The plasma instrument on Mariner 10 carried out measurements of electron density and temperature in the interplanetary medium between heliocentric distances of 0.85 and 0.45 AU. Owing to the stable coronal configuration and low solar activity during the period of observation (January 9 to March 30, 1974), the radial variations of these quantities could be obtained. The power law exponent of the core temperature was determined to be --0.3 ¿ 0.04, and the halo temperature was found to be almost independent of heliocentric distance. The exponent of the power law for the density variation was 2.5 ¿ 0.2, and the extrapolated value at 1 AU was consistent with measured values during the same period. Calculations of the core electron Coulomb collision time and the core-halo equipartition time were made as a function of radial distance. These measurements indicate a macroscale picture of a Coulomb collisional core and a collisionless isothermal halo. Extrapolating back to the sum, core and halo temperatures become equal at a radial distance of ~2--15 Rs. Temperature variations were observed at the time of passage of high-speed streams, and at the time of observation of stream interfaces, where the density falls before the temperature. Coulomb collisions in the core become much less important, and the halo density is observed to decrease by more than an order of magnitude.