Physical properties of aqueous solns: J. W. Mullin, M. M. Osman, J. Chem. Eng. Data 12, 516 (1967); and crystal growth rates: eidem, ibid. 18, 353 (1973). Toxicity study: E. L. Reagan, J. Am. Coll. Toxicol. 11, 694 (1992). Crystal structure of hexahydrate: T. H. Tahirov, T.-H. Lu, Acta Crystallogr. C50, 668 (1994); and thermo-stability and optical transmission properties: G. Su et al., J. Phys. D: Appl. Phys. 35, 2652 (2002). Review of toxicology and human exposure: Toxicological Profile for Nickel (PB2006-100005, 2005) 397 pp.