OA3 AND OASES 



GAS-L&HTING 



97 





accurately made by means of a Rtnall horizontal 



telescope, placed at a distance of about six led, ami 

 sliding on a vertical rod. Tin- height f the mer- 

 in II Millet MOW lie accurately del ei mined ; 

 HMci if fiiiiii tli** Muml.er thus read otl' the height 

 of the sixth division above the zero <f the scale in 

 II i^ deducted (the scale on II is not marked in 

 the figure), the remainder will express the true 

 volume of the ^as, no corrections being required 

 for variations of temperature, atmospheric presu- 

 me, tension of aiiiicoiis vapiMir, &<. 



Hydrogen, in tne proportion of half the volume of 

 the air u>ed, nm>t now l>e passed into I, and from 

 thence into F, when the volume of the mixed gases 

 must he again determined as hefore. An electric 

 spark must now he passed through the mixed gases 

 in F hy means of the platinum wires at m (near 

 the top of F). A slight explosion occurs, after 

 which we oliserve a considerable contraction in the 

 volume of the mixed gases, and one-third of this 

 .shrinkage represents the volume of oxygen. 



The objection to this kind of gas-analysis is its 

 mparative slowness. When we wish to control 

 the process of coal-gas-making, it is necessary to 

 collect a series of specimens during the progress of 

 the decomposition, but the" results of gas-analysis 

 are rarely available with useful expedition. Where 

 it is sufficient to trace up one special constituent, 

 such as sulphuretted hydrogen in coal-gas or car- 

 bonic acid in ventilation-experiments, results of 

 considerable value may be attained by passing 

 known volumes of the gas through a known quantity 

 of a test-liquid, or shaking it up with it, and measur- 

 ing by titration the amount of the reagent un- 

 affected by the particular constituent of the gas ; 

 or, more rapidly, by the gradual addition of one to 

 the other until the mutual reaction ceases. For 

 instance, 100 cubic cm. of crude coal-gas may 

 have successive instalments of a dilute solution of 

 iodine of known strength brought into contact 

 with it ; when the reaction ceases the iodine solu- 

 tion ceases to be decolorised by the sulphuretted 

 hydrogen, and if starch be present a blue tint will 

 l)e struck. 



