Samuelson's factor-price theorem has been elaborated to admit of reversals of factor-intensities, 202. — Actual industries in actual countries are not on the same production function, 203. — Production functions are assumed to be iso-elastic, 204. — The search for a production function in actual data is bound to be in vain, 204. — In the course of the search, however, much interesting information has been turned up, 205.