(1) Where in any proceedings for an offence under this Act, a question arises as to whether any thing or substance is a weapon, a hazardous, radioactive or a harmful substance, a toxic chemical microbial or other biological agent or toxin, a certificate purporting to be signed by an appropriate authority to the effect that the thing or substance described in the certificate is a weapon, hazardous, radioactive or harmful substance, a toxic chemical or microbial or other biological agent or toxin, shall be admissible in evidence without proof of the signature or authority of the person appearing to have signed it and shall, in absence of evidence to the contrary, be proof of the facts stated therein.