600 REPORT— 1881. 



mixture of air and coal-dust inflammable ; and that dust containing no combustible 

 matter produced flashes of flame in the apparatus when the air contained 3 to 3^ 

 per cent, of firedamp. The autlior attributes the higli percentage of firedamp 

 required in the first case to the coarseness of the dust employed, since he obtained 

 similar results on using coarse dust instead of very fine. The second result appears 

 to be a curious one, but some light will doubtless be thrown upon it by future 

 experiments. 



7. In July of the present summer the autlior made experiments, during the six 

 days of very warm dry weather ending on the 21st of that month, with an apparatus 

 of the following description. A sheet-iron cyhnder, 6 feet long by 2 feet in diameter,, 

 closed at one end and open at the other, has its open end bolted to a woodea 

 gallery, 126 feet long by 2 feet square inside. One end of the wooden gallery is 

 thus closed by the sheet-iron cylinder or explosion-chamber, and the other end is 

 open. Six sheets of newspaper are placed between the open end of the explosion 

 chamber and the gallery, and a tight joint is insured by means of the screws. Rather 

 less than two cubic feet of firedamp is carefully measured by means of water dis- 

 placement, and introduced into the explosion-chamber. The wooden gallery con- 

 tains only piu-e air. The firedamp contained in the explosion-chamber could not 

 find its way into the gallery except by passing through the six sheets of news- 

 paper, and if any part of it did so, it would be immediately carried away by a. 

 strong current of pure air, amomiting to 1,000 or 1,200 cubic feet per minute, 

 which enters the gallery just behind the paper, and traverses its whole length 

 towards the open end. It is thus doubly certain that the gallery contains nothing- 

 but pure air. The air and firedamp contained in the explosion-chamber are 

 thoroughly mixed by means of an appropriate mechanical arrangement, and the 

 mixture is exploded. The explosion bursts the sheets of paper, and the resulting 

 flame travels about 12 or 14 feet along the gallery, and as suddenly disappears. 

 The gallery is then strewed with a layer of fine coal-dust, fi-om ^ inch to ^ inch 

 thick, along its floor, and some is placed on shelves which stand in sets of three,, 

 one above the other, at distances of 10 feet from each other, along the gallery. 

 The same arrangement as before is then made in regard to preparing for a fire- 

 damp explosion, exactly the same quality of firedamp being measured, mixed, and 

 exploded. 



By the explosion of the firedamp mixture the coal-dust is raised in a cloud, 

 throughout the whole length of the gallery, part of it is projected out into the 

 air to a distance of 20 or 30 feet beyond the end, and, after the lapse of an appre- 

 ciable interval of time, the flame finds its way to the end of the gallery, and flashes 

 out through the cloud of dust to a greater or less distance, according to cir- 

 cumstances. The greatest lengths of flame thus obtained with coal-dust and pm-e 

 air was 147 feet on one occasion, and from 100 to 140 feet very often. 



The author considers that these results prove in the most convincing manner 

 that coal-dnst forms an inflammable mixture with pure air, and they settle once- 

 for all the question as to how an explosion, begun in one district of a dry and dusty 

 mine, can penetrate to the most distant parts of every other district of workings 

 in the same mine. 



If, then, water were sprinkled on the floor of all dry mines from time to time, 

 and always before fii-ing blasting shots, we should, in the author's opinion, have- 

 no more disastrous coUiery explosions, such as those with which we have become- 

 but too famOiar during the last ten or fifteen years. 



6. On the Double Iodide of Mercury and Copper, 

 By Professor Silvanus P. Thompson, B.A., D.Sc. 



Cuprous mercuric iodide, Cu^Hgl^, is prepared by the following process. Mercuric 

 biniodide is dissoh'ed in iodide of potassium ; the liquid is then raised to 100°, and 

 aqueous solution of cupric sulphate is added in excess, when the cuprous mercuric 

 iodide precipitates. When cold, it is of a fine scarlet colour ; but possesses the 

 property of becoming a deep dull black tint, between 80° and 90°, without suffering 

 decomposition, and of returning on cooling to its former hue. This remarkabla 



