Section 1. The Board of Trustees of any firemen's pension fund in any incorporated city or town in this State may, upon fulfilling requirements hereinafter stated, establish benefit eligibility for a fulltime employee who has been employed for as long as six (6) years, and thereafter becomes disabled or dies from heart or lung disease, based on a presumption that such death or disease was a consequence of his duties as a fireman, if the fireman shall have successfully passed a physical examination prior to the claimed disability or death, or upon entering upon his employment as a fireman, and the examination failed to reveal any evidence of the condition or disease of the lungs, hypertension or heart disease.