Section 13. When any such officer has, in exercise of the powers conferred by section 12, or by purchase, obtain

13. When any such officer has, in exercise of the powers conferred by section 12, or by purchase, obtained a sample of petroleum in the possession of a dealer, he may give a notice in writing to the dealer informing him that he is about to test the sample or cause it to be tested with the apparatus and in the manner described in the schedule to this Act, at a time and place to be fixed in the notice, and that the dealer or his agent may be present at the testing. Certificate as to result of testing.