86 



NA TURE 



[November 23, 1893 



of the area is built upon, the remainder being streets and open 

 spaces. 



The Paris Geographical Society has awarded the grand prize 

 for geographical research to M. Maistre, for his great journey 

 from the Congo to the Shari. 



FLAME} 

 'T'flE subject on which I have the honour to address you this 

 evening is, I am aware, one of the most hackneyed among 

 the topics that have served for popular scientific lectures. I can 

 only hope that it has not quite lost its charm. The chemist is 

 often twitted with having to deal with mere dead soulless things, 

 which at the best only set themselves into angular and unpalpita- 

 ting crystals. There may be a certain amount of truth in this, but 

 in fiamt'S we surely have phenomena of some liveliness. Our 

 flame must be fed ; it has its anatomy and varied symmetry ; 

 it is vigorous, mobile, and fleeting. I do not wish to make 

 extravagant claims, but I do think that one may be excused 

 for feeling almost as much interest in the study of flame 

 as, for example, in the contemplation of the somewhat torpid 

 evolutions ot an amoeba or the circulation of water in a 

 sponge. To our guileless ancestors, at any rate, flame was a 

 phenomenon of the rarest mystery ; unable as they were to 

 discriminate between the material and the immaterial, unable 

 to track the solid or liquid fuel to its gaseous end, this radiant 

 nothingness called flame became to them one of the primary in- 

 scrutable, irresolvable things of Nature — an all-devouring 

 element, often of peculiarly divine significance. 



The essential nature of flame appears to have been discovered 

 at the beginning of the seventeenth century by the Belgian, Van 

 Helmont. This remarkable man is well known to chemists as 

 one of the acutest and least superstitious of the whole band of 

 alchemists. He was somewhat speculative in the domain of 

 physiology, but in chemistry Van Helmont made discoveries of 

 fundamental importance From our immediate point of view, 

 one of the most important things he did was to sweep away the 

 mystery that had so long attached to the gaseous state of matter. 

 In so far as he distinijuished between different gases obtained from 

 different sources, he may be said to have been the first to bring 

 aeriform matter within the range of substantial things that might 

 be submitted to expeiimental investigation. It was in conse- 

 quence of this that he was led to the di>covery of the nature of 

 flame. I will quote the important passage from his writings. 



"But the flame itself, which is nothing but a kindled smoke, 

 being enclosed in a glass in the very instant perisheth into 

 nothing. 



"The flame indeed is the kindled and enlightened smoke of a 

 fat exhalation ; be it so ; but as the flame is such and true fire it 

 is not another matter, being kindled and not yet kindled, 

 neither doth it difler from itself; but that light being united in 

 its centre, hath come upon a fat exhalation which is the same as 

 to be inflamed. 



" Let two candles be placed which have first burned awhile, 

 one indeed being lower than the other by a span ; but let the 

 other be of a little crooked situation ; then let the flame of the 

 lower candle be blown out ; whose smoke, as soon as it shall 

 touch the flame of the upper candle, behold the ascending smoke 

 is enlightened, is burnt up into a smoky or sooty gas, and the 

 flame descendeih by the smoke even unto the smoking candle. 

 Surely there is there, the producing of a new being, to wit, of 

 fire, of a flame, or of a connexed light ; yet there is not a pro- 

 creation of some new matter or substance. 



" For the fire is a positive artificial death but not a privative 

 one, being more than an accident and less than a substance." 



We can best understand the meaning of this somewhat ora- 

 cular statement by repeating Van Helmont's experiment. We 

 take a bundle of lighted tapers so as to get a large flame, we 

 hold over "in a little crooked situation" another lighted 

 taper, and now blow out the lower flame. We note the ascend- 

 ing column of smoke, and observe that when it touches the upper 

 flame it ignites, and the flame descends several inches through 

 the smoke to the bundle of tapers. Flame therefore, says 

 Van Helmont, is burning smoke ; it is not a new substance nor a 

 mere chance occurrence, but the incandescence of a vapour or 

 smoke that already existed. 



Van Helmont only recognised in a vague way the important 

 part played by the atmosphere in the phenomenon. This was 



1 An evenins discourse to the British Association at tlie Nottingham 

 meeling, September 15, 1893, by Prof. Arthur Smithells. 



