1. Frey, H.: Anal. Chim. Acta5, 313 (1951).
2. Frey, H.: Anal. Chim. Acta5, 317 (1951).
3. Frey, H.: Diese Z.132, 276 (1951).
4. Frey, H.: Diese Z.133, 328 (1951).
5. Am. Soc. Testing Materials, 1949 Standards Part III B, Designation 297-43 T, pp. 816–45.