Influence of Pressure on the Luminosity of Flames

Frankland burned six stearin candles in Chamounix, and the same six afterwards in a tent on the top of Mt. Blanc. The average losses in weight were:

At Chamounix	9.4 grams per hour
Summit of Mt. Blanc	9.2 grams per hour

Attributing the small difference to variation in temperature, Frankland concluded that the rate of combustion of a candle is entirely independent of the density of the air.

He explained the result as follows: In the combustion of a candle, the radiant heat from the flame first melts the wax, and this, by the capillary action of the wick, rises into the flame. It is thus evident that the rate of consumption of the wax is entirely dependent upon the capillarity of the wick, provided the heat radiated from the flame is sufficient to maintain the supply of liquid fuel and to volatilise it on its arrival near the apex of the cotton. Since capillary action is not affected by variations in atmospheric pressure, and as the temperature is almost independent of the same influence, it is clear that neither factor will vary, and the above constancy in rate of combustion is to be anticipated.

During his experiments on Mt. Blanc, Frankland was impressed with the small amount of light emitted by the candles. The inner blue zone was extended, and the size of the luminous zone proportionately reduced. Upon returning to England he carried out a series of photometric measurements with coal-gas flame, and deduced the law that the diminution in illuminating power is directly proportional to the diminution in atmospheric pressure, down to a minimum of about 14 inches of mercury. For every inch fall in pressure of the atmosphere, the luminosity fell, under the particular conditions of the experiments, by 5.1 per cent. Thus, a quantity of coal gas which in London would yield a light equal to 100 candles would, if burned in Munich, give an illuminating effect equal to little more than 91 candles; whilst in Mexico its luminosity would be reduced to 61.5 candles - these numbers being independent of the change of volume of the coal gas by the reduced pressure.

Experiments were next carried out on the influence of compression. This was a difficult task to execute satisfactorily, for it was soon found that any considerable increase of pressure caused both candle and oil flames to smoke. Frankland therefore decided to employ flames that were but feebly luminous at ordinary pressure. In the experiments between atmospheric and two atmospheres' pressure, a lamp fed with amyl alcohol was used. As this smoked at somewhat higher pressures, a mixture of 5 parts ethyl alcohol with 1 of amyl was employed. This had no appreciable illuminating power at ordinary pressure.

It was found that the same law held as for diminution of pressure, an upper limit being reached at about three atmospheres, after which the observed luminosity rapidly increased.

The results obtained may be grouped as follows:

The Influence of Pressure upon Luminosity

Pressure in atmospheres.	Combustible.	Observed Luminosity.	Calculated Luminosity.
1	Coal gas	100	100
2	Amyl alcohol	262.5	253
3	5 parts ethyl alcohol with 1 part amyl alcohol	406	406
4	do	959	559

Analyses of escaping gases from two candles burning under different pressures yielded similar results, and showed that there was no escape of unconsumed combustible vapour. Consequently the diminution of light in rarefied atmospheres is not due to imperfect combustion.

On the other hand, compression tends to render the combustion less complete. Neither is the reduced luminosity due to a fall in temperature, for although a slight fall does take place, it is not sufficient to account for the whole change. It appears to depend "chiefly, if not entirely, upon the ready access of atmospheric oxygen to, or its comparative exclusion from, the interior of the flame."

Frankland also investigated the influence of pressure upon the combustion of hydrogen in oxygen. At 10 atmospheres the light is very bright, and the spectrum continuous from red to violet. Carbon monoxide, which normally burns with a feebly luminous flame, becomes even more luminous at high pressures than a flame of hydrogen of the same dimensions.