It is well known that the air of wells, mines, &c. is sometimes capable of extinguishing a flame, and recent researches have shown that this is almost always due to deficiency of oxygen, accompanied by the presence of a considerable percentage of CO21. The extinctive atmosphere is popularly known as “choke-damp” or “black-damp.” The source of the CO2 is not necessarily the same in all cases. If oxidisable organic matter is present it is reasonable to suppose that the CO2 may be derived from the carbon in this organic matter by the action on it of air; but the derivation remains a supposition. Carbonaceous materials may be oxidised and yet give no CO2. The other obvious source of CO2 is the decomposition of carbonates by an acid, the acid being produced by some process of oxidation. The commonest mineral which will yield an acid by oxidation with air is pyrites, and if direct oxidation of carbonaceous matter be ruled out, it may be assumed that the occurrence of CO2 is ultimately referable to pyrites in an easily oxidisable form, for example as marcasite.