In the absence of conclusive evidence in favor of the claim, disability or death from silicosis shall be presumed not to be due to the nature of any occupation within the provisions of this act, unless during the ten (10) years immediately preceding the date of disablement the employee or workman has been exposed to the inhalation of silica dust over a period of not less than five (5) years, two (2) years of which shall have been in this state: Provided, however, That if the employee or workman shall have been employed by the same employer during the whole of such five-year period, his right to compensation against such employer shall not be affected by the fact that he had been employed during any part of such period outside of this state.