520 



NA TURE 



\_April 2, 



air-space filled with that mixture, especially if the flame has 

 been turned down, as the latter is thereby brought nearer to the 

 reservoir. The sudden cooling of the glass, if it had become 

 heated by the burning of the lamp, may also cause it to crack if 

 it is n it well annealed, and this cracking, or fracture, which may 

 allow the oil to escape, may convey the idea that an explosion 

 has taken place. If the evidently common practice is resorted 

 to of blowing down the chimney with a view to extinguish the 

 lamp, the effects above indicated as producible by a sudden 

 coiling may be combined with the sudden forcing of the flame 

 into the air-space, and an explosion is thus pretty certain to 

 ensue, especia'ly if that air-space is considerable. If the flashing- 

 point of the oil used be below the minimum (73 Abel) fixed by 

 law, and even if it be about that point or a little above it, vapour 

 will be given off comparatively freely if the oil in the lamp be 

 agitated, by carrying the latter or moving it carelessly ; the 

 escape of a mixture of vapour with a little air from the lamp, 

 and its ignition, will take place more readily, but on the other 

 hand it will probably be feebly explosive, because the air will 

 have been expelled in great measure by the generation of 

 petroleum vapour. If the flashing-point of the oil be high, the 

 vapour will be less readily or copiously produced, under the 

 conditions above indicated, but, as a natural consequence, the 

 mixture of vapour and air existing in the lamp may be more 

 violently explosive, because the proportion of the former to the 

 latter is likely to be lower and nearer that demanded for the 

 production of a powerfully explosive mixture. If the quantity 

 of oil in the lamp reservoir be but small, and the air-space con- 

 sequently large, the ignition of an explosive mixture produced 

 within the lamp will obviously exert more violent effects than 

 if there be only space for a small quantity of vapour and air, 

 because of the lamp being comparatively full. If the wick be 

 lowered very much, or if for some other reason the flame becomes 

 very low, so that it is burning beneath the metal work which 

 surrounds and projects over the wick-holder, the lamp will 

 become much heated at those parts, and the tendency to the 

 production of an explosive mixture within the space of the 

 lamp will be increased, while, at the same time, heat will be 

 transmitted to the glass, and it will be correspondingly more 

 susceptible to the effects described as being exerted by its sudden 

 exposure to a draught. Experiments have demonstrated that a 

 lamp containing an oil of high flashing point is more liable to 

 become heated than a comparatively light and volatile oil, in 

 consequence of the much higher temperature developed by the 

 combustion, and of the comparative slowness with which the 

 heavy oil is conveyed by the wick to the flame. It therefore 

 follows that safety in the use of mineral oil lamps is not to be 

 secured simply by the employment of oils of very high flashing 

 point (or low volatility), and that the use of very heavy oils 

 may even give rise to dangers which are small, if not entirely 

 absent, with oils of comparatively low flashing points. The 

 occurrence of such an accident as that in the training-ship 

 Goliath, already referred to, which was brought about by a boy 

 letting fall a lamp which had been alight all night, and which 

 was so hot that he could no longer hold it, appears to be 

 primarily ascribable to the use of an oil of very high flashing 

 point ; and the accident at the Agricultural Hall furnished 

 another illustration of the kind of danger attending the use of 

 such an oil. 



The character of the wick very materially affects not only the 

 burning quality of the lamp, but also its safety. A loosely 

 plaited wick of long staple cotton draws up the oil to the flame 

 regularly and freely, and so long as the oil be not very heavy or 

 of very high flashing point, and therefore difficultly volatisable 

 or convertible into vapour (by so-called destructive distillation), 

 the flame will continue to burn brightly and uniformly, with but 

 little charring effect upon the wick — that is to say, the extremity 

 of the latter will only be darkened and eventually charred to a 

 distance of much less than a quarter of an inch downwards, and 

 it will not be until the partial exhaustion of the oil-supply 

 diminishes the size of the flame and induces the user to raise the 

 wick, that the latter will become more considerably charred. 

 But, if the wick lie very tightly plaited, and made, as is not 

 unfrequently the case, of a short staple cotton of inferior capillary 

 power, the oil will be less copiously drawn up to the flame ; as 

 a consequence, the length of exposed wick will be increased by 

 the user of the lamp, and as the evaporation of the oil will take 

 place more slowly from each portion of the wick which furnishes 

 the flame, the heat to which the cotton is exposed will be greater, 

 and the charring, which is fatal n the proper feeding of the flame 



by destroying the porosity of the end of the wick, will take place 

 more rapidly and to a much greater extent. 



