Section 14. Gasoline or any other volatile inflammable fluid which emits a vapor at a temperature below one hundred degrees Fahrenheit when tested in the open air shall, when in any motor vehicle which is in a building or other structure, be deemed to be kept in such building or other structure within the meaning of the preceding section; provided, that this section shall not apply to any building in existence on July first, nineteen hundred and eleven, in which not more than two automobiles or motor vehicles are kept, if such building or part thereof is not used either for human habitation or for holding gatherings of more than twenty persons, or for giving entertainments, instruction or employment to more than that number, or to any private garage not in existence on said July first which is an appurtenance to a dwelling and in which not more than three automobiles or motor vehicles are kept.