GA8-I.IOHTING. 



GAS- LIGHTING. 



portion* of nitrogen and lulphur. On exposure to the heat oi thu 

 retorti, the hydrogen escapes partly in a free state and partly combined 

 with the other elemenU, carbon, oxygen, nitrogen, and sulphur ; the 

 oxygen combine* partly with hydrogen, forming aqueoui vapour, and 

 partly with carbon, producing carbonio oxide and carbonic acid gate* ; 

 the nitrogen U evolved chiefly as ammonia, but partly also a* cyanogen ; 

 the sulphur chiefly an sulphuretted hydrogen, but likewise as bisulphide 

 of carbon ; whilst a considerable proportion of the most fixed element, 

 the carbon, remains in the retort as cute. Several of the gaseous or 

 volatile compounds thus formed unite with each other to form ueoond- 

 ary compounds. Thus, portions of the carbonic acid, sulphuretted 

 hydrogen, and cyanogen, unite with ammonia to form respectively, 

 carbonate of ammonia, sulphide of ammonium, and cyanide of ammo- 

 nium. After leaving the retorts, these volatile and gaseous matters 

 are cooled down nearly to atmospheric temperature, when nearly 

 all the vaporous matters condense, forming a liquid consisting of two 

 layers, a lower one called tar [COAL TAR], and an upper one con- 

 taining chiefly the ammoniacal compounds above mentioned dissolved 

 in water. [GAS LIQUOR.] The permanently gaseous product of the 

 operation is called crtuie or impure gat, and generally contains the 

 whole of the following ingredients : 



N'ume. 



Chemical Formula. 



/Olefiuitgu ..... C,II, 



Illuminating 1 Other hydrocarbons 

 constituents.) Benzole ... 



\Othcrhydrocarbons . . 



(Light carburettcd hydrogen 

 Hydrogen . . . 

 Carbonic oxide .. 



Cnllu 

 t',, 11 

 Cnll(n-C) 



( ',"i 



H 



CO 



["Carbonic acid .... CO, 



1 Sulphuretted hydrogen . . IIS 



Imparities .< Aqueous vapour UO 



I Ammonia ..... Nil, 



(_ Bisulphide of carbon . . . , CS, 



The total quantity of these constituents, as well as the relative pro- 

 portions in which several of them are generated, depends greatly upon 

 the temperature at which the distillation is conducted. As a general 

 rule, the lower the temperature the less gas is produced, but the 

 greater U its illuminating effect when burnt. On the other hand, when 

 a higher temperature is employed, a large volume of gas, but of inferior 

 quality, is obtained. 



Abstracting the impurities in the above list, it will be seen that gas 

 contains two classes of constituents : namely luminiferous consti- 

 tuent*, or gases yielding light on combustion ; and diluents, or non- 

 luminifcrous constituents, gases which piactically yield no light on 

 combustion. To the first class alone is the illuminating power of gas 

 due ; but one, at least, of the non-luminiferous gases is also necessary 

 in order to enable the first class of constituents to bum without smoke 

 and consequent loss of light. The members of the first class are all 

 decomposed slowly at a red, and rapidly at a white, heat, depositing a 

 large amount of carbon in the solid form, and being resolved into non- 

 luminous gases. It is therefore obvious that, in the process of gas- 

 making, more or less of these valuable constituents must be thus 

 decomposed ; the amount depending, on the one hand, upon the length 

 of time during which they are exposed to a high temperature, and on 

 the other, upon the number of the particles of such constituent* which 

 come into contact with the heated walls of the retort. Two methods 

 for the prevention of this decomposition present themselves. The first 

 consists in the rapid removal of the gases from the retort, and the 

 second in the dilution of the luminiferous gases, whilst still in the 

 retort, by the admixture or injection of non-luminous constituents. 

 The first of these remedies has been extensively applied in the form of 

 exhausters, which greatly facilitate the escape of the gases from the 

 retort*, whilst both remedial measures have been secured in White's 

 process of gas manufacture, in which a current of non-luminous gases 

 u made to sweep through the retort, and thus rapidly remove the 

 decomposable luminiferous constituents. This latter process, however, 

 though undoubtedly based upon sound philosophical principles, has 

 not come into extensive use, owing to certain mechanical difficulties in 

 carrying it out. 



The objects to be kept In view In the generation of gas for illumi- 

 nating purposes are the following : 



1. The formation of a due proportion of illuminating and non- 

 illuminating constituents ; so that, on the one hand, the combustion of 

 the gas shall be perfect, and without the production of smoke or 

 unpleasant odour ; and, on the other, the volume of gas required to 

 produce a certain amount of light shall not be too great. For the pro- 

 duction of an amount of light equal to that of twenty sperm candles of 

 six to the pound, a consumption of gas greater than five cubic fuel per 

 hour ought not to be required. 



2. The extraction of the largest possible amount of gaseous illumi- 

 nathutcom pounds from a given weight of materials. 



