556 GAS, COAL 



is not improbable that when gas is consumed in very largo burners this necessary 

 temperature is attained, and the light carburotted hydrogen contributes considerably 

 to the aggregate illuminating effect ; a view which is, to a certain extent, confirmed by 

 the fact, that a relatively much larger amount of light is obtained from coal-gas when 

 the latter is consumed in a large flame than when it is allowed to burn in a small flame. 

 Omitting light carburetted hydrogen and carbonic oxide, the remaining carbo- 

 niferous constituents of coal-gas yield, during combustion from suitable burners, an 

 amount of light directly proportionate to the quantity of carbon which they contain in 

 a given volume. 



In order to understand the nature of the combustion of a gas flame, it is necessary 

 to remember that the flame is freely permeable to the air, and that according to the 

 well-known laws of gaseous mixture, the amount of air which mixes with the ignited 

 gases will be increased, first, by an increase of the velocity with which the gas issues 

 from the orifice of the burner ; and secondly, by the velocity of the current of air 

 immediately surrounding the flame. It is well known that a highly luminiferous gas 

 may be deprived of all illuminating power either by being made to issue from the 

 burner with great velocity, or by being burnt in a very rapid current of air produced 

 by a very tall glass chimney. 



The foregoing considerations indicate the conditions best adapted for obtaining the 

 maximum illuminating effect from coal-gas. The chief condition is the supply of just 

 such a volume of air to the gas flame as shall prevent any particles of carbon from 

 escaping unconsumed. Any excess of air over this quantity must diminish the number 

 of particles of carbon deposited within the flame, and consequently impair the illumi- 

 nating effect. 



Dr. Frankland has shown that the luminosity of a flame is materially modified by 

 the pressure of the atmosphere in which it burns ; the denser the air the more 

 brilliant the light, the carbon combining more rapidly with the increased quantity of 

 oxygen. Hence the illuminating effect of the same gas will vary in different localities ; 

 thus, a quantity of gas which gives in London the light of 100 candles would give in 

 Munich (1,700 feet above the sea-level) only the light of 91 candles, and if the samo 

 gas were burned in the city of Mexico (about 7,400 feet above the level of the sea) 

 it would yield the light of only 61 candles. 



The following remarks on increasing the illuminating power of coal-gas by 

 carburetting or naphthalising, are derived from the Kev. W. K. Bowditch's work on 

 ' The Analysis, Technical Valuation, Purification, and Use of Coal-gas ' : 



' Carburetting gas signifies adding to it vapours of hydrocarbons, which contain a 

 large proportion of carbon in comparison of their other constituent, hydrogen ; and as 

 coal-naphtha was first used for this purpose, and is yet the chief substance employed, 

 the process was named naphthalising, from the fluid employed. Coal-naphtha is a, 

 mixture of hydrocarbons, and is made up of a solid (carbon) and a gas (hydrogen), 

 which, when united, form a fluid that contains a far higher percentage of carbon than 

 gas does. It is to this high percentage of carbon it owes its use in improving the 

 light of gas, to which its vapour is added. In fact, the light -giving power of gas 

 supplied to us for combustion is largely due to the pressure in it of vapours which, 

 when they are condensed into a fluid we call naphtha. We need not be chemists to 

 ascertain that naphtha is much richer in carbon than coal-gas is ; for if we light gas it 

 burns with a luminous, but not with a smoky flame, whereas if we apply a light to a 

 little naphtha in a saucer, it burns with a red flame, which affords but little heat, and 

 deposits large quantities of carbon in the well-known form of soot. The proportion 

 of carbon is so large that there is not heat enough to burn it, and hence the smoke. 

 The cause of this is obvious. When the naphtha-vapour is fired, both of its con- 

 stituents burn, but the heat-producing constituent, hydrogen, is relatively so small in 

 quantity that its heat cannot raise the carbon to the temperature required for its 

 union with oxygen or, as it is popularly called, burning, and therefore the unburnt 

 carbon is deposited in visible black particles, instead of being converted into an in- 

 visible gas (carbonic acid), as it would have been had the hydrogen been in sufficient 

 quantity to heat up the carbon to the temperature requisite for its combustion. Had 

 this been the case the flame would have been smokeless instead of smoky, and white 

 instead of red. The case of gas is very different. When that is lighted much more 

 than enough heat is produced to burn its carbon perfectly. The richness of naphtha in 

 carbon may also bo inferred from the fact that while a cubic foot of common pas 

 weighs but about 224 grains, a cubic foot of naphtha weighs more than 370,000 grains. 

 Now, this naphtha is composed of carbon and hydroLC' n <nly. Hydrogen is the lightest 

 substance known in nature, and a cubic foot of it weighs but 37*1 grains, from which 

 it is easy to see that the other constituent (carbon) gives the weight to the naphtha. 

 Why is it necessary to add to gas those highly carburet tod bodies? Because tho solid 

 (rarbon which, they contain is required to give light. A gas which does not contain a 



