July 7, 1887] 



NATURE 



221 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he under- 

 take to return, or to correspond with the writers of, 

 rejected manuscripts. No notice is taken of anonymous 

 communications. 



[The Editor urgently requests correspondents to keep their 

 letters as short as possible. The pressure on his space 

 is so great that it is impossible otherwise to insure the 

 appearance even of communications containing interesting 

 and novel facts.] 



Relation of Coal-Dust to Explosions in Mines. 



The suggestion in my former letter on this subject (vol. xxxiv. 

 P- 595) ^^^^ "keeping the ventilating air-current saturated with 

 aqueous vapour " might prove the most effective way of rendering 

 the dust in coal-mines innocuous, has, I am glad to see, been since 

 shown to be practicable, in a South Wales colliery. Since the 

 above date, I have considerably extended my research, with re- 

 sults that confirm the conviction therein expressed that many of the 

 most disastrous colliery explosions during the last seven years in 

 this northern district have been practically dust explosions, and 

 therefore preventable ; that the rough method of watering the 

 floors only, or floors and sides, of the mines is delusive, since it 

 leaves the most dangerous dust undisturbed, the upper and 

 flocculent dust ; and last, that probably the reasons why dust in 

 dry pits does not explode more frequently are now within grasp. 

 To this latter conclusion, with your permission, I will now briefly 

 address myself. That every firing of a shot that is accompanied 

 by flame in a dry and dusty pit does not produce an explosion is 

 well known ; that sometimes such firing of a shot does is un- 

 happily also well known. That the local presence of gas, even 

 in small amount, is sometimes the reason of this is universally 

 acknowledged. That the amount and condition of the dust 

 present (even in the practical absence of gas) is at other times 

 the reason is now believed by many. Setting aside the amount 

 of dust, which every one will allow must be an essential factor, 

 and also the varying energy which the shot, blown out or not, 

 develops, let us look at the other conditions. The temperature 

 and hygroscopic state of the air-current is one most important 

 factor, and consequently the concomitant temperature and hygro- 

 scopic state of the dust traversed by such current. Beyond this, 

 the degree of fineness and the constituents of the dust will have 

 much to say in the matter. The finer the particles the more 

 readily will they ignite, and the more completely will they place 

 their substance under the influences present. Thus ordinary 

 screen coal-dust will not ignite when a common match is lighted 

 and applied to it, but it will when finely pounded in a mortar. 

 Now the dust resting on the baulks and upper portions gener- 

 ally of the ways will invariably so light and burn when dry, 

 although the constituents vary greatly in different pits and in 

 different seams of the same pit. 



What are the ordinary constituents of coal-dust ? Two, per- 

 haps three, important substances, and others unimportant : 

 important, as being inflammable in varying degrees ; unim- 

 portant, either from their uninflammability or from their 

 excessively small amount. The three important are mother of 

 coal, ox dant ; coal ; and certain coloured bodies, ^xohzb\y spores. 

 The unimportant are shale or other stone dust, iron pyrites, lime 

 flakes, and incidentals, as animal and vegetable matters, and the 

 results of the wear and tear of the haulage and winning 

 apparatus, &c. Dismiss these last, as only one needs any 

 attention, the shale ; and that special, not general. 



Dant lights most readily ; the red end of a used match is often 

 sufficient to fire it, and then it burns itself out whether resting on 

 wood or stone. Burned in a retort, it loses little weight, and the 

 fumes it gives off will not ignite. Now, this dant is largely 

 present in upper and flocculent dust, reaching in some specimens 

 even 70 or 80 per cent. Dant clearly therefore is not it-elf 

 dangerously explosive, yet is admirably fitted to act the part that 

 tinder used to do, when it handed on the spark from the flint and 

 steel to the old-fashioned brimstone match. 



C^/ forms a considerable part of all upper and flocculent dust, 

 and constitutes the great mass of the bottom dust along intake 

 haulage roads. Coal-dust (got as free from dant as possible) 

 when pounded very fine ignites with some difficulty, burns at 

 fir.'it somewhat fiercely and with considerable smoke, but gener- 

 ally goes out leaving a portion of the heap unburned. Placed 

 on an iron plate and burned by heating the plate, it threw off 

 scintillations, its fumes readily took fire, and forty grains of dust 



were reduced to one grain of ash. In a retort it gave off first 

 much smoke which would not light ; soon, however, the smoke 

 lessened, when its fumes lit and burned with a long bright flame. 

