The radiation variety is calculated as one over c times the energy flux, and it was recently found that the absolute form of this quantity can be negative, explaining transpirational pull.  
This quantity can be defined as the derivative of internal energy with respect to volume at constant entropy.  It can be measured by Kiel probes, and structures sensitive to it in the body include Ruffini endings and Pascinian Corpuscles.  The radiation variety is calculated as one over c times the energy flux, and it was recently found that the absolute form of this quantity can be negative, explaining transpirational pull.  Gay-Lussac's Law says that, at constant volume, it is directly proportional to temperature, and Boyle's Law deals with its relation to volume.  FTP name this state function often measured in Pascals, defined as force per unit area.
This quantity can be defined as the derivative of internal energy with respect to volume at constant entropy.  
Pressure