1. Adler, L. H. (1891). Medical News, 59, 186.
2. Almkvist (1929). Acta. med. scand., 70, 464.
3. Bloomfield, J. J., Trasko, V. N., Sayers, R. R., Page, R. T., Peyton, M.F. (1940). U.S. Publ. Health Bull. No. 259, 199.
4. Christensen, H., Krogh, M., and Nielsen, M. (1937). Nature, 139, 626.
5. Dennis, L. (1876). Rep. Board Health, New Jersey, 2, 67.