The gases evolved in the manufacture of artificial manure are carbonic acid, sulphurous acid, sulphuretted hydrogen, hydrochloric acid, and, according to some, arseniuretted hydrogen and silicon fluoride, also sulphuric acid in the form of spray. These gases are certainly injurious, and in small quantities will lead to bronchial and other affections, and to poisoning if inhaled in large quantities. The operation of dissolving is, however, carried on in a closed mixer and pit in which a fan is placed, and the gases are drawn through towers in which blocks of wood loosely packed are arranged, and these blocks are kept wetted by water sprayed from above; by this means the harmful gases are dissolved out before the air is passed into the chimney. In a. properly constructed superphosphate works there will therefore be no nuisance from the escape of gases.