70 THE COLLIERY GUARDIAN. January 10, 1913. rod, which is then lowered into the glass cylinder. The finest wires will be found decidedly to glow with the greatest brightness, whilst the coarser ones will remain at red heat, or even below that limit. This effect is a result of convergence, because the section of the platinum wire against which the gaseous molecules impinge is the focus of convergence; and the smaller this focus the greater the concentration of the rays (as represented in fig. 7) and the higher the temperature. To sum up: The phenomenon of convergence is governed by two simple laws—(1) the velocity of con- vergence, and therefore the incandescence, is the greater in proportion as the gaseous mixture is more explosive; (2) the intensity of the impact of convergence is the greater in proportion as the radius of curvature of the incandescent solid diminishes. These laws approximate to those of electric incan- descent lamps, the velocity of convergence corresponding to the voltage and the fineness of the incandescent wire to that of the filament by which the electrical resistance is increased. In fact, if the fine platinum wire used in the experiments be given the same shape as a lamp filament, it will have exactly the same appearance as the latter when incandescent. The platinum wires used by the author were made of cast platinum, and are therefore non-porous and lack the catalytic properties of spongy platinum. Most solid bodies, which are not very oxidisable, possess the property of giving rise to the convergent combustion of gases, but in a very unequal degree and under different conditions. For the most part this Fig. 6. t— uJ z a id > o co z uu * ° z z ± < H O < Ea! Bunsen Burner, arranged for Investigating Conver- gent Combustion. Fig. 5. Platinum Wire intro- duced into Test Tube fitted with Gaseous Mixture. property is not manifested except at temperatures above 1,000 degs. Cent., whereas in the case of platinum, it begins to appear between 400 and 500 degrees Cent. The author has found that specially treated pure copper acquires properties similar in this respect to those of platinum. Ordinary sparks are due to the projection of particles of incandescent carbon from the flame; but in propor- tion as the spark is consumed and the nucleus diminishes in size, its brilliance increases, and it is the final brilliance which is the most vivid. The same applies to the particles of iron which fly off as sparks from a grindstone or under the blows of a hammer, their brightness increasing with the distance from the origi- nating source. This phenomenon is in accordance with the above-mentioned laws of convergent combustion. The white portion of the flame of gas, wood, candles and other illuminants is due to particles of carbon liberated by pyrogenation, and burning by convergent combustion. Some years ago this property was utilised for increasing the illuminating power of gas in the “ Albo-carbon ” system of lighting, in which the gas was enriched by passing it through a receptacle con- taining naphthalene. At high temperatures most oxides are capable of effecting the combustion of a combustible gaseous mixture and producing a brilliant light. The Drummond light, in which the oxy-hydrogen mixture burns in contact with a zirconium cylinder, was the first applica- tion of this principle, and this application formed the basis of the Auer system of lighting. It is very remarkable that empiricism, independently of any theoretical considerations, led to the zirconium cylinder —a large mass—being replaced by a fabric of the finest possible threads. The finer these threads, the more vivid the incandescence, and this fact constitutes a striking demonstration of the second law of convergent combustion. For a given gaseous mixture, the intensity of com vergence depends on the nature of the oxides used. Auer mantles are marked with ink before being fired- When the mantle is fired, the iron contained in that ink remains behind in the state of oxide, which, however, becomes merely red, whilst the rest of the mantle is a brilliant white, and the combustion of the gaseous mixture on this iron oxide goes on at a temperature which is not so high as on the other oxides composing the Auer mantle. As the oxides become incandescent they absorb the flame; but this does not imply that the combustion goes on in the interior of the oxides, the foregoing experi- Fig. 7. Attraction of the Molecules of a Gaseous Mixture during Convergent Combustion. The dark Central Discs represent the sectional areas of the incandescent wires. ments made with cast platinum, which is non-porous, demonstrating the contrary. It was found some years ago that if platinum be surrounded by an inert gas, it can be fused at 1,720 degs. by the electric current, whereas a temperature of 1,760 degs. is necessary in the Bunsen flame. These temperatures, however, were measured by means of the optical pyrometer, which enables the temperature to be estimated by noting the degree of incandescence of the surface. In the flame of the Bunsen burner (blue flame) platinum gives rise to convergent combustion, and the temperature indicated by the optical pyrometer is that of the incandescence of the gaseous stratum on the periphery of the wire. Similarly, the small rods of the “ Buisson Bella” system absorb the flame of the gas in which they are immersed. They are arranged at a distance of several millimetres apart, and therefore act at a distance on the gaseous mixture issuing from the burner. These remarks should be taken into consideration in studying the new problem of industrial heating by incandescence or convergent combustion. If the incandescent solid is of