22 



NATURE 



\Nov. 8, 1877 



mable gas has been present in the workings. The reports 

 of the Inspectors of Mines bear ample testimony to the 

 correctness of this statement. It has therefore been cus- 

 tomary in the absence of any other tenable hypothesis to 

 assume that a large volume of firedamp had been suddenly 

 poured into the workings. But these so-called "out- 

 bursts of gas " are entirely unknown in some localities in 

 which great explosions have occurred ; and therefore it is 

 much to be marvelled at that some other explanation was 

 not at least sought for. 



In September, 1844, before the appointment of inspec- 

 tors of mines, Lyell and Faraday were sent to Haswell 

 Colliery by the Home Secretary to report on an explosion 

 that had just taken place there. I am unable to quote 

 from their official report, but I am firmly convinced that 

 the following sentences taken from their article on the 

 subject in the Phil. Mag. 1845, is the true key to a solution 

 of the problem as regards both the mode of occurrence 

 and means to be used for the purpose of avoiding great 

 explosions in future ; and, moreover, I believe that it has 

 been highly unfortunate, both for the cause of the miner 

 and his employer, that these two philosophers were not 

 induced to prosecute their investigations further than they 

 did. 



The sentences referred to are these : — " In considering 

 the extent of the fire for the moment of explosion, it is 

 not to be supposed that firedamp is its only fuel ; the 

 coal-dust swept by the rush of wind and flame from the 

 floor, roof, and walls of the works, would instantly take 

 fire and burn, if there were oxygen enough in the air to 

 support its combustion ; and we found the dust adhering 

 to the face of the pillars, props, and walls in the direction 

 of, and on the side towards, the explosion, increasing 

 gradually to a certain distance as we neared the place of 

 ignition. This deposit was in some parts half an inch, 

 and in others almost an inch thick ; 1 it adhered together 

 in a friable coked state ; when examined with the glass it 

 presented the fused round form of burnt coal-dust, and 

 when examined chemically, and compared with the coal 

 itself reduced to powder, was found deprived of the 

 ■greater portion of the bitumen, and in some cases entirely 

 destitute of it." 



About three years ago JVl. Vital, Ingdnieur des Mines in 

 France, showed that a flame resembling that produced by 

 a blasting shot which blows out the tamping is greatly 

 lengthened in an atmosphere containing a cloud of coal- 

 dust ; and soon afterwards the writer ascertained that air 

 containing a small proportion of fire-damp (less than one 

 per cent, by volume) becomes highly inflammable when 

 coal-dust is mixed with it. 



These discoveries complete what Lyell and Faraday 

 began, ahd show how explosions of any conceivable mag- 

 nitude may occur in mines containing dry coal-dust. A 

 blasting shot or a small local explosion of firedamp, or a 

 naked light exposed when a cloud of coal-dust is raised up 

 by a fall of roof in air already containing a little fire- 

 damp is sufficient to initiate them, and, when once they 

 are begun, they become self-sustaining. 



These remarkable facts are either not yet sufficiently 

 well known or their true significance is not yet fully ap- 

 preciated. In conclusion I may state that out of many 



' In the reports of the Inspectors of Mines, human bodies, timber, and 

 coal, are described as being charred or burnt where they arc covered with 

 this deposit.— W. G. 



hundred collieries known to me there is not, to my know- 

 ledge, a single damp one in which a great explosion has 

 happened ; while, on the other hand, there is a con- 

 siderable number of very dry ones in which explosions 

 causing the deaths of from 12 to 178 men at a time have 

 occurred. W. Galloway 



THE SUN'S PHOTOSPHERE 



DR. JANSSEN has just made a communication to 

 the French Academy of Sciences, which will be 

 received with interest, not only by students of solar physics, 

 but by all who follow the various triumphs achieved 

 by modern scientific methods. It seems a paradox that 

 discoveries can be made depending on the appearance of 

 the sun's surface by observations in which the eye applied 

 to the telescope is powerless ; but this is the statement 

 made by Dr. Janssen himself, and there is little doubt that 

 he has proved his point. 



Before we come to the discovery itself let us say a little 

 concerning Dr. Janssen's recent endeavours. Among the 

 six large telescopes which now form a part of the equip- 

 ment of the new physical observatory recently established 

 by the French government at Meudon, in the grounds of 

 the princely Chateau, there is one to which Dr. Janssen 

 has recently almost exclusively confined his attention. It is 

 a photoheliograph giving images of the sun on an enormous 

 scale — compared with which the pictures obtained by the 

 Kew photoheliograph are, so to speak, pigmies, while the 

 perfection of the image and the photographic processes 

 employed are so exquisite, that the finest mottling on the 

 sun's surface cannot be overlooked by those even who are 

 profoundly ignorant of the interest which attaches to it. 



This perfection and size of image have been obtained 

 by Dr. Janssen by combining all that is best in the prin- 

 ciples utilised in one direction by Mr. De la Rue, and in 

 the other by Mr. Rutherfurd. In the Kew photohelio- 

 graph, which has done such noble work in its day that it 

 will be regarded with the utmost veneration in the future, 

 we have first a small object-glass corrected after the 

 manner of photographic lenses, so as to make the so- 

 called actinic and the visual rays coincide, and then the 

 image formed by this lens is enlarged by a secondary 

 magnifier constructed, though perhaps not too accurately, 

 so as to make the actinic and visual rays unite in a second 

 ■image on a prepared plate. Mr. Rutherfurd's beautiful 

 photographs of the sun were obtained in a somewhat 

 different manner. In his object-glass he discarded the 

 visual rays altogether and brought only the blue rays to 

 a focus, but when enlargements were made an ordinary 

 photographic lens— that is, one in which the blue and 

 yellow rays are made to coincide — was used. 



Dr. Janssen uses a secondary magnifier, but with the 

 assistance of M. Pragmowski he has taken care that both 

 it and the object-glass are effective only for those rays 

 which are most strongly photographic. Nor is this all ; 

 he has not feared largely to increase the apertures and 

 focal length, so that the total length of the Kew instru- 

 ment is less than one-third of that in operation in Paris. 



The largely-increased aperture which Dr. Janssen has 

 given to his instrument is a point of great importance. 

 In the early days of solar photography the aperture used 

 was small, in order to prevent over-exposure. It was 