 Such coal-dust is manifestly capable of producing an explosion. 

 Under favourable conditions it can produce a considerable amount 

 of ordinary illuminating coal-gas, whose presence would convert 

 the air-current into an explosive mixture. Therefore, adopting 

 the former simile, as the dant is the tinder, so this coal is the 

 sulphur match, as the shot flame or other initial cause is the 

 spark struck from the flint and steel. 



Spores. — Nearly all dusts (and I have examined many) have 

 shown under the microscope few or many orange, brown, or 

 reddish flakes, very often triangular in shape and with concoidal 

 fractures. I have not yet examined thin sections of these coals, 

 but the fragments present much the appearance presented by the 

 spores in the well-known spore coals of the Bradford "Better 

 Bed," and Leicestershire "Moira," If these coloured bodies 

 originate in Lycopodian and other microspores or macrospores, 

 they may play an important part, for the resinous nature of the 

 microspores of the Sclaginella selagittoidcs, &c., of our northern 

 hills is so well known that they were formerly used in theatres to 

 produce artificial lightning. As my experiments and inquiries in 

 this direction are yet incomplete, I will only suggest that their 

 presence may account for some dusts being so much more 

 dangerous (as the German experiments have conclusively shown) 

 than others, and add the hope that these words may lead others to 

 pursue this inquiry. ARTHUR Watts. 



Bede College, Durham, May 26. 



Science for Artists. 

 Of the various optical errors in this year's pictures, certainly 

 that in the elegant scene (624) of the Queen's Accession, in the 

 morning small hours of June 20, 1837, is largest and most hope- 

 less. Neither a source of light at 93,000,000 miles, nor one at 

 93 inches, could cast the bar-shadows. It is impossible to say 

 whether they are meant to be aerial in the dust or mist, or cast 

 on the walls and wainscot. But for either they are equally pre- 

 ternatural, though not by diverging perspectively. If cast on 

 the solids they would, instead of being straight, be crooking in 

 and out over the mouldings. But if they are in aerial mist or 

 dust, the error is in supposing the same eye can see more than 

 one of such shadows at a time. The eye requires to be very 

 nearly in the plane of the shadow seen, so that, of those 

 cast by parallel things, as window-bars, only one could be seen 

 by any single eye, and only as continuing the line of the bar 

 itself. The bar and its mist-shadow could never meet at an 

 angle, as they all do in this picture. Another error (now com- 

 mon) is in there being no more penumbra than if the sun were a 

 star, or a small electric arc-light. Edwd. L. Garbett. 



Weight, Mass, and Force. 



With reference to the extract, as to the language employed in 

 which Prof. Greenhill invites my criticism, I have no doubt that 

 to an engineer it would convey perfectly definite and intelligible 

 information, and that one who has mastered the fundamental 

 notions of dynamics as a science would be able to divine its 

 meaning, but Prof. Greenhill would hardly maintain that the 

 language is scientifically accurate, and that, however sufficient as 

 a shorthand for the trained engineer addressing engineers, it is 

 not full of pitfalls for the tyro. 



There is no need to object to the statement that " the weight 

 is 137,000 pounds," though it is just as easy to say, •' the mass 

 is 137,000 pounds." But that "the boiler carries 160 pounds of 

 steam," I find, means that the pressure of the steam is 160 

 pounds (weight) /^rr xi/z/ar^ i«^//, while " a 96-feet grade " means 

 "a gradient of 96 feet per mile." Surely, except as a recognized 

 shorthand for experts, the suppression of the words in italics is 

 unjustifiable and liable to lead into error. 



It is more important, however, to observe that (as in a great 

 majority of the cases an engineer has to deal with) the question 

 here discussed is essentially a statical one. The motion of the 

 train considered is uniform (30 miles per hour), and the variations 

 in pressure in the cylinders, &c., are avoided by taking the 

 "mean effective pressure," so that there are x\o cucelerations Xo 

 be considered, and only, in fact, a balancing of forces. The 

 question of mass therefore, (a purely kinetic notion), can hardly 

 arise, and there is no room for confusion between mass and 

 weight. R. B. Hayward. 