Gas, LIGHTING AND HEATING BY, depend 

 mainly on the presence of gaseous heavy hydro- 

 carbons in the gas. Pure hydrogen and even pure 

 methane give no light, and, volume for volume, 

 they give little heat, though their flames are flames 

 of high temperature. When illuminating gas is 

 ignited it burns with a flame which is luminous 

 for two reasons : (1 ) the hydrocarltons form acety- 

 lene, which upon becoming highly heated decom- 

 poses explosively with a bright flash ; and (2) the 

 hydrocarbons are partly decomposed, and leave 

 highly carbonaceous molecular residues which, 



1 mini: highly heated in the flame, incandesce 



and become luminous. I. Coal-gas is produced by 

 the simple distillation of dry coal. Anthracite 

 coal is unsuitable ; brown coal and lignite are 

 unsatisfactory : the greatest yield of the best gas is 

 obtained from highly bituminous coals, although 

 these are expensive and leave as residue inferior 

 coke, mainly ash ; practically the most useful gas- 

 eoal is that which will, either alone or mixed 

 with bituminous coal, yield a fair quantity of 

 good gas and leave good'coke in the retorts. 'The 

 very highly bituminous coals are only used for mix- 

 ing with ordinary coal : the ordinary bituminous or 

 cannel coals are sometimes used, especially in Scot- 

 land, for making richer gas of 25 to 30 candle- 

 power (in standard burners burning. 5 cubic feet per 

 hour), but are usually mixed with ordinary coal with 

 the view of improving the coke produced. Tha 

 ordinary caking coals of the north of England are 

 HIM inly used in England, mixed with a proportion 

 of cannel or of highly bituminous coal or shale in 

 order to improve the gas, which is generally sup- 

 215 



plied with an illuminating power of from 16 to 20 

 candles. The gnu-coal lined on (lie < Ontinent in inter- 

 mediate between caking coal and cherry coal, and 

 gives gas of from 12 to 17 candles. By fiituminouM 

 coal is not meant coal which actually contain* 

 bitumen, but coal which contains carbon and 

 hydrogen in a promotion suited to the formation 

 of heavy liyilrocnrhons when the coal is exponed 

 to heat : no bitumen can be dissolved bv alcohol 

 out of a so-called bituminoiiH coal. The pro- 

 portions of hydrogen and oxygen to the car- 

 Ma in various materials is shown in the following 

 table 



Carbon, Hjdrocvn, Osjcen, Hydro. pr Ozjr \<n 

 per otot. per oent. pr oni. 100 crb. 100 carb 



French anthracite 94 1-49 .. 1-6 



Glamorgan anthr.. 91-5 3-5 2*6 8-8 2-8 



Newcastle gaH-coal 82'1 6"8 6"7 6'4 6-t) 



Wigan caiinel 79-2 6-1 7-2 7*7 'l 



Boghead mineral .. WHS 8'86 4'70 13'8 7'4 



The hydrocarbons which enable the gas to give 

 a luminous flame depend for their formation upon 

 the presence of hydrogen : oxygen, on the other 

 hand, is detrimental ; it takes up hydrogen to form 

 water, and with carbon it forms carbonic acid and 

 carbonic oxide. Anthracite distilled gives no use- 

 ful result; Newcastle gas-coal gives, j>er ton, a little 

 over 10,000 cubic feet of gas, of an illuminating 

 power ranging between 14 and 20 candles; Scotch 

 cannel, 10,600 feet, of 30 candles ; Scotch Boghead, 

 distilled alone, 13,000 feet, of 40 candle, or 15,000 

 feet, of 35 candle ; and Australian Boghead, 14,000 

 feet, of 50 candle-gas. These are given merely as 

 typical examples; the results vary greatly accord- 

 ing to the temperatures employed and the duration 

 of the exposure to heat. Newcastle cannel coal, 

 for example, if distilled between 750 and 800 F., 

 yields, per ton, 68 gallons of crude oil (whereof may 

 be recovered paraffin spirit about 2 gallons ; lamp- 

 oil, 22 gallons ; heavy oil and paraffin, 24 gallons), 

 1280 Ib. of coke, and only 1400 cubic feet of gas; 

 whereas, when it is distilled for gas in the usual 

 way, it yields, besides the coal-gas. 18J gallons of 

 coal-tar (wherefrorn 3 pints benzol, 3 pints coal-tar 

 naphtha, and 9 gallons of heavy oils, naphthaline, 

 &c.), and 1200 Ib. of coke. Protracted distillation 



i at high heats causes the evolution of hydrogen 

 rather than of hydrocarbons ; high heats in general 

 cause the production of volatile rather than of con- 

 densable hydrocarbons, and this results, if notcarried 

 to excess, in a decided advantage viz. that the 

 gas produced, though of lower quality than the 

 smaller quantity produced at low heat*, is greatly 

 less liable to lose its illuminating power by conden- 

 sation and deposition of hydrocanons on the way 

 to the consumer. Very roughly, the candle-power 



i is, within a limited range, inversely proportional 

 to the number of feet of gas made (at a given 

 temperature) from a given quantity of coal. Tims, 

 if a ton of coal give 10,000 cubic feet of 15J candle- 



\ gas, then, if the distillation be protracted so that 

 10,500 feet are produced, the candle-power will 

 sink to 15. Tiettrunk calculates the percentage 

 composition (in volumes) of the gas which conies 

 off in successive hours thus : 



lit hour. iM hour. 3d hour. 4th hour. 5ih hour. 



Heavy hydro- \ ._ . , 7 



carbons / J 



Marsh-Rns 82 72 68 56 20 



Hydrogen 8-8 16 21-3 60 



Carbonic oxide.. 8'2 1-9 12-8 11 10 



Xitroj;i-ii 1-8 5'8 \t 4'7 10 



Relative volumes 1 0-685 0-887 0-105 



Distillation is thus after the fourth hour practically 

 disadvantageous to illuminating power. 



The products of distillation of coal, as usually 

 performed in gas-works, are very numerous. The 

 principal of them are marsh-gas, hydrogen, carltonic 

 oxide, carbonic acid, nitrogen, oxygen, sulphuretted 

 hydrogen, ammonia, hydrocyanic acid, bisulphide 

 of carbon, and other organic sulphur compounds ; 



