December 29, 1910] 



NATURE 



291 



During this period of smoke production no soot is 

 formed, and the physical properties of the cloud of vapour 

 are an interesting study, as it explains one of the secrets 

 of the lasting power of smoke and the way in which it 

 acts. A most beautiful and instructive experiment is one 

 ievised by Mr. F. Hovenden, which shows to perfection 

 rhe structure of smoke as it escapes from a burning object. 

 A puff of smoke blown through a small glass cell 

 illuminated from below by an oxyhydrogen or arc light, 

 and examined under a low-power microscope, reveals the 

 fact that it consists of excessively minute vesicles which 

 ire in a marvellous condition of motion, and which, owing 

 o the gas within them being lighter than air, remain 

 iloating in the stream of air or gas until impact with a 

 olid surface causes a bursting of the little liquid envelope, 

 rorming a microscopic drop of tar on the solid against 

 vhich it has struck, and liberating the contained gases. 



The wonderful movement of these vesicles is the most 

 beautiful realisation that I know of our conception of 

 nolecular motion, and the marvellous way in which they 

 ceep up a continuous bombardment would be a perfect 

 ;cture illustration of kinetic energy if only it could be 

 n rejected on the screen. 



Given proper conditions, most condensing vapours seem 

 assume this form, and the small vesicular masses seem 

 o retain the molecular activity of the particles that build 

 hem up. and there is little doubt that in fog or cloud it 

 i^ this formation that gives the floating power, as the 

 vater vapour contained by the vesicle is only a little 

 nore than half the weight of air, and also explains the 

 formation of rain by gun-fire and the dispersion of fc^ 

 y electrical discharges, the bursting of the vesicle in each 

 :ase leading to precipitation. 



The tar vapour which escapes during the distillation 



:f coal, either in the gas-maker's retorts or upon an open 



nre, consists of a mass of vesicles of this character, and 



his period is the one in which the most serious waste 



akes place, as not only is the greatest amount of heat 



leing rendered latent by the distillation from the coal of 



:'iese products, but they also escape unburnt up the 



himney. .After a while sufficient heat finds its way 



hrough the coal to the top of the fuel to ignite some of 



he escaping vapours, and the bright luminous flame then 



nakes its appearance above the fire. This flame radiates 



': considerable amount of heat owing to the incandescent 



oarticles within it, and the waste of heat diminishes ; but 



;: will be seen that a large amount of vapour is still 



-caping unburnt, owing to the dilution of the hydro- 



arbon gases by steam and the cold air sucked in over 



"he surface of the fire, which lowers their temperature 



elow the point of ignition. 



The appearance of the flame itself is worthy of notice, 

 as the chemical changes taking place within ' it make it 

 red and lurid towards the top, and the particles of oily 

 carbon which form the soot escape from it. 



Flame is caused by the combustion of gaseous matter, 

 and when the air supporting the combustion is supplied 

 xternally to the combustible gas, the resulting flame is 

 always holk)w, consisting of at least two parts, an outer 

 zone in which combustion is taking place, and an inner 

 zone in which, there being no oxygen to carry- on the 

 combustion, no such action can take place. The' ordinary 

 luminous flame, such as is employed for illuminating pur- 

 poses, is divided into four parts, but for present purposes 

 our fire flame may be looked upon as consisting of only 

 ^■hree, the inner zone being an area in which no combus- 

 on is taking place, but in which the gases are subjected 

 the baking action of the heated envelope that surrounds 

 it, and undergo many decompositions, the most important 

 chemical ^ change being the conversion of any hydro- 

 carbons into acetylene. In the outer zone combustion 

 takes place in contact with air, giving the hottest part 

 of the flame, and as the result carbon dioxide, carbon 

 rnonoxide, and water vap>our are formed ; whilst between 

 the inner and outer zones is a brilliantly luminous sheath 

 givmg the major portion of the cheerful firelight, whilst 

 higher up in the flame, if combustion is not complete, this 

 uminous portion becomes dull red and gives out far less 

 5ht. and above this again smoke begins to appear in 

 onsiderable quantities. These gradations in appearance 

 ye due to the acetylene and kindred bodies formed by 

 ne baking action of the outer zone on the hvdrocarbons 

 NO. 2148, VOL. 85] 



