This method of test is applicable only to asphaltic products derived from petroleum and should not be applied to natural asphalts containing nonbituminous matter insoluble in xylene. Materials, which by use of standard solvent are classed as positive, may be further tested to determine their degree of positiveness by means of their “xylene equivalent.” The xylene equivalent shall be the lowest percentage by volume of xylene in a solvent composed of xylene and standard naphtha or xylene and normal heptane, as specified, that produces a “negative spot” for the material in question. These shall be known as the “naphtha–xylene equivalent” and “heptane–xylene equivalent,” respectively. The percentage of xylene in the solvents shall be stated in even 5.0 percent increments. When neither xylene equivalent is specified, the standard naphtha only shall be used as the solvent.