422 



COMPOUNDS OF CARBON. 





FIG. 43. containing from 784 to 800 holes in the square 

 inch. A crooked wire, which works tightly in a 

 narrow tube passing upwards through the body 

 of the lamp, affords the means of trimming the 

 wick, without undoing the wire-gauze cover of 

 the lamp. When the lamp is carried into an 

 atmosphere charged with fire-damp, a blue flame 

 is observed within the gauze cylinder, from the 

 combustion of the gas, and the flame in the centre 

 of the lamp may be extinguished. The miner 

 should then withdraw, for although the gauze has 

 often been observed to become red-hot, without 

 inflaming the external explosive atmosphere, yet 

 -j the texture of the gauze may be destroyed, if 

 li JjB retained long at so high a temperature. It has 

 always been known, since this lamp was first pro- 

 posed, that when it is exposed to a strong cur- 

 rent of the explosive mixture, the flame may pass too quickly 

 through the apertures of the gauze to be cooled below the point 

 of ignition, and, therefore, communicate with the external atmo- 

 sphere. But this is easily prevented by protecting the lamp 

 from the draught, and an accident from this cause is not likely 

 to occur in a coal mine.* 



Carburetted hydrogen does not explode when mixed with air 

 in a proportion much above or below the quantity necessary for 

 its complete combustion. With 3 or 4 times its volume of air 

 it does not explode at all, with 5| or 6 volumes of air it detonates 

 feebly, and with 7 to 8 most powerfully. With 14 volumes of 

 air, the mixture is still explosive, but with larger proportions of 

 air, the gas only burns about the flame of the taper. The large 

 quantity of air which is then mixed with the gas, absorbs so 

 much heat as to prevent the temperature of the gaseous atmo- 

 sphere from rising to the point of ignition. 



Coal gas. The products of the distillation of coal in an iron 

 retort are of three kinds : a black oily liquid, of a heteroge- 

 nous nature, known as coal-tar, of which a considerable con- 

 stituent, according to M. Dumas, is benzin ; a watery fluid, 



* For additional information respecting the safety lamp, the reader is referred to 

 Davy's Essay on Flame, to Dr. Paris's Life and Dr. J. Davy's Life of Sir H, Davy, 

 and to ihe Report of the Parliamentary Committee on accidents in mines, 1835. 



