1. Belt, T. H., and Ferris, A. A. (1942). Spec. Rep. Ser. med. Coun. Lond., No. 243.
2. Thesis for degree of M;Black, J.,1951
3. See Haldane, J. S. (1931), p. 426.
4. Report of H. M. Chief Inspector of Mines for the Year 1948;Bryan, A.M.,1950
5. Cochrane, A. L., Davies, I., and Fletcher, C. M. (1951). British Journal of Industrial Medicine, 8, 244.