GAS, COAL 557 



-solid gives" but a mere "trace of light, however great the heat produced by its com- 

 bustion. This is well seen in the case of the gas hydrogen. When it burns, the heat 

 produced is very intense. Solid substances held in the flamo become strongly ignited 

 and afford light ; but without the solid the burning gas gives so little light that we 

 cannot imagine a flame sufficiently large to afford us useful illuminations ; and, if we 

 could produce such a flame, the heat of it would be unbearable. Hydrogen does not 

 contain a solid, and the sole product of its combustion is water. If we pass hydrogen 

 over naphtha we find by the smell that it has carried with it a quantity of the vapour ; 

 and if we then burn the hydrogen so mixed with vapour, we obtain light, and the 

 light increases as the quantity of vapour mixed with the hydrogen increases. In this 

 case the hydrogen burns as soon as it comes into contact with the air, and produces 

 intense heat. This heat decomposes the naphtha-vapour and sets free its carbon in 

 the solid form, and then heats it to whiteness, at which temperature it affords light ; 

 the quantity of light being regulated, first, by the quantity of carbon present in the 

 flame; and, secondly, by the temperature to which the solid carbon is raised. The 

 greater the quantity of carbon and the higher its temperature, the greater the light. 

 Another change is observed to take place when the hydrogen is mixed with the 

 vapour. The flame of hydrogen alone is small in comparison of the flame produced 

 by the same quantity of coal-gas, but if the hydrogen be passed over naphtha and 

 burnt at the same rate as before, the flame is considerably expanded by the white-hot 

 carbon contained in it. That which occurs in this case occurs universally. An in- 

 crease of luminosity is always attended by an increased size of flame when the gases 

 compared are burnt at the same rate. 



What has now been said applies to the case of coal-gas thus : The principal 

 portion of coal-gas consists of heat-producing substances which do not give light, and 

 which, like hydrogen, require to be mixed with bodies rich in carbon before they can 

 be made useful. In gas itself we find some of these richly-carburetted bodies, but 

 their quantity is small even in the best gas, and of course much smaller in gas of 

 inferior quality. Professor Bunsen of Heidelberg has published an analysis of 

 Manchester gas, in which he shows the percentage of illuminants and non-illuminants ; 

 and although the Manchester gas is made principally from cannel coal, and has an 

 illuminating power of from 20 to 22 sperm candles for a consumption of five feet per 

 hour, the quantity of non-illuminating gas is far larger than is suspected by persons 

 not familiar with the subject. The following is Professor Bunsen's statement, 

 (" Gasometry," p. 113): 



Analysis of Manchester gas. In 100 parts. 



Non^iUuminating I ^ dr -g en ' 48>58 1 



llhuninant*. 



f Sulphuretted hydrogen 



Impurities* { Nitrogen 



I Carbonic acid 



34-90 1 87* 12 per cent. 

 6-64 J 



4-08 



0-29 

 2-46 

 3'67 



100-00 



^gg 1 6 -46 per cent. 



* This is' cannel-gas. In common gas, such as that of London which has little 

 more than half the illuminating power of Manchester gas the proportion of illumi- 

 nants is smaller, and of non-illuminating heat-producers is larger. The mean of 

 results obtained by many chemists shows that common 12-candle gas does not con- 

 tain more than 4 per cent, of illuminating constituents, the other 96 per cent, being 

 non-illuminating heat-producing compounds ; that is, 4 feet in a hundred yield light 

 and 96 feet yield heat. It seems extraordinary, that when we buy and burn 100 

 cubic feet of cannel gas we obtain and use only 6 feet of useful illuminants, and 

 are obliged to take 87 feet of gases which do not furnish any light. An unscien- 

 tific person is apt to exclaim " Can this be ? and if it be, is it necessary? Cannot 

 the illuminants be made and sold without the heat-producers?" This is impossible. 

 Gas cannot be made without its bulk being non-illuminating. But still something 

 can be done to increase the proportion of illuminants, and this is what is called 

 naphthalising or carburetting. We find by experiment if we pass gas over the 

 highly-carburetted substance called naphtha, that every foot of gas carries away 

 with it as vapour a part of the naphtha ; that the naphtha-vapour burns with the 

 gas; and that the light-giving power of the gas increases with the quantity of 

 naphtha- vapour so removed and burnt. We can measure the gas, weigh the naphtha 

 removed, determine the increase of light obtained, and thus ascertain exactly the 



