Letter from Howard Walter Florey to R V Christie, St Albans Discusses work done on the calcium salt of penicillin - compares methods and dosages used by Herrell with those used at Oxford - impurities differ according to where the penicillin is produced - likely that some of the American penicillin is not of the same chemical structure as the British stuff - safe to use calcium salt by continuous intravenous drip - would not use intramuscularly. 16 October 1943