The Mystery of Flame
Flame and Combustion in Gases. By W. A. Bone and D. T. A. Townend. (Longmans. 32s.) Tax phenomenon of flame occurs in connexion with a much wider range of circumstances than is probably contemplated by the average man. We have the quietly burning flame of coal gas, either luminous, as familiar in the old days of gas lighting, or non-luminous, as in the burner of the gas stove, where the gas is mixed with air before being burnt. In a coal or wood fire we have the same sort of thing going on, the gases released from the hot fuel burning with more or less air mingled with them. All explosions are manifestations of flame, either propagated with enormous rapidity in mixtures of combustible gases, as in coal-mine or lighting-gas explosions, or released by the instantaneous burning of a solid explosive. Every working stroke of an internal-combustion engine, such as that of the ordinary motor car, is due to a generation of flame in the cylinder. The economical utilization of fuel for raising steam involves many problems of flame : the smelting of iron has initiated prolonged investigations of the combustion in the hot blast. These are but a few of the practical aspects of the study of those leaping streaks of hot and coloured vapours whose beauty has charmed and mystified man since the first fire.
From the purely scientific point of view flame is a problegi which raises issues involving the most complicated considera- tions of physics and chemistry. The difficulties of the inquiry are not far to seek. It is hard to find out what gases are being formed by chemical combination in an ordinary flame, since the combining bodies may unite and break up in a variety of ways, and if we collect the products and examine them when they are cold we have no guarantee that changes have not taken place as they cooled. No flame is uniform : there are different zones, each of which represents a different process. The part played by the radiations of the flame, and of the electrically charged corpuscles which are found in it, is hard to disentangle. A gas flame is really a region of rapid chemical action which is travelling along a mixture of combustible gases, but is kept in its position at the end of the tube by the upward flow of gases in the pipe. If the gas is mixed with air, and we diminish the rate at which the mixture is issuing, the flame may run down the pipe until it reaches gas unmixed with air, which will, of course, not burn. If we turn to study the travelling of flame through a long tube filled with a stagnant explosive mixture, we find that very queer things take place, for, after travelling a certain distance quietly, at the rate of a few yards or so per sec. ond, the flame accelerates until it is propa- gated with explosive violence at relatively enormous speed, depending upon the nature of the mixture, but always exceed- ing a mile per second. We speak of a detonation wave. All thesc things have to be studied, but the layman can well understand that to find out what is going on in a transforma- tion which occurs at a temperature and p-mssure abnormally high, compared to those which we can maintain steadily, and is all over in a small fraction of a second, is not the easiest thing in the world.
A store of information as to the nature of flame problems is contained in the volume just produced, by Professor Bone and Dr. Townend. It may be said at once, as indicating the com- plexity of modern science, that although the book, entirely devoted to flame, is a large one, many interesting phenomena which have been the subjects of prolonged research, ench'ai the
conduction of electricity in flames and the explosion of solids in closed vessels, are hardly mentioned. The authors deal mainly with the branches of the subject in which Professor Bone, his pupils, and his master, Professor H. B. Dixon, have carried out researches, and it says much for the activity of this school that the scope is as wide as it is.
The book opens with a very good historical sketch of the early work on the subject, and passes on to consider the conditions necessary to start a flame, and the fortunes of the flame once started in a long pipe. The study of the explosion and detonation waves by the help of very rapidly moving photographic plates or films is thoroughly described and illustrated. Ordinary—stationary—flames are considered in detail. A section is devoted to the explosion of gases in closed vessels strong enough to withstand the pressure deve- loped, a pressure which can now be followed in its rise and fall, and measured pretty accurately. We then have an extensive treatment of one of the most intriguing features of flame study, namely, the effect of traces of moisture on the combustion of gaseous mixtures. Gases which normally combine with violence on the passage of a feeble electric spark refuse to combine if they have been carefully dried by a prescribed ritual. This is but one aspect of a very general rule, in revealing which Professor Baker has been so promi, nent : that the most familiar chemical reactions do not take place if every trace of moisture has been removed—and what a task it is to remove the last trace ! Everyone knows how phosphorus catches fire in air, but really dry phosphorus can be heated in really dry oxygen without any action taking place. The book concludes with an account of a peculiar form of combustion, known as surface combustion, associated with Professor Bone's name, which has proved of considerable economic importance. A very extensive appendix of tables is provided.
Are we to think of flame, then, as something which science. has taken in hand and explained ? By no means. Coal gas is a complicated mixture of chemically distinct gases—methane and other hydrocarbons, together with hydrogen and carbon monoxide. Suppose, therefore, we take something simpler, pure methane ; do we know what takes place when it burns in oxygen ? As a result of long experiment, detailed in this book, we know that theories long held are wrong, and have some idea of the very complicated process that must actually take place with these simple gases, but we cannot speak with certainty. " Knock " in internal-combustion engines, a phenomenon known to the motorist, is one of the things which the book discusses. There are ten theories as to why certain chemicals, in small quantities, stop this knock; a few of these are considered critically, but the authors end by advising an open mind. Ignition temperature, so long a subject of discus- sion, proves to have no precise meaning. The presence of a trace of moisture is necessary for gaseous combustion in some cases, but other cases are well known, such as that of cyanogen, where combustion is not effected in this manner. We know next to nothing of the true causes of the so-called catalytic combustion at surfaces. The book before us gives a. careful and valuable account of an immense body of painstaking research, which has cleared up many complicated minor points, but when we turn to the main questions there is, as our authors do not attempt to hide, scarcely one to which we can answer " That is settled."
E. N. DA C. ANDRADE.
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