MIXED GASES. 413 



3. SOME PECULIAR PROPERTIES OF THE FIRE DAMP. 



(a.) The most explosive mixture of this gas with common air ', was 

 found to be 1 measure of the inflammable gas to 7 or 8 of air ; it ex- 

 plodes feebly with 5 or 6 volumes of air, and with only 3 or 4, it does 

 not explode at all ; it is still explosive with 14 volumes of air, but 

 with more, a taper burns in it only with an enlarged flame. 



(6.) Charcoal in active combustion, and iron heated to redness or 

 even to whiteness, did not kindle this mixture ; it was, however, ex- 

 ploded by iron in a state of brilliant combustion, and the smallest 

 point of flame, owing to its high temperature, produced instant ex- 

 plosion. 



(c.) The fact which led immediately to the discovery of the safe- 

 ty lamp, had been observed before by Dr. Wollaston, and was this, 

 " that an explosive mixture cannot be kindled in a glass tube so nar- 

 row as one seventh of an inch in diameter." 



(d.) Two separate reservoirs filled with an explosive mixture, be- 

 ^ing connected by a metallic tube one sixth of an inch in diameter, 

 a*nd one and a half inch in length the explosion could not be made 

 to pass into the one, when the other was set on fire. 



(e.) It was also discovered that fine wire sieves, or wire gauze 

 being in fact only short tubes, form, upon the same principle, an effec- 

 tual barrier between two portions of explosive gas, which will not 

 communicate through such a partition. 



(/.) It was found also that " a mixture of fire damp and air, in ex- 

 plosive proportions, was deprived of its power of exploding by the 

 addition of about one seventh of its bulk of carbonic acid or nitrogen 

 gas."* 



(g.) Sir Humphry Davy was thus led to an attempt to combine 

 both these principles by the construction of a lamp, which being fed 

 with only a limited supply of air, might be occupied more or less by 

 carbonic acid and nitrogen, and which, by having small metallic aper- 

 tures, might prevent the spreading of combustion into the surround- 

 ing atmosphere, should that be in an inflammable or explosive state. 



(A.) After various modifications and improvements, the safety lamp 

 is now constructed of wire gauze, that is, the flame is surrounded by 

 a wire sieve, so fine as to have at least 625 apertures in a square 

 inch. 



(i.) It is a cylinder 2 inches in diameter ; it rises 10 or 12 inches 

 above the flame ; the wire gauze is double at the top, where the 

 greatest heat exists, and no part of it is impervious to air, except 

 that which contains the oil, and which is furnished with means of 



* Many miners perish from the prevalence of these gases after the explosion ; 

 carbonic acid being formed and mixed with the nitrogen which is left. 



