Several months were required to calibrate the various apparatus, and to make analyses of the available natural gas to determine the correct method of proportioning it with air, so as to produce exact mixtures of 2, 4, 6, or 8% of methane with air. Tests of existing explosives were made in air and in inflammable mixtures of air and gas, with a view to fixing on some standard explosive as a basis of comparison. Ultimately, 40% nitro-glycerine dynamite was adopted as the standard. Investigative tests having been made, and the various factors concerning all the explosives on the market having been determined, a circular was sent to all manufacturers of explosives in the United States, on January 9th, 1909, and was also published in the various technical journals, through the associated press, and otherwise. On May 15th, 1909, all the explosives which had been offered for test, as permissible, having been tested, the first list of permissible explosives was issued, as given in the following circular: “EXPLOSIVES CIRCULAR NO. 1. “DEPARTMENT OF THE INTERIOR. “United States Geological Survey. “May 15, 1909. “LIST OF PERMISSIBLE EXPLOSIVES. “Tested prior to May 15, 1909. “As a part of the investigation of mine explosions authorized by Congress in May, 1908, it was decided by the Secretary of the Interior that a careful examination should be made of the various explosives used in mining operations, with a view to determining the extent to which the use of such explosives might be responsible for the occurrence of these disasters. “The preliminary investigation showed the necessity of subjecting to rigid tests all explosives intended for use in mines where either gas or dry inflammable dust is present in quantity or under conditions which are indicative of danger. “With this in view, a letter was sent by the Director of the United States Geological Survey on January 9, 1909, to the manufacturers of explosives in the United States, setting forth the conditions under which these explosives would be examined and the nature of the tests to which they would be subjected. “Inasmuch as the conditions and tests described in this letter were subsequently followed in testing the explosives given in the list below, they are here reproduced, as follows: “(1) The manufacturer is to furnish 100 pounds of each explosive which he desires to have tested; he is to be responsible for the care, handling, and delivery of this material at the testing station on the United States arsenal grounds, Fortieth and Butler streets, Pittsburg, Pa., at the time the explosive is to be tested; and he is to have a representative present during the tests, who will be responsible for the handling of the packages containing the explosives until they are opened for testing.