Chap, i.] Cboak and Fire Damp* 363 



workmen experience occafionally in mines to the eman- 

 cipation of this fpirit. On the fame principle he ac- 

 counts for the eructations from the ftomach and 

 bowels, and for the floating of drowned bodies j and 

 he concludes by determining, that this gas is a fluid of 

 a nature quite different from that of our common air. 



The exigence of two different kinds of vapour, or 

 elaftic fluids, had been previoufly obferved in mines 

 and coal-works : the one was obferved to affect ani- 

 mals with a fenfe of fuffocation, and to extinguifh life, 

 and it therefore obtained the name of the cboak-dampi 

 the other, from the dangerous property of catching fire 

 when a candle or any ignited body was brought in 

 contact with it, was termed the fire-damp. 



A fpecimen of the tire-damp, or inflammable air, 

 was collected from a coal-mine of Sir James Lowther, 

 in Cumberland, and brought up in bladders to be ex- 

 hibited to the Royal Society at London, in the year 

 1733; and in the year 1736 Mr. John Maud pro- 

 cured, from the folution of iron in oil of vitriol, a 

 quantity of the very fame fpecies of inflammable air, 

 and demonftrated that the fame might be procured 

 from moil of the metals in certain circumftances. 



The experiments of Van Helmont were greatly im- 

 proved upon by the fagacious Boyle. He changed 

 the name of gas to that of artificial air; he demon- 

 ftrated, that this artificial air was not always the fame; 

 for inftance, that the air produced by fermentation is 

 efientially different from that which is formed from 

 the explofion of gunpowder. He was, I believe, the 

 firft who perceived that the volume of air was di- 

 minifhed by the eombuftion of certain fubftances. 



This laft obfervation of Mr. Boyle feems particu- 

 larly to have attrafted the attention of the indefatigable 

 Dr. Hales, and he invented inftruments for determin- 

 ing 



