From 1842 to 1848 he studied physics , chemistry , botany , mineralogy , philosophy and medicine at the University of Giessen . From 1842 to 1848 he studied physics , chemistry , botany , mineralogy , philosophy and medicine at the University of Giessen , where he graduated in 1848 with a dissertation entitled BeitrÃ ge zur Hall ` schen Lehre von einem excitomotorischen Nervensystem ( Contributions to the Hallerian Theory of an Excitomotor Nervous System ) .