müller, the chair of anatomy and physiology, which that man had held, was divided into a chair of human and comparative anatomy, which was given to Karl Bogislaus Reichert (1811–1883), and a chair of physiology, which naturally fell to Du Bois-Reymond. This he held to his death, carrying out his researches for many years under unfavourable conditions of inadequate accommodation. In 1877, through his influence, the government provided the university with a proper physiological laboratory. In 1851 he was admitted into the Academy of Sciences of Berlin, and in 1876 became its perpetual secretary.
For many years Du Bois-Reymond and his friend Hermann von Helmholtz, who like him had been a pupil of Johannes Peter Müller, were prominent scientists and professors in the Prussian capital. Acceptable at court, they both used their position and their influence for the advancement of science. Du Bois-Reymond, as has been said, had in his earlier years wandered into fields other than those of physiology and medicine, and in his later years he went back to some of these. His gave occasional discourses, dealing with general topics and various problems of philosophy.
Du Bois-Reymond is now remembered also in terms of the ignorabimus, to which he gave common currency