The coal gas had another element besides tar, ammonia and naphthalene. The hydrogen sulfide, also known as Sulfur. Its odour was so intense that it prevented the lighting of indoor spaces. So the hydrogen sulfide was removed from the coal gas through large tanks filled with a special soil with alkaline capacities. So the coal gas flowed through these tanks and the soil was sort of a filter for the hydrogen sulfide. So, this was the end of the purification stage.