irregular shape it cannot be raised to a uniform temperature throughout Fig. 8. Apparatus for Observing the Flame of Convergent Combustion. its entire mass, since the incandescence will be greater in proportion as the radius of curvature of any one part is the smaller. In fig. 8, which illustrates an apparatus designed for exhibiting various manifestations of convergent combus- tion, S is a trellis of fine platinum wires which give a dazzling white incandescence. L is a sheet of platinum bent round in the shape of a cone, and merely gets red hot. Its purpose is to stabilise the phenomenon, and to prevent ignition of the flame at S. In this apparatus the gaseous mixture does not burn completely, because the platinum trellis S is not sufficiently closed. Conse- quently, the flame F is produced, which burns at the apex of an iron rod, which acts like a wick. The author has made an exhaustive examination of this flame, which exhibits certain interesting peculiarities which, however, cannot now be discussed. Hence, in view of the appli- cation of convergent combustion for heating purposes, regularity of shape of the incandescent solid cannot be too strongly recommended. It is not always advisable to attain a very high degree of incandescence, this being liable to affect the material too rapidly, produce local overheating, or give rise to an excessive drop in the thermal potential during heating or evaporation. In order to obtain the most favourable combustion in practice, a suitable choice must be made of the diameter of the heating bodies, the nature of the oxides and the relative distance between the incandescent solids. This distance should not be so great as to prevent complete combustion, nor so small as to hinder the free circulation of the gaseous mixture around the heating members. Since convergent combustion entails the employment of explosive gaseous mixtures, every cause which tends to lessen the intensity of the convergence diminishes its stability, and is liable to give rise to explosions. These causes may be due, either to the current of gas which supplies the burning lamp, or else to external currents of air. Variations in the Gas Supply.—If the rubber tube supplying the gas be pinched suddenly, a slight explosion will be heard, and the incandescence will diminish progressively. If the tube be suddenly released while the platinum .wire is still incandescent, an explosion occurs and the flame will ignite. On the other hand, slow constriction of the tube prevents an explosion on the extinction of the flame; and releasing the tube in the same manner will prevent explosion and ignition, the incandescence being re-established. Effects of External Air Currents (Application in Coal- mining).—When the lamp is in full incandescence, all that is necessary to produce explosion and ignition is the presence of a sudden current of air, such as may be caused by a rapid gesture, the flick of a handkerchief, the opening or closing of a door, &c., or even by blowing from a distance in the direction of the lamp and placing on the glass an object which momentarily deflects the ascending air current. Again, if the lamp be grasped by the bottom and swung gently, no ignition will occur, whereas moving the lamp rapidly will cause this phenomenon. The glass with which the lamp is provided stabilises the incandescence (convergent combustion), but if the glass be removed, the influence of the external causes will become far more apparent, Naturally, the gaseous mixture will be the more prone to ignite in proportion as its explosive character increases, the slightest external cause disturbing its flow and composition. The facts set forth above approximate to others long known in coalmining practice : the swinging of lamps, moving them about brusquely in air charged with fire- damp, and variations in the composition of the gaseous mixture, being all causes favouring the production of an explosion. Up to the present, no suitable explanation has been found for the action of these causes, but the foregoing experiments with coal gas, the explosive limits of which are much wider than those of firedamp, throw a new light on the question. Calendars.—Calendars, almanacs, &c., have been received from Messrs. W. B. Brown and Co. (Bankhall) Limited; Peckett and Sons, Bristol ; The Hunslet Engine Company Limited, Leeds; Mavor and Coulson Limited Glasgow; The Westinghouse Brake Company Limited, King's Cross; and the British Thomson-Houston Company Limited, Rugby. Colliers and Medical Relief.—At the meeting of the Tynemouth Insurance Committee, held on Monday night, at North Shields, a letter was read from Drs. Mears and Fraser, requesting that the whole of the employees of the Preston Colliery should be treated as persons living in a rural area, regardless of the 1-mile limit, in connection with their medical benefits. Dr. Fraser said that, as president of the North of England branch of the British Medical Association, he could inform the committee that similar claims were being made in every colliery district throughout the country. It was being made in cases exactly similar to that of Preston Colliery, where numbers of the employees lived within the prescribed mileage limit. Mr. Lloyd George had informed the deputation from the doctors which waited upon him that colliery doctors would be placed upon a different footing to ordinary medical men. That promise had been clearly laid down. He might say it was the desire of the Preston Colliery men that they should be excepted. After a long discussion, it was agreed to send the letter of Drs. Mears and Fraser (who are the colliery doctors) to the Commissioners for their consideration and advice.