GAS-LIGHTING 



99 



M.I <>t a roil, t here is a candle ; at theothd .-nd 1 1 !: 

 is a gas-burner, ll "d a nieter t<> measure tin- supply 

 Inn in-i and the candle arc dm- ..i a 

 li\.'.l di-tance I'niiii mi- another. Between them 

 tin-re moves, sliding on a graduated har, a disc of 

 prepared paper; this is slipped up and down until 

 its two -.ides i or rather tho linages of its respective 

 sides in two little mirrors which travel with it) 

 appear "i[iially illnminatetl. This is ascertained by 

 the disappearance of a grease-spot or rather, in the 

 newer models, by the vanishing of all difference in 

 appearance between an ungreased centre and the 



d rim of the. disc. In t.he Let-son disc there 

 an- three thicknesses of paper, of which the middle 

 MM- is much the thickest, hut is perforated at its 

 centre; and this form of disc works better in the 

 comparison of light of somewhat different colours. 

 Tip- Luinniel-Brodluin photometer is an idealised 



n plmtt -ter, in which the place of the 



paper with its central grease-spot' is taken by a 

 purely optical arrangement of totally reflecting or 

 partially reflecting prisms. The bar may be 

 graduated in one of two ways : ( 1 ) Equal intervals, 

 -i drii die' respective distances between the disc 

 and tin- gas-burner and candle may be measured; 

 then the ratio between the intensities is the inverse 

 ratio of the squares of the respective distances; 

 for example, that the respective distances of 

 the candle and gas-burner are 20 inches and 80 

 inches; then the gas- burner's intensity: the candle's 

 : = (A) 2 = (A) 2 > = : 16 : 1. (2) The bar may be 

 so graduated as to anticipate and save this calcula- 

 tion, on which principle the mid-point of the bar 

 would be marked 1, and a point one-lifth of the bars 

 length from either end would be marked 16 ; the 

 figures si i marked show directly the ratios sought 

 for. The pressure of gas must be measured by a 

 gauge and regulated by a governor; the consumpt 

 of the candle must be weighed ; the gas used must 

 be exactly 5 cubic feet per hour ; the burner is a 

 standard Sugg's London Argand No. 1 for common 

 coal-gas, a standard Steatite Batswing burner for 

 cannel gas ; the candles are sperm candles, of six 

 to the pound, each burning 120 grains per hour; 

 and the quantity of gas used is to be corrected for 

 temperature and barometric pressure. The candle 

 is a very unsatisfactory unit of light; it varies as 

 much as 6 per cent., and its colour is not the same 

 as that of the gas-flame. Other standards have 

 been proposed ; of these the principal are the (ier- 

 inan standard candle 1'065 English sperm candle; 

 the French Carcel lamp (648 grains colza-oil per 

 hoar )=- 10*441 English sperm candles; Mr Vernon 

 Harcoiirt's pentane lamp, air + pen tane- vapour, 

 \ cubic foot per hour, nearly equal to the English 

 standard candle; Mr Methven s and Mr Fiddes's 

 standard, in principle a given area of the bright 

 part of gas-flame, this being, singularly, an almost 

 uniform standard of illumination, not with any 

 kind of illuminating gas, as was at first believed, 

 but quite accurately so with pentane-vapour ; 

 M"t'ner-Alteneck's amyl-acetate lamp, with the 

 flame turned up to a height of 1 '6 inch, equal to 

 i>*77 English standard candle; and the Dutch 

 ethei ben/.ol standard (1893) = 1*48 English stand- 

 ard candle. Other photometers (Elster's, with 

 movable standard light, &c.) have been proposed. 

