Sentence:  This project was carried out under sponsorship of the Ballistic Missile Division of the Air Research and Development Command , U. S. Air Force , and had as its goal the investigation of the <WSD>transport</WSD> by diffusion of the heat energy of chemical binding . 
Meaning of the word marked with <WSD> tags:  an exchange of molecules (and their kinetic energy and momentum) across the boundary between adjacent layers of a fluid or across cell membranes