1 9 0 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 where J denotes the magnetization, the potential of the permanent magnet, dV the volume element of the piece of iron. The mechanical work dA produced by an infinitely small motion of the permanent magnet therefore comes out as equal to the increase d from for constant J: , hence the mechanical work produced upon cyclic magnetic reversal per unit volu- me of the iron is equal to , where vector J now should be regarded as a function of the vector . Nowadays we tend to write: , which for a closed cycle of the magnetization comes down to the same thing, of course. After such important advances made by the kinetic theory in the case of gases, the question of how far the theoretical conceptions held out for highly compressed gases was of high interest. One of the most notable conclusions of this theory sup- ported by experiment, that the coefficient of friction was independent of density, was therefore tested by Warburg and Babo (1882) for carbon dioxide at high den- sities. The outcome was that although the coefficient of viscosity increased, it was only by about 9%, if the density rose to about 500 times the normal density (at atmospheric pressure and regular temperature). The conclusion from this is that the basic conceptions of the theory of gases apply up to high densities. We have no cer- tain explanation for what this slight increase is based on.[4] Perhaps it is based on that in dense gases the molecule’s apparent diameter is smaller than in less dense ones because the molecular forces by neighboring molecules on the one under con- sideration partly compensate each another. From 1887 Warburg’s research is concentrated on the study of electrical conduc- tion in gaseous, fluid, and solid bodies, the inquiry into electromotive forces and chemical reactions produced by electrical processes in gases. These latter studies then led him to his pioneering articles in the area of photochemistry. Reading the papers on gaseous discharge, one is astonished by the amount of meticulous exper- imental research which initially was not yet guided by the hypothesis of ions. From among the abundance of these articles I pick out only those that strike me as of spe- cial importance. In 1887 and 1888 Warburg and Tegelmeyer found that rock crystal heated to 200° conducts electrolytically, specifically, parallel but not orthogonally to the ϕ ϕ ϕ dA dΦ(J) J dh)dV , ( – = = A J dh –= h A + h dJ =