222 METTALLOIDS. 



550. EXPLOSIONS IN MINES. Marsh gas 



Explain the . , . 



cause of explo- forms, with air, an explosive mixture be- 

 *ion in mines? fore alluded ^ w hich is often the occa- 



sion of fearful accidents in mines. The experiment 

 may be made with olefiant gas, which has the same 

 explosive property. This property belongs, indeed, to 

 most gases and vapors which contain hydrogen ; as for 

 example, to the vapors of ether, alcohol, camphene, 

 and " burning fluid/' 



551. DAVY'S SAFETY LAMP. The dis- 



Descnbe Da- . . -.-i t i r-v -i i 



vy's safety tinguished English chemist, Davy, devised 

 lamp. a method O f security against these explo- 



sions. It consists in surrounding the 

 miners' lamp with wire gauze, which 

 will admit air through its insterstices, 

 but will not let out flame to ignite the 

 explosive atmosphere of the mine. 

 This effect may be illustrated, by 

 holding down a piece of wire gauze upon the flame of 

 a candle. If the gauze is fine, the flame will not 

 pass through it. This effect is owing to the reduc- 

 tion of temperature which the wire occasions. The 

 subject will be better understood by reference to the 

 paragraphs which follow, on the nature of flame. 



HEAVY CARBURETTED HYDROGEN. 



552. DESCRIPTION. Heavy carburetted 



What are the { 



properties of hydrogen is a colorless gas, of peculiar 

 defiant gas? ^ezl\$\\ odor, also known as olefiant gas. 



