Ammonia is objectionable in acetylene because it corrodes brass fittings and pipes, and because it is partially converted (to what extent is uncertain) into nitrous and nitric acids as it passes through the flame. Sulphur is objectionable in acetylene because it is converted into sulphurous and sulphuric anhydrides, or their respective acids, as it passes through the flame. Phosphorus is objectionable because in similar circumstances it produces phosphoric anhydride and phosphoric acid. Each of these acids is harmful in an occupied room because they injure the decorations, helping to rot book-bindings, [Footnote: It is only fair to state that the destruction of leather bindings is commonly due to traces of sulphuric acid remaining in the leather from the production employed in preparing it, and is but seldom caused directly by the products of combustion coming from gas or oil.] tarnishing "gold-leaf" ornaments, and spoiling the colours of dyed fabrics. Each is harmful to the human system, sulphuric and phosphoric anhydrides (SO_3, and P_4O_10) acting as specific irritants to the lungs of persons predisposed to affections of the bronchial organs. Phosphorus, however, has a further harmful action: sulphuric anhydride is an invisible gas, but phosphoric anhydride is a solid body, and is produced as an extremely fine, light, white voluminous dust which causes a haze, more or less opaque, in the apartment. [Footnote: Lewes suggests that ammonia in the gas burnt may assist in the production of this haze, owing to the formation of solid ammonium salts in the state of line dust.] Immediately it comes in contact with atmospheric moisture phosphoric anhydride is converted into phosphoric acid, but this also occurs at first as a solid substance. The solidity and visibility of the phosphoric anhydride and acid are beneficial in preventing highly impure acetylene being unwittingly burnt in a room; but, on the other hand, being merely solids in suspension in the air, the combustion products of phosphorus are not so easily carried away from the room by the means provided for ventilation as are the products of the combustion of sulphur. Phosphoric anhydride is also partly deposited in the solid state at the burner orifices, perhaps actually corroding the steatite jets, and always assisting in the deposition of carbon from any polymerised hydrocarbons in the acetylene; thus helping the carbon to block up or distort those orifices. Whenever the acetylene is to be burnt on the incandescent system under a mantle of the Welsbach or other type, phosphorus, and possibly sulphur, become additionally objectionable, and rigorous extraction is necessary. As is well known, the mantle is composed of the oxides of certain "rare earths" which owe their practical value to the fact that they are non-volatile at the temperature of the gas-flame. When a gas containing phosphorus is burnt beneath such a mantle, the phosphoric anhydride attacks those oxides, partially