This declassified Porton Down film, which is Crown Copyright, shows a Valetta aircraft making a number of passes in front of the camera - all the time spraying the Biological Warfare simulant - Zinc Cadmium sulphide. These particular experiments were conducted during March 1958, and were conducted to determine the characteristics of an aircraft mounted Zinc Cadmium sulphide dispenser. The resulting information was then used in Porton Down’s later public area BW experiments, some of which (the Large Area Coverage or LAC) contaminated vast swathes of the UK. A BW simulant is a supposedly harmless substance which mimics the physical properties of a real BW agent, in this case, size (between 1-5 microns). BW simulants are used in BW experiments in which, for safety reasons, a real BW agent could not be used.  A similar procedure was adopted for the 1963/64 Norwich Trials. An aircraft, this time a Devon, sprayed the BW simulant Zinc Cadmium sulphide, at a rate of 2-3 lbs per mile, as it flew along a 62 nm track