GAS, COAL 555 



It hag boon already mentioned that, in addition to sulphuretted hydrogen and car- 

 bonic acid, which are readily removed by the processes just described, there also 

 exist in coal-gas, as impurities, various quantities of bisulphide of carbon and probably 

 sulphuretted hydrocarbons. Now all these sulphur-compounds produce sulphurous 

 acid during the combustion of the gas, and where the quantities of these impurities is 

 considerable, as is the case with much of the gas now manufactured, the atmosphere 

 of the apartments in which such gas is used becomes so strongly impregnated with 

 sulphurous acid, as to be highly offensive to the senses and very destructive to 

 art decoration, bindings of books, &c. It becomes, therefore, a matter of con- 

 siderable importance to prevent, as far as possible, the occurrence of these injurious 

 constituents ; in fact, until this is effected, gas will never be more than very partially 

 adopted as a means of illumination in dwelling-houses. When once generated with 

 coal-gas all attempts to remove these constituents have hitherto proved ineffectual, 

 and there seems little ground for hope that any practicable process will be devised for 

 their abstraction. Attention may, therefore, more profitably be directed to the con- 

 ditions which tend to diminish the amount generated in the retorts, or altogether to 

 prevent their formation. Mr. Wright, who has paid considerable attention to this 

 problem, finds that the employment of a moderate heat for the generation of the gas 

 has the effect of greatly reducing the relative quantity of these noxious ingredients, 

 and thus by simply avoiding excessive heat in the retorts, and rejecting the last 

 portions of gas, he has, to a great extent, prevented their formation. Unfortunately, 

 however, this remedy is not likely to find favour amongst gas -manufacturers in 

 general, inasmuch as it considerably reduces the yield of gas. ' A few well-directed 

 chemical experiments could scarcely fail to discover the conditions necessary for the 

 non-production of these sulphuretted compounds. Probably the proper admixture of 

 salt or lime with the coals before carbonisation would have the desired effect. The 

 subject is one of so much importance to the future of gas illumination, that it ought 

 not to be suffered to rest in its present unsatisfactory condition. 



On the consumption of gas. The proper consumption or burning of illuminating 

 gas depends upon certain physical and chemical conditions, the due observance of 

 which is of great importance in the development of a maximum amount of light. The 

 production of artificial light depends upon the fact that, at certain high temperatures, 

 all matter becomes luminous. The higher the temperature the greater is the intensity 

 of the light emitted. The heat required to render matter luminous in its three states 

 of aggregation differs greatly. Thus solids are sometimes luminous at comparatively 

 low temperatures, as phosphorus and phosphoric acid. Usually, however, solids 

 require a temperature of 600 or 700 Fahr. to render them luminous in the dark, and 

 must be heated to 1000 Fahr. before their luminosity becomes visible in daylight. 

 Liquids require about the same temperature. But to render gas luminous, they 

 must be exposed to an immensely higher temperature ; even the intense heat generated 

 by the oxyhydrogen blowpipe scarcely suffices to render the aqueous vapour produced 

 visibly luminous, although solids, such as lime, emit light of the most dazzling splen- 

 dour when they are heated in this flame. Hence it has generally been maintained that 

 those gases and vapours only can illuminate which produce, or deposit, solid or liquid 

 matter during their combustion. This dependence of light upon the production of 

 solid matter is strikingly seen in the case of phosphorus, which when burnt in chlorine 

 produces a light scarcely visible, but when consumed in air or oxygen emits light of 

 intense brilliancy. In the former case the vapour of chloride of phosphorus is pro- 

 duced, in the latter, solid phosphoric acid. 



Several gases and vapours possess this property of depositing solid matter during 

 combustion, but a few of the combinations of carbon and hydrogen are the only ones 

 capable of practical application ; these latter compounds evolve during combustion 

 only the same products as those generated in the respiratory process of animals, viz. 

 carbonic acid and water. The solid particles of carbon which they deposit in the 

 interior of the flame, and which are the source of light, are entirely consumed on 

 arriving at its outer boundary ; their use as sources of artificial light, under proper 

 regulations, is therefore quite compatible with the most stringent sanitary rules. 



The constituents of purified coal-gas have already been divided into illuminating 

 and non-illuminating gases ; amongst the latter will be found light carburetted hydro- 

 gen, which, although usually regarded as an illuminating gas, has been proved by the 

 experiments of Frankland to produce, under ordinary circumstances, no more light 

 than hydrogen or carbonic oxide, and therefore for all practical purposes it must be 

 regarded as entirely destitute of illuminating power. This is owing chiefly to the 

 temperature required for the deposition of its carbon being higher than that attained 

 in an ordinary gas burner ; for Frankland has proved that, if the temperature of 

 the light carburetted hydrogen flame be increased by previously heating the gas and 

 air nearly to redness, then the flame becomes luminous to a considerable degree. It 



