98 



GAS-LIGHTING 



aqueous vapour ; ethylene, propylene, butylene, 

 acetylene, ditetryl, and allylene ; caproyl, capryl 

 and rutyl hydrides; caproylene, cenanthylerie ; ben- 

 zol, toluol, xylol, cymol ; paraffin, naphthaline, 

 anthracene, chrysene, pyrene ; acetic acid, carbolic 

 acid, cresol, phlorol, rosolic acid ; aniline, pyridine, 

 picolin, and several other nitrogenous alkaloid sub- 

 stances ; with some hydrochloric and sulphurous 

 acids. These substances have very different vola- 

 tilities and solubilities ; a large number of them 

 may be separated from the gas by mere cooling, 

 and together these form coal-tar, which is a black 

 viscous liquid, sp. gr. 0'98 (from carinel) to I'lo 

 (from ordinary coal), the yield of which is, from 

 coal, up to 12 gallons, and from cannel up to 17 

 gallons per ton distilled, the average yield being 

 scarcely 11 gallons. By careful distillation coal- 

 tar yields successively the following products, the 

 percentages of which vary widely in different gas- 

 works : 2-4 per cent, of water, ammonia (which 

 may be extracted from the tar by cold water), 

 and volatile hydrocarbon vapours ; 1 '5 to 16 per 

 cent, of light oils, including carbolic acid ; 20-35 

 per cent, of heavy oils (creasote oils); 10-20 per 

 cent, of anthracene oils, and a residue of 28-64 

 per cent, of pitch. The reason of this wide range 

 of variation in the tar lies partly in the nature of 

 the coal used, the temperature of distillation (the 

 higher the heats the thicker the tars), and partly 

 in the mode and temperature of condensation. 



After the tar has been inostly deposited the 

 gas is washed with water, .which is converted into 

 ammoniacal liquor, containing ammonia, carbonate 

 of ammonium, sulphide of ammonium and some sul- 

 phite, chloride, and sulphocyanide of ammonium, 

 and salts of nitrogenous allcaloids. After being 

 cooled and washed the gas still contains carbonic 

 acid, sulphuretted hydrogen, some hydrocyanic 

 acid, and some bisulphide of carbon, and other 

 sulphur compounds. Slaked lime, moistened so 

 as to form a porous mass, will absorb the carbonic 

 acid or sulphuretted hydrogen, but not the hydro- 

 cyanic acia and bisulphide of carbon so long as 

 there is free carbonic acid present. Oxide of iron 

 absorbs HaS, becoming sulphide ; and this, when 

 re-exposed to the air, is re-oxidised, the oxide being^ 

 regenerated, while free sulphur is formed mixed 

 with the oxide ; the oxide may be used over and 

 over until the percentage of free sulphur rises to 

 50 or 56, after which the oxide is 'spent,' and is 

 transferred for the sake of its sulphur to the manu- 

 facturing chemist. Spent oxide also contains 

 Prussian blue, or ferrocyanide of iron, FerCyis ; 

 this, together with sulphocyanide of iron, is 

 formed from the hydrocyanic acid. Further, the 

 free sulphur in the oxide arrests bisulphide of 

 carbon and other sulphur compounds. The re- 

 generation of the oxide can be brought about 

 by admitting a percentage, say 2, of air into the 

 gas-stream. The oxygen of the admitted air is 

 taken up in continuous regeneration of the purify- 

 ing oxide. The disadvantage of this is that the 

 residual nitrogen of the air tells against the illum- 

 inating power of the gas ; but recently, since pure 

 oxygen has become cheap, oxygen gas alone has been 

 employed with very favourable results. One result 

 of continuous revivification is, that the evil smells 

 associated with the opening of purifiers have become 

 unfamiliar in most works. When continuous re- 

 generation is resorted to, the oxide does not become 

 spent until it contains a considerably higher per- 

 centage (as much as 75) of sulphur. Iron oxide, 

 however, does not remove carbonic acid, and Mr 

 R. H. Patterson showed that complete purification 

 might be secured by removing ( 1 ) COz by means 

 of lime ( the carbonic acid having a stronger affinity 

 for lime than sulphuretted hydrogen has, is retained 

 in the first lime purifier, while H 2 S either passes 



on directly or is driven off by the succeeding CO? 

