§ 320. Standard of purity. The gas furnished or supplied by any corporation, association, partnership or person in any city of the second class shall be free from sulphuretted hydrogen, to be determined by exposing for thirty seconds a slip of white paper saturated with acetate of lead to a jet of gas flowing about five feet per hour, and each one hundred cubic feet shall not contain more than ten grains of ammonia nor twenty grains of sulphur.