1882,] on Some of the Dangerous Projoerties of Dusts. 101 



a large quantity of each jmrticular coal dust the property of exploding 

 throughout the gallery. By these experiments the samples were 

 classed in the order of their sensitiveness to explosion, and it was 

 found that those which were very rich in pure coal, and which 

 contained the highest proportion of very fine dust were the most 

 sensitive, i. e. required the lowest proportions of fire-damp in air to 

 bring them to explode readily when suspended in a dense cloud. But 

 with the samjDles containing larger proportions of non-combustible 

 matter the order of sensitiveness did not necessarily harmonise with 

 the comparative richness of a sample in pure coal, nor with its com- 

 parative fineness, and this was strikingly illustrated by a sample of 

 dust from one of the roads in Seaham Colliery, which contained more 

 than half its weight of non-combustible matter, yet ranked only third 

 in order of sensitiveness, while another sample, containing considerably 

 more coal and a somewhat larger proportion of the finer dust, ranked 

 fifth. 



Another point clearly established, and confirming by more accurate 

 data the observations of earlier experiments, w^as, that the proportion 

 of fire-damj) required in a mine to bring dust into operation as a 

 readily exploding material when thickly suspended in the air is 

 bordering upon and even below the smallest amount which can be 

 detected in the atmosphere of a mine, by the most practised observer, 

 with the use of the Davy lamp, the only means of searching for gas 

 which has until quite recently been employed in mines. The highest 

 proportion which can be thus detected by an experienced operator is 

 stated to be about 2 per cent. Explosions were produced by dusts 

 suspended in air travelling at a velocity of 600 feet per minute, w^hen 

 fire-damp was present in proportions ranging from to 2 to 2 • 75 per 

 cent. ; in currents of low velocity the same result was produced with a 

 sensitive dust in the presence of only 1 • 5 per cent, of fire-damp, and 

 ignitions which approached explosions in their nature and extended to 

 considerable distances, were obtained with this dust in air containing 

 still smaller proportions of gas. Mixtures of fire-damp and air 

 bordering upon those which will ignite upon the approach of flame, 

 were found to be instantaneously fired by a lamp if they contained only 

 a few particles of dust in suspension, and in connection with this fact 

 the interesting observation was made that such dust particles need not 

 be inflammable nor combustible to produce the result named. Mixtures 

 of air and gas which passed a naked flame without any symptom of 

 ignition, were inflamed when particles of a fine light powder, such as 

 calcined magnesia were suspended in them. The action of certain of 

 the pit dusts which contain comparatively little coal, in determining 

 the ignition of mixtures of air and small proportions of fire-damp, is 

 possibly of the same character as the behaviour of such a dust as 

 calcined magnesia. The power of favouring the ignition of mixtures of 

 fire-damp and air was not exhibited by some other powxlers similar in 

 fineness to the latter, but differing in structure and density from this 

 and one or two other non- combustible dusts which may be called active ; 