 Lowe and Sugg's jet-photometer depends on this, 

 that assuming the height of the flame to be kept 

 constant, the lighting-power of a jet is inversely 

 proportional to the consumpt or otherwise, that 

 the consumpt being kept constant, the height of 

 the jet-flame is direct ly proportional to the lighting- 

 power. In (Jiroud's jet-photometer the height of 

 the flame at constant pressure is taken as the 

 measure of illuminating power; when the flame 

 is about 6 inches high, a variation of about | inch 

 corresponds to a variation of one-candle power, 



when the whole lighting-power in from 10 to 14 

 candles per cubic feet. A Committee appointed 

 by the Board of Trade in 1891, reported in inr, that 

 a flame of some kind must be used as the standard ; 

 that the -perm candle is unsatisfactory ; that Mr 

 Vernon Harcourt's |>entane-vapour and air-name in 

 constant in brightness and easily reproducible 

 when used an directed, ami that it in accurately 

 equal to an average standard candle ; and that tln- 

 should be made the ba-i- of comparison, and called 

 a candle ; that for actual work with gas-flames it in 

 better to compare these with more powerful sources 

 of light than a candle, and that for this purpose a 

 Dibdin 10-candle standard (an air and pentane- 

 vapour Argand flame with a Methven screen) 

 should be used, with the Methven screen fixed so 

 as to expose 2*lf> indie- of the flame. They also 

 recommend that instead of burning gas at f> cubic 

 feet per hour, the gas should be burned at just such 

 a rate as will give the required number of candles, 

 and that the illuminating power becalculated back, 

 and be stated as so many candles per 5 cubic feet. 

 Photometrically the lime-purified gas of the south 

 of England is greatly inferior to the iron-oxide 

 purified gas of the north of England, and yet 

 an impression of greater brightness is often ex- 

 perienced, for the name is white instead of yellow. 

 Gas-work apparatus falls under thirteen heads. 

 The Retort-house contains the benches or sets of 

 retorts in which the coal is distilled. The retorts 

 were formerly small, and of cast-iron only ; they are 

 now generally larger and of fireclay ; though the use 

 of iron is again becoming familiar in cases where the 

 last retort or two of a set are more easily heated if 

 made of iron than when made of fireclay. Retorts 

 are made round, oval, and D-shaped; the first of 

 these is the strongest and most durable ; the oval 

 and the D-shaped are better carbonisers. Clay 

 retorts are usually 2 to 3 inches thick, oval, with 

 diameters 15 and 21 inches inside, and 9 feet 4 inches 

 long; but 'through' retorts are often used, corre- 

 sponding to two ordinary retorts joined together so 

 as to form one tube, some 20 feet long, with a 

 mouthpiece at each end a form which is more 

 readily manipulated and more readily kept clear of 

 coke-deposit. Even these diameters are somewhat 

 too great, and the result is better with narrower 

 retorts ; and in small works smaller and shorter 

 retorts are generally used. Of late years through- 

 retorts, inclined at an angle of some 30, have come 

 greatly into use, especially in conjunction with 

 mechanical appliances for charging and discharg- 

 ing the retorts: the coal slides down the retort 

 from a hopper and is promptly spread out into a 

 layer of uniform thickness, and the spent coke is 

 easily drawn from the retort in a stream. To an 

 increasing extent the coal is first raised to a height 

 and then lowered in the successive operations to 

 successively lower levels, so that manual labour is 

 economised. The Dinsmore retorts are Z-shaj>ed, 

 and the tarry products are subjected to continued 

 distillation in the upper bends. Mr Isaac Can's 

 modification of this process has been very successful 

 in his own hands at Widnes ; but it seems that the 

 process has not been successful elsewhere. Five or 

 seven retorts, and sometimes ten or more are built 

 into each oven ; and all the retorts of one oven are 

 heated from the same source. This may be a coke 

 furnace, in which case some 3i cwt. of coke are 

 used in distilling each ton of coal i.e. about 25 per 

 cent, of tin- coke made a proportion which sinks 

 in large works to 20 or 18 per cent. or tar may be 

 used as fuel, either dropped on hot plates or blown 

 in by air or by steam as spray : or generator 

 furnaces may be employed in which the fuel is 

 first half-burned (CO being formed), and the hot 

 furnace gases thus produced are burned under the 

 retorts ; or regenerative furnaces, in which the 



