TESTING GAS.] 



UNDULATOKY FORCES. LIGHT. 



119 



might be expected, there is a difference in the quality of 

 these products, according as the tar is obtained from 

 common Newcastle coal, or from the richer sorts of 

 canneL* 



Of the other imparities or products of gas-making, 

 cyanogen is the most important ; already it has been 

 extracted from the impure gas, and converted into 

 Prussian blue. It is said that a ton of Newcastle coal 

 will yield enough cyanogen to produce seven pounds 

 of Prussian blue a quantity that will, at the present 

 market-price of the pigment, almost cover the original 

 cost of the coaL 



The mode of obtaining this compound is very simple. 

 When the raw or impure gas is purified by hydrated 

 oxide of iron, according to the patents of Croll, Laming, 

 and Hills, the cyanogen combines with the iron, and 

 produces the pigment in question. But when thus 

 made it is largely contaminated with sulphuret of iron 

 and other impurities. These may be got rid of 1'V 

 washing the mixture with dilute sulphuric or muriatic 

 acid. Or if the iron compound is treated with a solution 

 of potash, it gives up its ferro-cyanogen, and produces 

 prussiate of potash, which is an equally valuable com- 

 pound. 



The following table, which has been constructed from 

 the experiments of Messrs. Barlow and Wright, will 

 afford some idea of the relative proportions of gas, tar, 

 atnmoniacal liquor, ami coke, furnished, per ton, by 

 different varieties of coal : 



Tefttfor the Impurities in Coal Oat. Notwithstanding 

 that so much trouble is taken with the purification of 

 coal gas, yet it is found that a greater or less proportion 

 of the several noxious compounds already described, will 

 escape absorption, and will find their way into the street 

 mains. The most important of these are ammonia, car- 

 bonic acid, sulphuretted hydrogen, bisulphuret of carbon, 

 and tarry matter. 



The first of these i objectionable from the circum- 

 stance that it attacks the fittings, corrodes the meters, 

 and fixes the stopcocks ; besides which, it has the pro- 

 perty of holding tar in suspension. Ammonia ought not 

 to exist in coal gas to a greater extent than one part in 

 about 50,000 : that Ls, 100 cubic feet of gas ought not to 

 contain more than about 3 '5 cubic inches, or rather 

 more than half a grain, of ammoniacal vapour. Any- 

 thing approaching to this quantity may be readily dis- 

 covered by means of turmeric paper, which is immedi- 

 ately reddened by the impure gas. 



The presence of carbonic acid may be known by col- 

 :ig a bottleful of the gas, and shaking it up with a 

 little lirne-wator ; if the impurity be present, the lime- 

 water will 1... nn. lured milky. The great objection to 

 carbonic acid is, that it reduces the illuminating power 

 of the gas, and thus lowers its value. Mr. Wright and 

 Mr. Lewis Thompson say, that every one part of car- 

 bonic acid in a hundred of gas, reduces its illuminating 

 power to the extent of eight or ten per cent. 



Sulphuretted hydrogen may be discovered by a solu- 

 tion of sugar of lead, a little of which ought to be 

 dropped on a strip of wliite paper, and then held in the 

 gas for a period of not less than ten minutes or a quarter 

 of an hour. If the paper becomes discoloured, sulphu- 

 retted hydrogen is present. There are several reasons 

 why this impurity should not exist in coal gas : it gives 

 to the gas a foetid odour ; it tarnishes silver, and destroys 

 the beauty of paint ; and in the act of burning, it geiie- 



Oa Ur in larjfljr employed by calico printen and dyen, in producing 

 maure, Maf rnta, and other eolouri. Milor. 



rates sulphuric acid, which acts injuriously on books, 

 linen goods, and other textile fabrics. 



Bisulphuret of carbon is not so easily detected, for 

 the gas must be burnt under a platinum rosette, and 

 the products of combustion collected in a vessel contain- 

 ing a little ammonia. Mr. Wright, of the Western Gas 

 Company, has contrived an apparatus for this purpose. 

