It is necessary to distinguish between burning and the mere appearance of it. A gas flame is gas in a state of combustion, whereas the electric light is no example of it, although the wire within the glassen cylinder is red hot, and to all appearance burning. Combustion generally takes place through the strong affinity of some element, such as carbon in a substance for the oxygen in the atmosphere. In coal gas, for instance, the carbon contained in it unites with the oxygen in the air to form a colorless substance called carbonic acid gas. The latter is unable to support life, and may be called, therefore, poisonous. It is the presence of this gas which makes it unhealthy to burn many jets without proper ventilation.

Also, carbonic acid gas is given off by the lungs. It may seem curious, but it is none the less true, that breathing is a process of combustion. The blood brings to the surface of the lungs the carbon, which has resulted from the waste of the internal organs of the body. When drawing in a breath the oxygen present in the atmosphere meets the impure blood at the surface of the lungs, and purifies it by uniting with the carbon in it. Then, though oxygen has been breathed in, carbonic acid gas has been breathed out.

To prove this will be interesting: Obtain from a chemist a little lime water—two cents worth will do. It looks like ordinary water, being perfectly transparent and colorless. Pour some into a clean glass, and through a glass tube blow steadily into the water. In half a minute the hitherto colorless liquid will become milky and opaque. If allowed to stand there will fall down at the bottom of the glass a white powder.

What has happened in this case? The carbonic acid gas from the lungs has formed with the lime in the lime water a substance called carbonate of lime, which, being insoluble in water, falls to the bottom of the glass as a white powder.

If carbonic acid gas were not present in the air blown from the lungs, this milkiness would not appear, for no other gas, except this, would alter the lime water’s clearness.