1. A. V. Grosse et al, Matls. Res. and Standards (1965) 173.
2. J. A. Sheward, NPL Report IMS Ex 1 (1966).
3. J. A. Champion, private communication.
4. R. E. White and T. W. Higgins, Tappi 41 (1958) 71.
5. R. T. Jones and E. H. Swift, Analyt. Chem. 25 (1953) 1272.