ANALYSIS OF COAI, GAS.] 



UNDULATORY FORCES. LIGHT. 



121 



of discovering the quality of coal gas. The reactions of 

 bromine on gas are exactly the same as those of chlorine ; 

 but it has an advantage over the latter, in the circum- 

 stance that it is much more manageable, that it is more 

 likely to be pure, and that the admission of light does 

 not affect the results. In manipulating with this liquid 

 we fill a graduated vessel, called a Cooper's tube, with 

 gas, and then pour into the shorter leg of the instrument 

 a small quantity of a saturated solution of bromine in 

 water, taking care to use enough to give the gas an 

 orange-red colour. After the mixture has been shaken, 

 the tube is allowed to stand for about ten minutes, and 

 then the excess of bromine is to be absorbed by means of 

 potash ; after which, the amount of absorption is noted. 

 As in the last case, this will range from three to twenty 

 per cent. , according to the quality of the gas. 



4th. Tlw. Sulphuric Acid Test. Professor Faraday long 

 since observed, tliat when concentrated sulphuric acid 

 was brought into contact with the condensible hydro- 

 carbons, it speedily absorbed them. Relying upon this 

 fact, Professor Bunsen has recommended that fuming 

 or anhydrous acid should be employed for the purpose 

 of ascertaining how much of these agents is present in 

 coal gas. Messrs. Leigh and Frankland have spoken 

 well of the results obtained in this manner ; but however 

 successful the process may be in their hands, it is open 

 to many fallacies, and cannot, therefore, be recommended 

 to the unskilful operator. The mode of experimenting 

 is this : The gas is to be collected in a graduated tube 

 over mercury ; and then a piece of coke or pumice-stone, 

 fastened to a platinum wire and moistened, or rather 

 saturated, with the acid, is to be passed up into the 

 gas ; after remaining in contact with it for ten minutes 

 or a quarter of an hour, the coke is to be withdrawn ; 

 and, as a small quantity of sulphurous acid will have 

 been formed by the action of the coke on the mercury, 

 the gas is to be washed with a little potash, and then 

 the amount of absorption noted. Sulphuric acid does 

 not, however, attack all the hydro-carbons; for it is 

 found that chlorine or bromine will effect a further con- 

 densation after the action of the acid. This, with other 

 circumstances, renders the process objectionable. 



5th. The Expiation Test. Dr. Henry noticed that there 

 was a direct relation between the value of a gas for 

 illuminating purposes, and tho quantity of oxygen 

 required to burn it, or of carbonic acid produced thereby. 

 In fart, as the illuminating power of any gas is dependent 

 on the quantity of carbon contained in a given bulk of 

 it, it follows that tho products of its combustion must 

 furnish a sure indication of its value. This will be 

 manifest from the following table : 



OnTo.meof ^M""* *$%* 



Marsh gas (CH*) .... 2 volumes . 1 volume. 

 Olenantgas(C'H') ... 3 . . 2 ,, 

 Siiper-olenantga8(CH) . 4'5 . .3 

 Faraday's gas (CH 9 ) . . 6 . . 4 

 Bicarburettedhyd. (O'H 3 ) . 7-5 . .6 

 So that if we mix a known quantity of any gas with 

 about three times its bulk of oxygen, and explode them 

 in an eudiometer by means of electricity, or make them 

 combine by the aid of spongy platinum, as Dr. Henry 

 suggested, the amount of oxygen consumed, and of 

 carbonic acid produced, will serve as indications of the 

 quality of the gas. Dr. Henry found that the best 

 description of coal gas requires two-and-a-quarter times 

 its bulk of oxygen for combustion, and gives one-and-a- 

 quarter of carbonic acid ; while the worst gas of his time 

 took only eight-tenths of its bulk of oxygen, and gave 

 but time-tenths of carbonic acid. The amount of car- 

 bonic acid produced is to be determined in the usual 

 way by means of a solution of caustic potass. 



