FIRE-DAMP INDICATOR 



393 



tho chamber is firmly hold in position by a lever fitted for the purpose ; when the 

 hammer is raised the lever is removed, and the chamber is released. So long as the 

 hammer remains at half-cock, the chamber is free and can be loaded at pleasure. 

 Revolvers by Daw, by Adams and Dean, and others, have been introduced. They are 

 all so similar in principle, that they need not be described. 



FIRE-BLENDE. A rare mineral containing about 62 per cent, of silver, with 

 sulphur and antimony. It is found at Andreasberg in the Hartz, at Freiberg iu 

 Saxony, and at Przibram in Bohemia. 



FIRE-BRICKS. See Biucxs. 



FIRE-CLAY. Those clays which are termed fire-clays are such as are of a very 

 refractory character resisting the highest temperatures of tho blast-furnace without 

 melting. Tho conditions which are necessary for the production of a good fire-clay 

 have rarely been sufficiently explained. The best fire-clays are hydrated silicates of 

 alumina, not unfrequently containing silica uncombined ; they should not contain soda, 

 potash, or lime in any quantities, as those alkalis combine at high temperatures with 

 the silica, and form glasses. The presence of iron is also exceedingly injurious to a 

 fire-clay. Under the direction of Dr. Percy, an extensive series of analyses of fire-clays 

 were made in the laboratory of the Museum of Practical Geology. From these, as pub- 

 lished in Dr. Percy's 'Metallurgy,' the following more important have been selected : 



See CLAY. 



, the carburetted hydrogen of coal-mines, produced, in some cases, 

 by the slow decomposition of the coal itself ; in others, it is probably the result of the 

 changes in the constitution of the vegetable matter of which the coal itself is formed, 

 which has been confined under great pressure in the interstitial spaces of the coal 

 or rocks immediately in connection with them. The accumulation of this gas in the 

 'goaf,' or waste spaces of a coal-mine, is probably due to the changes which the 

 coal itself undergoes. The sudden outbursts of this gas, known as ' blowers,' are no 

 doubt the result of the liberation of the gas by suddenly removing the pressure under 

 which it has been confined. This gas is the constant product of the decomposition of 

 carbonaceous bodies under water ; it has hence been also called ' marsh gas.' It is a 

 protocarburetted hydrogen, its formula being C 2 H 4 (CH 4 ). 



This carburetted hydrogen gas does not explode when mixed with air in a propor- 

 tion much above or below the quantity necessary for complete combustion. With 

 three or four times its volume of air it does not explode at all, with five and a half or 

 six volumes of air it detonates feebly, and with seven or eight most powerfully. 

 When mixed with fourteen volumes of air the mixture is still explosive, but with 

 larger proportions of air the gas only burns about the flame of the taper. See SAFETY 

 LAMP and MINING. 



FIRE-DAMP INDICATOR. Fire-damp or Grisou of the French is a great 

 source of danger to the coal-miner, for, when mixed with certain proportions of air, 

 it forms a mixture which, meeting with, a naked light, explodes, occasionally causing 

 great destruction to life and property, while its products of combustion poison those 

 who, although stunned by the concussion, would yet recover if conveyed into a pure 

 atmosphere ; therefore its presence is a twofold source of danger. To make these two 

 sources of danger more easily comprehended, it may be explained that if fire-damp be 

 present to a very large extent, it naturally reduces to such an amount the oxygen, 

 because two substances cannot occupy the same space at one time, and oxygen being 

 thus excluded leaves in its place fire-damp, which is not poisonous, but simply in- 

 capable of supporting life, that is of oxidising the blood, since life simply goes out. 

 The other source of danger is, however, of a different character, for when an explosion 

 of such a mixture takes place the carbon which is hidden in the compound called fire- 

 damp or mine-gas becomes converted, by its combustion with oxygen, into carbonic 



