1 8 8 D O C . 2 3 1 W A R B U R G A S R E S E A R C H E R step? The content of a science can, no doubt, be understood and assessed without entering into the individual development of those who have created it. But with such a one-sidedly objective depiction, the individual steps sometimes appear to be guided by chance.[2] An understanding of how these steps were possible—indeed necessary—can only be attained from following the intellectual development of the individuals who defined the direction as they joined in the effort. Let us try to sur- vey the work of this contemporary of ours from this point of view. We must, how- ever, confine ourselves to what appears especially important to us today because the four weighty volumes of Warburg’s original papers[3] lying here in front of me address the most disparate topics of physics and do not all let themselves be effort- lessly subsumed under uniform aspects, which really is imperative for our survey here. For it, though, I refer to the list of publications, partly with brief indications of the content, at the end of this article, to facilitate for experts the use of E. Warburg’s abundant scientific findings. Warburg’s first works (including the Latin dissertation 1868) concern them- selves theoretically and experimentally with the mechanics of acoustic vibrations (Oscillations of rods, determination of the speed of sound in soft bodies by attach- ing them to virtually undamped vibrating systems. Reversible oscillatory change in the magnetization of iron rods through vibration deformations heating from sound vibrations muffling of tones from internal resistance in solid bodies). In 1870 Warburg demonstrated with experiments on the discharge of mercury from glass capillary tubes that no slippage of observable amounts occurs along the glass as the mercury flows out. This work supplied the natural point of departure for an important investigation that Warburg, together with A. Kundt, had Helmholtz present to the Berl. Acad. of Sci. in 1875 (on friction and heat conduc- tion in rarefied gases). Whereas for flowing liquids a noticeable slippage of the layer directly bordering the wall does not occur, for gases a perceptible slippage is required by the kinetic theory of gases in the case where the gas molecules’ free length of path is not practically negligible against the dimensions of the vessel under consideration. According to the theory at the wall another flow rate prevails for the gas that would take place at a distance of 0.7 ( = free length of path in the gas) from the wall without the slippage phenomenon. Thus at the wall a discon- tinuous change in the rate of flow takes place that is larger, the greater the length of path, i.e., the smaller the gas density. The explanation for this phenomenon is simple. The thermally agitated mole- cules that hit against the wall had last collided in a deeper layer, so they have a mean one-sided translational velocity (flow) parallel to the wall. After the collision λ λ