NO. 1256, VOL. 49] 



realised much more perfectly soon afterwards by Hooke, whcv 

 speaks of "that transient shining body which we call flame '\as 

 "nothing but a mixture of air and volatile sulphureous parts^of 

 dissoluble or combustible bodies which are acting upon each 

 other whilst they ascend," an action so violent, he says, " that 

 it imparts such a motion or pulse to the diaphanous parts of the 

 air" as was requisite to produce light. 



Without entering further into early historical details I may 

 say that it was only towards the end of last century that the ■ 

 essential chemistry of the phenomenon was fully expounded by 

 the great Lavoisier. He showed that, as Hooke had surmised, 

 flame is the region in which combination attended by the evolu- 

 tion of light takes place between the components of a gaseous 

 substance and the oxygen of the air. 



The next step in the history of our knowledge of flame brings 

 us to the memorable researches of Humphry Davy, whose name 

 more than that of any other man is associated with this sub- 

 ject. Of Davy's work I shall have moe.to say presently ; but 

 at this moment I will only make one allusion to it, an allusion 

 which will provide us with a proper starting-point this evening. 

 It is interesting to note that Davy's discoveries concerning 

 flame were the consequence and not the cause of the discovery 

 of the miners' safety-lamp. In this case practical application 

 preceded purely scientific discovery. 



I need not describe the safety-lamp to you in Nottingham, 

 v;here it has recently received such important improvements at 

 the hands of Prof. Clowes. When the lamp is placed in an 

 exjilosive mixture, you know what happens — the explosive 

 mixture burns with a quiet flame within the lamp, but the flame 

 cannot pass through the wire gauze to ignite then ixture outside the 

 lamp. I can demonstrate this by means of this large gas-burner, 

 which is primarily a Bunsen burner, that is, a burner which by 

 means of holes at the base of the tube draws in sufficient air to 

 enable the gas to burn with a practically non-luminous flame. 

 Il I turn on the gas and apply a light to the top of the burner, you 

 observe that I get a flash and a small explosion within the 

 tube, but no continuous flame. The fact is that the mixture of 

 gas and air within the tube is highly explosive. Placing a 

 gauze cap over the burner and applying a light, I now get a 

 steady flame. The explosive mixture made in the tube passes 

 through the gauze and is inflamed, or, if you like, exploded ; 

 but the explosion cannot pass through the gauze, because the 

 metallic wires withdraw the heat so rapidly that the mixture 

 below it never reaches the temperature of ignition. Above 

 the gauze we have the continuous flame. 



" These results are best explained," says Davy, " by con- 

 sidering the nature of the fl ime of combustible bodies, which in 

 all cases must be considered as the combination of an explosive 

 mixture of inflammable gas or vapour and air ; for it cannot 

 be regarded as a mere combustion at the surface of contact of 

 the inflammable matter." 



Davy, then, regarded flame as being essentially the same as 

 explosion ; it was, in fact, a kind of tethered explosion. 



Since Davy's time we have learned much about the nature of 

 gaseous explosions, and we now know that such explosions, 

 when fully developed, proceed with enormous rapidity and are 

 of great violence, incapable of arrest by such simple means as 

 we have just used. Still there is not much to correct in what I 

 have said. I think I cannot do better than show you the transi- 

 tion of flame into explosion by an experiment which was first 

 shown by Prof Dixon in the lecture w hich he gave at the 

 meeting of the British Association in Manchester in 1887. 

 I The apparatus before you consists simply of a Bunsen burner 

 surmounted by a long glass tube. If I turn the gas on and 

 light it I obtain at the top of the glass tube a steady flame. 

 The mixture ascending the tube can scarcely be called ex- 

 plosive at present, but if I aber the proportions of gas and air 

 suitably it becomes distinctly explosive. Observe what happens 

 when this is the case. The flame can no longer keep at the top 

 of the glass tube ; it passes within it, and descends with uniform 

 veli)city till at a certain point it flickers and then shoots down 

 almost instantaneously to the bottom. This sequence of events 

 is exhibited in all cases when flame develops into explosion, 

 We are concerned only with the first phase, viz. that of com- 

 paratively slow inflammation and a flame, we may say, is a 

 gaseous explosion brought to anchor in the period of incubation. 

 There is one other point connected with explosion that we 

 must note on account ol its important bearing on the chemistry 

 ol flame. When we are dealing with explosive mixtures of 

 gas and air, we find practically that the composition of the 



