If a cold substance, metal or non-metal, be placed in a flame, whether it be luminous or non-luminous, it will be observed that there is a clear space, in which no combustion is taking place, formed round the cool surface, and that as the body gets heated so this space gets less and less until, when the substance is at the same temperature as the flame itself, there is contact between the two.  Moreover, when a luminous flame is employed in this experiment the space still exists between the cool body and the flame, but you also notice that the luminosity is decreased over a still larger area although the flame exists.

This meaning that, in immediate contact with the cold body, the temperature is so reduced that the flame cannot exist, and so is extinguished over a small area; while over a still larger space the temperature is so reduced that it is not hot enough to bring about decomposition of the heavy hydrocarbons with liberation of carbon to the same extent as in hotter portions of the flame.  Now, inasmuch as when water is heated or boiled in an open vessel, the temperature cannot rise above 100 deg.C., and as the temperature of an ordinary flame is over 1,000 deg.C., it is evident that the burning gas can never be in contact with the bottom of the vessel, or, in other words, the gas is put out before combustion is completed, and the unburned gas and products of incomplete combustion find their way into the air and render it perfectly unfit for respiration.

The portion of the flame which is supposed to be the hottest is about half an inch above the tip of the inner zone of the flame, and it is at this point that most vessels containing water to be heated are made to impinge on the flame; and it is this portion of the flame, also, which is utilized for raising various solids to a temperature at which they radiate heat.

In order to gain an insight into the amount of contamination which the air undergoes when a geyser or cooking stove is at work, I have determined the composition of the products of combustion, and the unburned gases escaping when a vessel containing water at the ordinary temperatures is heated up to the boiling point by a gas flame, the vessel being placed, in the first case, half an inch above the inner cone of the flame, and in the second, at the extreme outer tip of the flame.

         GASES ESCAPING DURING CHECKED COMBUSTION.

|    Bunsen flame.      |     Luminous flame.
+-----------+-----------+-------------+----------
|   Inner.  |   Outer.  |     Inner.  |  Outer.
+-----------+-----------+-------------+----------
Nitrogen            |   75.75   |    79.17  |     77.52   |   69.41
Water vapor         |   13.47   |    14.29  |     11.80   |   19.24
Carbon dioxide      |    2.99   |     5.13  |      4.93   |    8.38
Carbon monoxide     |    3.69   |     Nil.  |      2.45   |    2.58
Marsh gas           |    0.51   |     0.31  |      0.95   |    0.39
Acetylene           |    0.04   |     Nil.  |      0.27   |    Nil.