While endeavoring to leave out of these articles most of the chemistry and as much of the technical detail as is consistent with clearness, the chemistry of combustion and the “gas producer” is so interesting that it will be well to explain that carbon (coal, coke, wood, etc.) can burn either in one or two stages. Nearly every one has noticed the blue flame with which coal burns in the parlor coal heater or in other furnaces where little draft is used and most of us remember that the gas which is given off from such a fire has asphyxiated many who were unfortunate enough to be sleeping in a closed room, when through insufficient chimney draft or a leaky stove some of the unburnt gas filled the room. This gas, which is carbon monoxide, is labeled CO in books on chemistry. It is the result of burning the coal with insufficient air. Chemically it is explained by the second of the chemical “equations” which follow. The third equation explains the second stage of the burning which would occur were further air or oxygen admitted to the upper part of the furnace. _The usual one-stage combustion with plenty of air_: 1. C (carbon) + 2O (oxygen) burns to CO_{2} (carbon dioxide). Non-poisonous. _The two-stage combustion with insufficient air_: 2. C + O burns to CO (carbon monoxide). Poisonous. 3. CO + O burns to CO_{2}. Non-poisonous. Carbon monoxide asphyxiates by forming a chemical compound with the hæmoglobin of the blood, which therefore is prevented from supplying the body with the oxygen that is required for the sustenance of life. Carbon dioxide is no such poisonous product, as may be inferred when we remember that it is the gas with which our carbonated waters are charged and which is so commonly served with ice cream in ice cream soda.