 from any temporary lodgment it may have gained 

 in the first purifier) ; (2) H 2 S by a second lime 

 purifier, the resulting sulphide of calcium uniting 

 with the bisulphide of carbon to form thiocarbonate 

 of calcium (CaS + CS 2 =CaCS3, analogous to car- 

 bonate of calcium, CaCOa), or rather a basic com- 

 pound CaCS3,CaH 2 O2,7H 2 O, and also with other 

 sulphocarbon compounds; and (3) if necessary 

 any remaining H 2 S may be taken up by iron 

 oxide. In 1888-89 Mr Valon found that if 0'6 

 per cent, of oxygen be added to crude gas, and if 

 lime be used alone as the purifying agent, there is 

 complete and simultaneous removal of the carbonic 

 acid, sulphuretted hydrogen, and sulphide of car- 

 bon, the sulphur being separated in the free state 

 and the gas-lime produced being entirely devoid 

 of smell ; while, owing to complete separation of 

 the carbonic acid and through not introducing 

 nitrogen, the lighting-power of the gas is at least 

 1| candle better than when iron oxide is employed 

 alone. 



Purified gas contains, in percentages by volume : 



London London Boghead 



common Gas. Cannel Gas. Gas. 



Heavy hydrocarbons 3-8 13 24-5 



Marsh-gas 39'5 50 58'4 



Hydrogen 46 27 '7 10'5 



Carbonic oxide 7'5 6-8 6'6 



Carbonic acid 8'7 O'l 



Nitrogen 0'5 0'4 



Aqueous vapour 2 2 



London cannel gas is no longer made ; and true 

 Boghead mineral is no longer obtained in Great 

 Britain, though large quantities of an equivalent 

 substance are now shipped from Australia. 



When coke is made in a beehive oven, the ga 

 evolved is largely contaminated with nitrogen ; but 

 when coke is made from moderately bituminous 

 coal in a by-products oven, the gas produced is 

 practically equivalent to a somewhat poor coal-gas 

 or to a rich fuel-gas. It is understood that the 

 manufacture of this by-products coke-gas is likfely 

 to be undertaken on a large scale in Massachusetts 

 and at Pittsburg, where the supply of natural gas 

 shows symptoms of exhaustion. 



The illuminating power depends on the ' heavy 

 hydrocarbons ; ' of these benzol is the most effective 

 (3 parts of it being equal to 25 of ethylene), and in 

 ordinary English gas is present to the amount of 

 from 5 to 10 grains per cubic foot, while ethylene 

 and propylene are together from four to twelve 

 times that quantity. If carbonic acid, sulphuretted 

 hydrogen, and nitrogen be absent, the heavier gas 

 is generally the richer, though a high percentage of 

 carbonic oxide may also make a gas heavy. The 

 specific gravity of coal-gas is from 0'4 to - 55 (air= 

 TOO). There are two rough tests for the value of 

 gas : (1) its durability i.e. the time taken to burn 

 1 cubic foot of gas in a jet of 5 inches high ; this 

 ranges from 50' 40" for English caking-coal gas, to 

 84' 22" for Boghead gas; (2) the percentage of 

 volume which is condensed by chlorine or bromine, 

 which attack the heavy hydrocarbons. If any 

 carbonic acid remain in the gas, it will diminish 

 the illuminating power about one candle for every 

 1 per cent, of carbonic acid. If gas be mixed with 

 air the illuminating power rapidly falls off: with 1 

 per cent, of air, the loss of lighting-power is 6 per 

 cent. ; with 2, 1 1 ; 3, 18 ; 4, 26 ; 5, 33 ; 10, 67 ; 20, 93 per 

 cent. ; 45, total loss of lighting-power. Ordinary 

 gas mixed with more than 4 and less than 12 times 

 its bulk of air is explosive ; most so when mixed 

 with 8 volumes of air or somewhat more (up 

 to 11 volumes) if the gas be richer. Alone, it is not 

 'explosive. For ascertaining the illuminating power, 

 the Bunsen photometer (the open 60-inch Bunsen- 

 Letheby photometer, or the enclosed 100-inch Evans 

 photometer) is generally employed. In this, at one 



