Residual and Extinctive Atmospheres

Closely connected with the foregoing study is that of the composition of the residual atmosphere in which a substance has been burning, and of one which just extinguishes a flame - the so-called extinctive atmosphere. Their composition is not determined solely by the percentage of oxygen; the nature of the inert or diluent gases also exerts an important influence. Thus, for example, nitrogen has a less powerful extinctive effect than carbon dioxide. Theoretically extinctive and residual atmospheres are the same, but, owing to the difficulty caused by the products of combustion raising the temperature of the contents of the containing vessel, the flame is apt to continue burning for a longer time than if the surrounding air remained at the original temperature. The percentage of oxygen in the residual air is thus slightly lower than that in an extinctive atmosphere. Under theoretically ideal conditions the results would be the same for both residual and extinctive atmospheres under identical conditions. The following results are interesting:

Combustible.	Residual Atmospheres (Volume per cent.).
Candle flame	15-16 oxygen. 80-81 nitrogen. 3 CO2.
Do	13-15 oxygen. 4-6 oxygen 4-6 CO2
Alcohol, burning on cotton wool	11 oxygen. 82.5 nitrogen. 6.5 CO2.
Wood charcoal glowing to extinction	9 oxygen. 83 nitrogen. 8CO2
Sulphur burning	13.5 oxygen.
Glowing wood	16 oxygen

It will be observed that the residual atmosphere for a candle closely resembles that exhaled by human beings. It may be inhaled by most people for a considerable time without producing any noticeable ill effects.

It is interesting to note that decrease of pressure raises the oxygen limit of the residual atmosphere as shown in the following table:

Residual Atmospheres

Combustible.	otal Pressure, mm.	Oxygen per cent, by Volume in Residual Atmosphere.
Candle	736.7 91	16.1 19.9
Ethyl alcohol burning from asbestos wick	736.7 129	15.1 19.0

The oxygen in residual atmospheres from jets of various combustible gases has been determined by Rhead as follows:

Combustible	Oxygen per cent.
Methane	15.6
Propane	15.9
Butane	16.0
Pentane	16.4
Cyanogen	15.3
Hydrogen	5.7
Carbon monoxide	10.2

The slight increase in oxygen noticeable on ascending the methane series of hydrocarbons is probably due to the highly extinctive effect of the increasing proportion of carbon dioxide, to which reference has already been made. The low oxygen content in the case of hydrogen is noteworthy.

In the case of burning gases, the composition of the extinctive atmosphere is affected by several factors, notably the speed at which either the combustible gas or the atmosphere surrounding it is allowed to move. Rhead found that for a constant speed of combustible gas issuing from a jet, and fed at constant speed by air passing into an encircling container, the extinctive atmospheres of the four lower hydrocarbon gases were identical, and contained approximately 16.6 per cent, of oxygen. This suggests a similar primary reaction in each case. With increased speed the oxygen content falls to a minimum. This is probably due to the fact that with a slow speed of atmosphere the oxygen is consumed too quickly, but upon increasing the speed the combustible mixture is formed sufficiently rapidly to maintain flame. The limiting speed will be reached when the rate of inflammation of the mixture is balanced by the upward movement. This is a point of considerable practical import, inasmuch as an atmosphere, too poor in oxygen to maintain a flame under ordinary conditions or when fed to it at a slow speed, may be able to maintain that flame at a higher speed.

Coal gas, on the other hand, was found to behave quite differently. The oxygen content of the extinctive atmosphere was almost independent of the speed of the atmosphere itself, but fell with increasing speed of the gas stream. This is undoubtedly due to the fact that coal gas is a mixture consisting mainly of hydrogen and methane, the extinctive atmospheres of which possess widely different oxygen contents. Hence, upon occasion, the atmosphere might contain sufficient oxygen to support a hydrogen flame, but not one of methane. With slow gas streams the hydrogen would be burned too quickly. By increasing the speed of the gas and hence of the hydrogen, the combustible mixture of oxygen and hydrogen is produced with sufficient rapidity to maintain a flame. The data in the following will serve to illustrate the foregoing conclusions:

Extinctive Atmospheres

Combustible.	Diameter of Jet, mm.	Speed of Gas. cm. /min.	Speed of Atmosphere, cm. /min.	Oxygen in Extinctive Atmosphere. Per cent.
Methane	4	288	684	16.6
Propane	4	288	715	16.6
Propane	4	288	715	16.6
Propane	4	288	937	15.0
Coal gas	3	514	500	13.2
Coal gas	3	514	291	13.1
Coal gas	3	514	253	13.5
Coal gas	3	514	500	13.2
Coal gas	3	1028	500	12.3

Closely allied to this is the very important problem of the inflammability of hydrogen gas as used for the inflation of balloons and airships. The only non-inflammable gas that could be used economically for the purpose is helium, but, as this gas is twice as dense as hydrogen, its lifting power is somewhat reduced. It follows, therefore, that if a mixture of the two gases could be found which is non-inflammable, the result would be more efficient quite apart from financial aspects of the subject. It appears that, under favourable conditions, a jet of helium containing more than 14 per cent, of hydrogen can be ignited in air; but in the case of a gas issuing from an orifice under conditions prevailing in balloon practice, a mixture containing even 18 to 20 per cent, of hydrogen will not burn with a persistent flame, and might, therefore be employed for military purposes with safety. Mixtures containing upwards of 20 per cent, of hydrogen would be dangerous.