WIRE-GAUZE SAFETY. LAMP. 



explofive point ; and in extinguifhing a flame by blowing 

 upon it, the effeft is probably produced principally by this 

 caufe, affifted by a dilution of the explofive mixture. 



If a piece of wire-gauze fieve is held over a flame of a 

 lamp, or of coal-gas, it prevents the flame from paffing it, 

 and the phenomenon is precifely fimilar to that exhibited 

 by the wire-gauze cylinders: the air paffing through is 

 found very hot, for it will convert paper into charcoal ; 

 and it is an explofive mixture, for it will inflame if a lighted 

 taper is prefented to it ; but it is cooled below the ex- 

 plofive point, by paffing through wires even red-hot, and 

 by being mixed with a confiderable quantity of air com- 

 paratively cold. The real temperature of vifible flame is, 

 perhaps, as high as any we are acquainted with. Mr. 

 Tennant was in the habit of fliewing an experiment which 

 demonftrates the intenfity of its heat. He ufed to fufe a 

 fmall filament of platinum in the flame of a common candle ; 

 and it is proved by many fafts, that a ftream of air may 

 be made to render a metallic body quite hot, yet not be 

 itfelf luminous. 



A confiderable mafs of heated metal is required to in- 

 flame even coal-gas, or the contaft of the fame mixture 

 with an extenfive heated furface. An iron-wire of yVth of 

 an inch, and eight inches long, red-hot, when held perpen- 

 dicularly in a ftream of coal-gas, did not inflame it, nor did 

 a (hort wire of one-fixth of an inch produce the effeft held 

 horizontally ; but wire of the fame fize, when fix inches of 

 it were red-hot, and when it was held perpendicularly in a 

 bottle containing an explofive mixture, fo that heat was 

 fucceffively communicated to portions of the gas, produced 

 its explofion. 



A certain degree of mechanical force, which rapidly 

 throws portions of cold explofive mixture upon flame, pre- 

 vents explofions at the point of conta£k. Thus, on preffing 

 an explofive mixture of coal-gas from a fyringe, or a gum 

 elaftic bottle, it burns only at fome diftance from the aper- 

 ture from which it is difengaged. 



Taking all thefe circumftances into account, there ap- 

 pears no difficulty in explaining the combuftion of explofive 

 mixtures within, and not without the cylinders : for a cur- 

 rent is efl.abli(hed from below upwards, and the hotteft part 

 of the cyhnder is where the refults of combuftion, the 

 water, carbonic acid, or azote, which are not inflammable, 

 pals out. The gas which enters is not fufficiently heated 

 on the outfide of the wire to be exploded ; and as the 

 gafes are no where confined, there can be no mechanical 

 force preffing currents of flame towards the fame point. 



Two papers by fir H. Davy, connefted with this fub- 

 jeft, virere afterwards publifhed in the Philofophical Tranf- 

 aftions for 1817, entitled " Some Refearches on Flame." 

 In thefe papers, a number of new and extremely interefting 

 experiments on the properties of flame are detailed. The 

 praftical application of the refults to fafety-lamps we ftiall 

 briefly ilate, as they explain more clearly the principle on 

 which their fafety depends, and the circumftances eflentially 

 rtquifite to their proper conftruftion. Sir H. Davy com- 

 mences the paper by informing us, that the intenfity of the 

 light of flames depends principally upon the produftion and 

 ignition of folid matter in combuftion ; and that the heat 

 and light in this procefs are in a great meafure independent 

 phenomena : and he afterwards defines flame to be gafeous 

 matter, heated fo highly as to be luminous, and that to a 

 degree of temperature beyond the white heat of folid bodies, 

 as is fliewn by the experiment ; that air not luminous will 

 communicate this degree of heat ; for if we hold a fine 

 platina wire one-twentieth of an inch from the exterior of 

 the middle flame of a fpirit-lamp, and conceal the flame by 



9 



an opaque body, the wire will become of a white heat in a 

 fpace where there is no vifible light. 



