Section 109. The gas of every company which supplies more than fifty consumers shall be inspected at least twice a year and as much oftener as the department may determine. The department may, from time to time establish a new standard of purity for gas, after a public hearing, determine how many grains of sulphur and ammonia per hundred cubic feet of gas may be permitted, but not more than thirty grains of sulphur per hundred cubic feet and no sulphuretted hydrogen shall be allowed.