Electrical Resistance of Eighteen Elements.---The paper contains a brief summary of an extensive series of measurements which are to be published in detail elsewhere made to determine the effect of pressures up to 12000 kg. per sq. cm. and of temperatures from 0\ifmmode^\circ\else\textdegree\fi{} to 275\ifmmode^\circ\else\textdegree\fi{} $C$. on the resistance of lithium, sodium, potassium, gallium, bismuth, mercury, calcium, strontium, magnesium, titanium, zirconium, arsenic, tungsten, lanthannum, neodymium, carbon (amorphous and graphitic), silicon, and black phosphorus. The data for tungsten and magnesium are improvements on data previously published; the data for the other substances are new. The first six of these elements were studied in both the liquid and the solid states. The pressure coefficients of solid calcium, solid strontium, and both solid and liquid lithium are positive; the coefficient of bismuth is positive in the solid state, but negative in the liquid.Modified Electron Theory of Metallic Conduction.---A previous theoretical discussion of measurements of the effect of pressure on resistance suggested most strongly that in metallic conduction the electrons pass through the substance of the atoms, and that the mechanism by which resistance is produced is intimately connected with the amplitude of atomic vibration. This view is here given quantitative form. The classical expression for conductivity, $\ensuremath{\sigma}=(\frac{{e}^{2}}{2m})(\frac{\mathrm{nl}}{v})$, is retained; the number of free electrons is supposed to remain constant, their velocity is taken to be that of a gas particle of the same mass and temperature, and their mean free path is supposed to be many times the distance between atomic centers. The variations of path are then computed in terms of the variations of amplitude, and thus the variations of resistance are obtained and checked with experimental results. It is shown that the theory in this form explains Ohm's law, gives the correct temperature coefficient and the most important part of the pressure coefficient, avoids the difficulty of the classical theory with reference to specific heats, indicates a vanishing resistance at low temperatures, leaving open the possibility of super-conductivity, and retains the classical expression for the Wiedemann-Franz ratio. Besides these quantitative checks, the theory is shown to be entirely consistent qualitatively with all the new data; in fact, many of these new results, particularly the effect of pressure and temperature on the relative resistance of solid and liquid, seem to demand uniquely this conception of metallic conduction.