 Having consumed about twelve cubic feet of gas, at the 

 rate of half a cubic foot per hour, it will be found that 

 six or seven ounces of water will have condensed in the 

 receiver. This is to be treated with a solution of nitrate 

 of baryta, that has been rendered acid by a little nitric 

 acid. If, in the course of a few hours, a white powder 

 settles to the bottom of the liquid, then bisulphuret of 

 carbon was present in the gas ; and if the precipitate is 

 collected and weighed, the quantity of impurity present 

 can be determined ; for every 234 grains of sulphate of 

 baryta represent 38 grains, or nearly 46 cubic inches, of 

 the vapour of bisulphuret of carbon. By proceeding in 

 this way, it will be found, that, 100 cubic feet of coal 

 gas yield from 50 to 300 grains of sulphate of baryta 

 quantities that represent from 8'1 to 48 '6 grains of 

 bisulphuret of carbon. The presence of this impurity in 

 coal gas is a most serious affair : for it has been shown 

 by Dr. Letheby, in several of his reports to the Corpora- 

 tion of London, that bisulphuret of carbon, in the act of 

 burning and oxydisiug, forms sulphuric acid ; a great 

 portion of which escapes in a corrosive form, and does 

 enormous damage to every kind of textile fabric. He 

 states, that the books in almost every library in the 

 kingdom where gas is used, are falling to pieces from 

 the action of this acid upon the covers; and he makes 

 reference to the libraries of the Athenaeum Club-house, 

 the London Institution, the Royal College of Surgeons, 

 the Portico Library at Manchester, and that of the 

 Literary Society at Newcastle-upon- Cyne, for examples 

 of the mischief done. In most ol these places, the 

 injury has been so great that a remedy has been called 

 for. This remedy consists in burning the gas in such a 

 way, that the products of combustion shall be carried off 

 as soon as they axe generated. 



As regards the quantity of sulphuric acid which is 

 produced in this manner from bisulphuret of carbon, we 

 may say that there is some discrepancy of opinion. Mr. 

 Lewis Thompson states that coal gas rarely contains less 

 than the one-thousandth of its bulk of bisulphuret of 

 carbon ; and if so, 100 cubic feet of the gas can rarely 

 produce less than 29 - 8 grains of anhydrous sulphuric 

 acid. In another place he states, that he has never ob- 

 tained less than 40 grains of such acid from 100 cubic 

 feet of gas. On the other hand, Dr. Letheby asserts 

 that he rarely obtains more than 20 grains of the acid. 

 But let the quantity be what it may, it is evident that 

 mischief must arise from its presence, and that the at- 

 tention of chemists and gas manufacturers ought to be 

 especially directed to the subject. The quantity of this 

 impurity is small when the gas has been generated at a 

 low heat ; but it is large when the temperature of the 

 retort has approached a full red, or when the charges 

 have been kept in beyond five hours. If, therefore, no 

 means can be adopted for the removal of the impurity 

 after it has once been formed, it is manifest that care 

 should be taken not to produce it 



Tarry matter is held in suspension in coal gas by 

 means of ammonia; consequently, if we pass the gas 

 through a small bottle or tube containing fragments of 

 flint, moistened with dilute sulphuric acid, we shall 

 absorb the ammonia, and arrest the tar. When coal gas, 

 containing much tar in suspension, escapes through the 

 fissures in the street-pipes, it impregnates the soil of the 

 neighbourhood, and gives it a most offensive odour. 



Atmospheric air is sometimes present in coal gas when 

 the exhauster has been doing its work too energetically, 

 or when there have been leakages in the vessels or pipes 

 between the retorts and the exhauster. This is recog- 

 nised by the blueness or thinness ">f the flame, and by 

 its low illuminating power. 



The Analysis of Coal Gas ig rather a complicated 

 undertaking ; though a general idea may be formed of 



