YOUR contributor J. S. G. in reviewing (NATURE, December 20, p. 913) the report of the Ministry of Agriculture and Fisheries on oyster mortality in 1920, and dealing with the finding of 3.7 parts of arsenic per million in oysters from certain beds, comments on the serious questions raised by such a fact. A definite pronouncement is needed as to what constitutes danger. As is well known, the Royal Commission on Arsenical Poisoning, in its report in 1903, recommended that no substance used in the preparation of food should contain more than 1/100 grain per 1b. This recommendation has been adopted as a standard for years, and many prosecutions have been successful for quantities but little in excess of this. Surely the position is now more illogical than ever it was. Is the fishmonger to be prosecuted if his oysters have three parts per million (1/50 grain per 1b.), and, if not, why should a grocer be charged if, say, a baking powder—probably not made by himself—contains this amount?