Ludwig wrote a classic monograph on renal secretion. He theorized that urine formation could be explained purely in terms of the laws of physics and chemistry, and that under the hydrostatic pressure of the blood in the capillaries of the glomerulus, protein and cell-free fluid is separated from the blood by a simple physical process of filtration. This theory contradicted Bowman’s contention that the glomerulus secretes fluid. See also Ludwig’s habitation thesis, from which the above work was expanded: De viribus physicis secretionem urinae adjuvantibus. Marburg, Elwert, 1842. Digital facsimile of th 1843 work from Goethe Univerität at this link.