396 



FIRE-DAMP INDICATOR 



instrument consists of a porous cell, surmounted by a cylinder, in which is placed a 

 piston of brass, tho piston being free to move, just as in the case of steam. Imme- 

 diately that this instrument is placed 



914 in an atmosphere of coal-gas, diffu- 



sion commences; tho gus p.-i 

 into tho porous cell through its walls 

 more rapidly than the air passes out, 

 causes an increase of volume, which 

 exerts its pressure on tho lower side 

 of tho piston and carries it up through 

 the cylinder. 



Fire-damp and choke-damp accu- 

 mulate slowly under some conditions, 

 and rapidly under others ; there are, 

 therefore, two main divisions to be 

 met, and wo will first explain how 

 Mr. Ansell proposes to indicate the 

 existence of a slowly -accumulating 

 mass of gas in a goaf, or other place. 

 For tho purpose of indicating by 

 signal a slowly-accumulated mass of 

 fire-damp, or of carbonic acid, ho 

 uses a balloon of thin India-rubber ; 

 for, singularly enough, he found that 

 both these gases cause an expansion 

 of the balloon. At first sight one 

 can understand how marsh gas ex- 

 pands the balloon, by the law of 

 diffusion ; but that carbonic acid 

 should also expand it, is so surpris- 

 ing, that the effect will require to bo 

 explained by a law not yet dis- 



_J L covered. 



The balloon is filled with atmo- 

 spheric air, and its neck tied tightly with silk, or wax-end, and a piece of linen 

 is bound round tho equator of tho balloon to prevent lateral expansion. The balloon 

 c, fig. 914, so prepared, is placed under a small lover A upon a stand of wood, so that 

 it exerts a gentle pressure on the lever ; if now any fire-damp or carbonic acid 

 accumulate round it, either of these gases passes through the substance of the India- 

 rubber balloon, and, accumulating inside, causes it to expand, thus to press against 

 the lever, and raising it releases a detent B, E, by which the terminal poles of 

 a battery F are connected, whereby we get telegraphic communication with a distant 

 place, or a warning on tho spot at will. It must be stated that tho temperature 

 of a given place in a mine does not vary from year's end to year's end. The 

 action of gases through India-rubber has been explained, by assuming that the gases 

 dissolve in tho outer coats of tho India-rubber, and in solution pass through and 

 evaporate from the inside. This proposition is rather fascinating, but Mr. Ansell 

 refuses to regard it as tho true explanation ; he is, therefore, engaged in investigating 

 tho facts in a larger field, and at a future time hopes to discover tho law which 

 regulates tho passage of gases through thin India-rubber. 



These balloon instruments can bo so arranged as to toll if tho accumulation bo still 

 free from danger, or if it bo explosive. 



In the event of a sudden irruption of fire-damp, ho applies tho law of diffusion in 

 all its simplicity ; for by an instrument,^. 915, ho shows at once, if there bo gas, no 

 irruption can be so sudden that this instrument cannot toll of its approach say, in 

 from 5 to 10 seconds according to tho percentage of tiro-damp contained in tho 

 dangerous rush. It may bo so delicately set as to give warning if the mixture be 

 still below the explosive point. Tho instrument consists of an iron funnel, B, whoso 

 stem is bent into a U, tho funnel being closed with a plate, A, of unglazod Wedgwood 

 ware (but, in his first experiments, he used a broken flower-pot), the stem being closed 

 by a cap of brass, c, through which is passed a platinum-tipped copper-wire, capable 

 of just dipping into tho mercury, D, previously placed in the bond of the funnel. 

 The distance between the platinum-pointed wire and the mercury regulates tho point 

 at which the indication shall bo given as regards the irruption ; that is to say, if a 

 non-explosivo mixture is to give its warning, tho wire must bo brought almost to 

 touch the mercury ; but if it be intended to give its alarm for an explosive amount, 

 then it may Btand a little farther off; but in no case to exceed the thickness of a 