Even with wicks of the higher qualities, considerable differ- 

 ences exist in the rapidity with which the oil is raised to the 

 flame. In Mr. Redwood's experiments, conducted with a speci- 

 men of English wick of good quality and with a very superior 

 American wick, of corresponding dimensions, the quantity of oil 

 siphoned over by the latter in a given time, was from 35 to 47 

 per cent, greater (according to the nature of oil experimented 

 with) than that carried over by the English wick. 



If the wick be at all damp when taken into use, its power of 

 conveying the oil to the flame will be decidedly diminished, the 

 capillaries of the fibre being more or less filled with moisture, 

 and similarly, if the oil accidentally contain any water, the latter, 

 passing into the wick, will interfere with the proper feeding of 

 the flame. As the oil is very thoroughly filtered or strained 

 during its transmission through the body of the wick to the 

 flame, it is obvious that any impurities suspended in the liquid 

 will be deposited within the wick and will gradually diminish its 

 porosity. For this reason the same wick should not be used for 

 a great lengih of time, and it is decidedly objectionable to use a 

 much greater length of wick than is necessary to reach to the 

 bottom of the reservoir, and to continue its use until it has 

 become too greatly shortened by successive trimmings. On the 

 other hand, the wick should always be of sufficient length to be 

 immersed to a considerable distance in the oil. It is evident 

 that the copious supply of oil to the flame will become reduced 

 as the column of liquid which covers the wick in the reservoir 

 becomes reduced in height ; hence the supply of oil in the lamp 

 should never be allowed to get very low, not only because it is 

 undesirable to have a large air-space which may be filled with 

 vapour and air, but also because the burning of the lamp is 

 injuriously affected thereby. 



Some lamps of patterns first constructed in the United States 

 are provided with what may be called a feeding wick, in addition 

 to the wick or wicks which furnish the flame. This wick is 

 generally simply suspended from the lower surface of the burner, 

 and reaches nearly to the bottom of the reservoir, being so 

 placed that it hangs against one fiat side of the regular wick, 

 and thus aids considerably the copious and uniform absorption 

 of oil by the latter. In certain lamps of recent construction the 

 reservoir which contains the main supply of oil is so arranged 

 (upon the principle of the old study- or Queen's oil-lamp) that 

 it regularly maintains at a uniform level the supply of oil, which 

 surrounds the wick in a small central reservoir or cylinder, sepa- 

 rated from the main reservoir (excepting as regards a small 

 channel of communication) by an air-space, which presents the 

 additional advantage of preventing the transmission of heat to 

 the oil vessel. This kind of lamp is constructed entirely of 

 metal ; thi, is the case now with a very large proportion of the 

 lamps in use, and unquestionably adds greatly to the safety of 

 lamps, which, if constructed of glass or porcelain, are always 

 liable to accidental fracture, quite apart from the question of 

 possible explosion. 



It has been proved experimentally that if the reservoir of a 

 burning lamp be warmed, so as to favour the emission of vapour 

 into the space above the oil, and a small opening in the top of 

 the reservoir be then uncovered, air will be drawn into the latter 

 and form an explosive mixture with the vapour, which, escaping 

 from the lamp clo e to the wick-holder, will be fired, and pro- 

 duce an explosion in the lamp. It is an interesting illustration 

 of the very imperfect appreciation, by some lamp-designers, of 

 the conditions which, in the construction of a lamp, secure safety 

 or determine danger, that the reservoirs of some petroleum- 

 lamps are actually furnished with an opening in the upper 

 surface, which is closed with a more or less badly-fitting metal 

 cap, and is intended to be used for filling the lamp with oil. 

 Independently of the great element of danger which this fitting 

 presents, in consequence of the obvious temptation to the users 

 to replenish the reservoir while the lamp is actually burning, it 

 is very likely sooner or later to be the means of admitting to the 

 reservoir, in the manner above indicated, the supply of air 

 necessary to determine the explosion of vapour therein existing. 

 Another source of danger introduced in the construction of 

 lamps which should be sufficiently obvious, and to which refer- 

 ence was made when first discussing the causes of lamp explo- 

 sions, consists in the provision in many lamps, of openings of 

 considerable size close to the burner, apparently with the object 

 of affording a passage for the air or vapour in the reservoir, which 

 may expand as the lamp becomes somewhat warm. Other 