Cth. Tht. Nprcific Gravity Test. This U founded on the 

 fact that the rich hydro-carbons are much heavier than 

 the poor ones ; for example, if a given bulk of marsh gas, 

 or li',;lit carbureted hydrogen, weighs 10 grains, the 

 same bulk of ol.-lUnt g.w will weigh 17i grains; and, 

 in the case of the other hydro-carbons, the increase in 

 weight is still greater. A knowledge of this fact will 

 L I. 



enable us to ascertain the value of any description of 

 gas. We take a glass globe, or flask, fitted air-tight to 

 a stopcock, and exhaust it with great care, by means of 

 an air-pump; then let in pure and dry hydrogen, and 

 again exhaust. Do this a third or even a fourth time, 

 so as to get the flask as empty of air as possible ; then 

 weigh in a delicate balance, and note the amount : pure 

 and dry atmospheric air is now to be admitted, and the 

 flask is to be weighed again. In this manner we ascertain 

 how much of pure dry air, at a temperature of 60 Fah. 

 and a pressure of 30 inches of the barometer, it contains. 

 When we wish to take the specific gravity of any gas, 

 the globe is to be exhausted as before, then filled with 

 the gas, and weighed; corrections are to be made for 

 any abnormal difference of temperature or pressure ; and 

 then we say, as the weight of the vesselful of air is to 1, 

 so is the weight of the gas to its specific gravity. In 

 practice it will be found convenient to have a globe with 

 two stopcocks, one opposite the other ; so that after the 

 first exhaustion and weighing, the globe can be easily 

 filled with gas without the aid of an air-pump, by simply 

 allowing the gas to pass through it for about a quarter 

 of an hour. Mr. Wright has constructed an apparatus 

 which still further simplifies this calculation. It consists 

 of an oiled silk balloon that holds 1,000 cubic inches of 

 gas ; and, as coal gas is lighter than air, he determines 

 its specific gravity by ascertaining the number of grains 

 which the balloon will carry up. A book that accom- 

 panies the apparatus contains instructions for the 

 management of the experiment. The specific gravity of 

 coal gas ranges between 390 and 750. The former is 

 about the weight of the worst gas from Garoticld coals, 

 and the latter of the best from Boghead cannel ; a good 

 average of specific gravity is 450. In conducting experi- 

 ments of this kind, it must be ascertained that the gas 

 does not contain carbonic acid, carbonic oxide, or atmo- 

 spheric air ; for if it does, the specific gravity of the gas 

 is sure to be high, notwithstanding that the illuminating 

 power may be very low. 



Before we leave this part of the subject, it may be 

 remarked, that if the specific gravity of a gas be taken 

 before its condensation by bromine, and then again 

 afterwards, the difference in weight will afford a moans 

 of ascertaining the specific gravity of tho condensed 

 portion ; and if this be multiplied by the amount of 

 condensation, we obtain a number that represents very 

 nearly the illuminating power of the gas in sperm candles, 

 as it is usually expressed. For example : a gas of specific 

 gravity 441, has a condensation of 5 per cent. , and tho 

 residual gas has a specific gravity of 328. Now since 

 100 cubic inches of the former weigh 13 'G3 grains, and 

 95 cubic inches of the latter 9 '5 grains, the 5 cubic inches 

 of the condensed portion must have weighed 4 "13 grains, 

 anil it must have had a specific gravity of 2 '7. This, 

 multiplied by 5, the amount of condensation, gives 13 '5 

 as the illuminating power of gas. Experiment showed 

 it to be 14. 



?th.The Durability Test. Or. Fyfe, who is an autho- 

 rity in matters of this description, is accustomed to 

 estimate the value of a gas, not only by noting its amount 

 of condensation with chlorine, but also by observing tho 

 time that it takes to bum a given bulk of it from a jet of 

 a given size, with a flame of a given height. The jet 

 which he employs has an aperture of the ^rd of an 

 inch in diameter, and the flame is four inches in height. 

 The first of these tests he calls the quality test, and the 

 latter the durability. "I consider," he says, "both of 

 these circumstances absolutely necessary; for, though 

 some have insisted only on the one, and some on the 

 other, yet, unless both be taken into account, we do not 

 arrive at the true value of the gases, and, consequently, 

 cannot compare one with another for the purpose of 

 illumination." Ho thinks it possible that two gases may 

 afford, by combustion, the same amount of light for the 

 same height of flame, but that one may burn away half 

 as fast again as the other; and, consequently, if no 

 regard is paid to this circumstance, there will be a 

 false estimate of their relative values. His mode of 

 amalgamating these two powers is to multiply the 



