A quantitative Schlieren analysis, using the shadow method in conjunction with a helium-neon laser, has been made of positive streamers in a point-plane corona discharge in air. Changes in the intensity profile of the laser beam, after it passed through the discharge gap, were measured and analysed to provide radial profiles of neutral density at different times following streamer onset. The profiles reveal that energy is transferred to the neutrals from streamer electrons in two stages. The first stage lasts for ca. 1 $\mu s$ after streamer onset, during which time energy may be transferred directly from electrons and from rotationally excited molecules. The second stage lasts for ca. 100 $\mu s$, during which time energy may be transferred from vibrationally excited metastables. The resultant temperature rise causes reductions in neutral density of up to 13.5% below the atmospheric value.