in the gases and vapours passing through the dark inner 

 zone, entering the heated zone of combustion, when the 

 acetj'lene suddenly splits up under the influence of heat 

 into carbon and hydrogen, the latter of which burns and 

 adds to the general heat of the flame, whilst the carbon 

 raised to incandescence partly by the heat generated 

 during its own formation from the endothermic acetylene 

 and partly by heat from the flame, as well as by its own 

 combustion, gives out the light. If the combustion were 

 completed no smoke would be formed, but the diluting 

 influence of the nitrogen and other products from the fire 

 beneath and the cooling influence of the chimney draught 

 so check and hamper the completion of the combustion 

 of the products from the decomposed acetylene that the 

 top of the flame is cooled to a dull red, and the flame 

 is finally extinguished before all the carbon particles can 

 be consumed, this producing the sooty smoke which passes 

 up the chimney. The smoke does not consist merely of 

 the liberated carbon particles, but contains tar vapour, 

 water vapour, products of combustion, and excess of air, 

 together with the residual nitrogen from that portion of 

 the air that has been used in the combustion, as well as 

 particles of ash sucked up by the draught of the chimney. 



In time the fire burns clearly, the amount of flame 

 becoming extremely small, and consisting mainly of carbon 

 monoxide, and practically smokeless combustion is 

 attained. No further pollution of the atmosphere takes 

 place until more coal is fed on to the fire, whilst the 

 incandescent fuel is radiating out the heat given by the 

 combustion of the carbon, and is doing more heating work 

 than at any other period. 



Such details of chemical and physical action as I have 

 attempted to bring before j-ou seem absolutely superfluous 

 to the lay mind, but until they are recognised it is prac- 

 tically impossible to arrive at any true solution of the 

 difliculty. 



Take an iron flask, half fill it with pieces of bitu- 

 minous coal the size of peas, and heat it up to the highest 

 temperature you can obtain with an atmospheric burner, 

 and you will find that, as the heat penetrates the mass 

 of coal, first white and then brown vapours distil from 

 the mouth of the flask. Ignite these brown vapours, and 

 you will see the same phenomena that are shown by the 

 luminous flame above the fire ; stop the flame for a 

 moment by closing the mouth of the flask by a damp 

 plug, and, having extinguished the flame, pass the brown 

 vapours through a condenser, and you find that black 

 liquid tar condenses and a clear, colourless coal gas 

 escapes, which when ignited gives a luminous flame with 

 little or no formation of carbon. Moreover, if, having 

 ascertained this fact, you remove the condenser and re- 

 ignite the mixture of gas and tar vapour, you find it 

 gives a flame which steadily becomes less and less 

 luminous, and finally assumes the character of a yellowish 

 flame incapable of forming smoke, and from which no 

 tar can be condensed. 



This flame gradually dies away, and if the residue in 

 the flask be examined, it is found to be ordinary' gas 

 coke, which when burnt in air gives no smoke or soot, 

 and only such flame as is due to the formation of carbon 

 monoxide by the passage of air through the incandescent 

 carbon, and which, escaping from the mass, meets more 

 air and burns with a small non-luminous flame. 



From the fact I have brought befwe you several points 

 are clear : — 



(i) That the smoke-forming portion of bituminous coal 

 is the hydrocarbons, which on destructive distillation form 

 the tar. 



(2) That the true coal gas contains but little of these, 

 and can easily be burnt with smokeless combustion. 



(3) That the residue left after the destructive distilla- 

 tion of the coal, i.e. coke, burns without the formation 

 of smoke. 



(4) That tar vapour and white smoke escape in the form 

 of minute vesicles, which will float in air until burst by 

 violent contact with some surface, on which they then 

 deposit as tar. 



(5) That what we speak of as smoke consists of a 

 mixture of (a) tar vapour ; (b) water vapour ; (c) tarry 

 carbon particles ; (i) products of combustion other than 

 water vapour ; (e) fine particles of ash. 



Amongst the gaseous products of combustion also are 



