De corporis humani viribus conservatricibus dissertatio.

Young's thesis for his medical degree from Göttingen on the conservation of strength in the human body, which also contains, on the final four pages, the only extant fragment of Young's brief thesis on the human voice prepared for his oral examination. It was this last, as Young's biographer tells us, that began Young on his career in physics: "[The fragment] gives an alphabet of forty-seven letters designed to express, by their combination, every sound which the organs of the human voice are capable of forming. . . . Here we see his early and sustained interest in languages combined with his interest in anatomy. From this combination he developed an interest in the production and propagation of sound...." (Wood / Oldham, Thomas Young, pp. 49-50). Digital facsimile from wellcomecollection.org at this link.