When an attempt is made to pafs flame through a very • 

 fine mcfli of wire-gauze at tlie common temperature, the 

 gauze cools each portion of the elaftic matter that pafles 

 through it, fo as to reduce its temperature below that 

 degree at which it is luminous ; and the diminution of tem- 

 perature muft be proportional to the fmallnefs of the mefti 

 and the mafs of the metal. The power of a metallic or 

 other tifl'ue, to prevent explofion, will depend upon the heat 

 required to produce the combuftion, as compared with that 

 acquired by the tiflue ; and the flame of the moft inflam- 

 mable fubftances, and of thofe that produce moft heat in 

 combuftion, will pafs through a metallic tiflue that will in. 

 tercept the flame of lefs inflammable fubftances, or thofe 

 that produce little heat in combuftion. Or the tiflue being 

 the fame, and impermeable to all flames at common tem- 

 peratures ; yet when heated it will become permeable to 

 each different kind of flame at diff'erent temperatures : 

 thofe which produce moft heat will moft readily pafs 

 through it. A tiflue of one hundred apertures to the 

 fquare inch, made of wire of one-fixtieth part of an inch, 

 will, at common temperatures, intercept the flame of a 

 fpirit-lamp, but not that of hydrogen ; and when ftrongly 

 heated will no longer arreft the flame of the fpirit-lamp. 



The ratio of combuftibihty of the different gafes is to 

 a certain extent proportionate to the mafles of heated matter 

 required to inflame them. Thus, an iron-wire of one-for- 

 tieth of an inch heated cherry-red will not inflame olefiant 

 gas, but will inflame hydrogen gas : and a wire of one- 

 eighth of an inch heated to the fame degree will inflame 

 olefiant gas ; but a wire of one-five-hundredth part of an 

 inch muft be heated to whitenefs to inflame hydrogen. 



Thefe circumftances will explain why a mefli of much 

 finer wire is required to prevent the explofion from hydro- 

 gen and oxygen from paffing ; and why fo coarfe a texture 

 of wire is fufficient to prevent the explofion of the fire-damp^ 

 the leaft combuftible of the known inflammable gafes. 



The following experiments afford a fatisfaftory and Am- 

 ple explanation of the caufe of the ftoppage of flame by the 

 wire-gauze lamp. Let the fmalleft poffible flame be made 

 by a fingle thread of cotton immerfed in oil, and burning 

 immediately on the furface of the oil ; it will be found to 

 be about one-thirtieth of an inch in diameter. Let a fine 

 iron-wire one-hundred-and-eightieth part of an inch be 

 made into a circle of one-tenth of an inch in diameter, and 

 brought over the flame. Though at fuch a diftance it 

 will inftantly extinguifh the flame if it be cold ; but if it 

 be held above the flame, fo as to be flightly heated, the 

 flame may be paffed through it without being extinguifhed. 

 The effeA depends entirely on the power of the metal to 

 abftraft the heat of the flame. This is (hewn by bringing 

 a glafs capillary ring of the fame diameter and fize over the 

 flame : this being a much worfe conduftor of heat will not 

 extinguifli it even when cold. If its fize, however, be made 

 greater, and its circumference fmaller, it will aft like the 

 metalhc wire, and require to be heated to prevent its ex- 

 tinguifliing the flame. 



Suppofe a flame to be divided by the wire-gauze into 

 fmaller flames, each flame muft be extinguifhed in paffing its 

 aperture, till that aperture has attained a temperature fuf- 

 ficient to produce the permanent combuftion of the explo- 

 five mixture. Where rapid currents of explofive mixtures 

 are made to aft upon wire-gauze, it is of courfe much more 

 rapidly heated, and therefore the fame melh which arrefts 

 the flames of explofive mixtures at reft will fuffifr them to 

 pafs when in rapid motion ; but by increafing the cooling 



furface. 