' The presence of the largest possible proportion of hydrogen 

 amongst the non-illuminating constituents, to the exclusion of light 

 carburetted hydrogen and carbonic oxide, so as to produce the least 

 amount of heat and atmospheric deterioration in the apartments where 

 th> gas is consumed. 



11. Purijimtio* / OVu. A reference to the list of substances con- 

 tained in crude or impure gai, given above, show* that there are five 

 distinct compounds, ail of which must be regarded as impurities. Of 

 these, however, two namely, bisulphide of carbon and aqueous vapour 

 may be left out of consideration, since the first, although highly 

 objectionable in gas, as the cause of the sulphurous odour always per- 

 ceived when gas is burnt in unventilated apartments, cannot be removed 

 by any practicable process; whilst the second, which does little harm 

 except diminish to some extent the illuminating power of the gas, 

 cannot readily be abstracted, owing to the hydraulic nature of the 

 apparatus used for the storage and measurement of gas. The processes 

 used for the purification of gas are therefore restricted to the removal 

 of sulphuretted hydrogen, carbonic acid, and ammonia. One hundred 

 volumes of crude gas contain on an average the following quantities of 

 these impurities : 



Carbonic acid 

 Sulphuretted hydrogen 

 Ammonia 



2-5 

 8 

 10 



The two chie ingredients used for their removal, with more or less 

 success, are hydrate of lime and hydrated peroxide of iron. 



The first was formerly used suspended in water as milk of Him-, 

 through which the gas was made to bubble ; but it is now almost 

 universally employed in the solid pulverulent form. By direct chemical 

 affinity, the hydrate of lime removes only sulphuretted hydrogen and 

 carbonic acid from the impure gas ; bvit the water which it contains 

 enables it also to remove the greater part, at least, of the ammonia. 

 The sulphuretted hydrogen forms, with hydrate of lime, water and * 

 non-volatile sulphide of calcium, according to the following equation : 



CaOlIO + H8 = CaS + 2HO. 



Carbonic acid is rapidly absorbed by hydrate of lime, forming a 

 basic carbonate, thus : 



2(CaO HO) + CO, = CaO CO, + CaO HO + HO. 



Basic carbonate of lime. 



In both these reactions water is set at liberty, which assists in 

 absorbing and retaining ammonia as mentioned above. 



When hydrated peroxide of iron is used for the purification of 

 gas, the carbonic acid is totally unacted upon, but the ammonia is 

 perhaps more completely absorbed than by the lime process. The 

 carbonio acid is either subsequently absorbed by slaked lime or, as is 

 more frequently the case, it is suffered to remain in the gas, occasioning 

 a considerable loss of light, but no additional nuisance to the consumer. 

 The action of hydrate and peroxide of iron in removing sulphuretted 

 hydrogen consists in the formation of sulphide of iron, water and free 

 sulphur, thus : 



Fe.O,, 3HO + SHS = 2Fe8 + 8 + 6HO. 



The cost of hydrated peroxide of iron would effectually previ 

 use, if, like lime, it were only once available for the purpose ; but the 

 peculiarity and value of this oxide as a purifying agent consist 

 capability of rcririjtcation ; that is, restoration to its original con- 

 dition, or uearlyso. This is effected by simple c 



air, the oxygen of which is to rapidly absorbed by the sulphide of 

 iron as to occasion considerable risk of ignition unless the supply of 

 air be moderate. Hydrated oxide of iron is regenerated, and the 

 sulphur previously combined with the iron is set at liberty in the 

 following manner : 



SFeS + SO + SHO = Fc,O,, SHO + 28. 



This alternate absorption of sulphuretted hydrogen and revivification 

 might probably be carried on for an infinite number of times, were 

 it not that the free sulphur ultimately accumulates to such an 

 extent as to greatly reduce the efficiency of the mixture, and the 

 sulphur therefore requires occasional removal cither by distillation or 

 roasting. 



As it reaches the consumer, gas is very rarely contaminated with 

 even a tracu of sulphuretted hydrogen, but it frequently e.< 

 carbonio acid and invariably bisulphide of carbon, to which latter 

 substance, giving as it does sulphurous acid on combustion, nearly all 

 the annoyance experienced from the use of gas in dwelling -houses is 

 due. In conclusion, sulphuretted hydrogen may be readily d< 

 in gas by allowing a jet of the latter to blow upon a piece of white 

 paper moistened with a solution of acetate of lead ; the slightest dis- 

 coloration of the paper shows thu presence of sulphuretted hydrogen. 

 Carbonic acid is best detected by allowing the gas to bubl.lc through 

 clear and transparent lime-water which will become turbid if the acid 

 gas be present. Ammonia is recognised by allowing a jet of the gas 

 to blow against paper tinted yellow with infusion ot turmeric- ; urn 

 changes the yellow of this paper to reddinh brown. Bisulphide of 

 carbon is detected by condensing the aqueous vapour formed by n g. 

 flame : if the condensed product redden litmus paper this impurity is 



HI . :>.. 



III. The combutliun of gat. 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 



