We propose to measure the far infrared fine structure lines of [CII] 158 mu, [OI] 63 mu and [NII] 122 mu as well as the dust continuum to study the cooling of the diffuse cold (neutral) and warm (ionized and neutral) interstellar gas phases. We have selected lines of sight were the densities are well determined from absorption or recombination studies and pulsar dispersion measurements. The goal of this research is a DIRECT measure of the cooling rate of the cold and warm phases of the ISM which will lead to a better understanding of the ISM. These measurements will allow to characterize the physical conditions of the interstellar gas and provide an important test for the various heating mechanisms proposed for the interstellar medium.