ix] Doctrines of Respiration. 235 



" with that of the Grotto del Cane in Italy (but he supposed 

 " the identity, because both are deadly), for he had examined 

 " neither of them chemically, nor did he know that it was the 

 "air disengaged in the effervescence of alkaline substances 

 " with acids. I convinced myself of the fact by going to a 

 "brewhouse with two phials, one filled with distilled water, 

 "and the other with lime water. I emptied the first into a 

 " vat of wort fermenting briskly, holding the mouth of the 

 " phial close to the surface of the wort. I then poured some 

 " of the lime water into it, shut it with my finger, and shook it. 

 " The lime water became turbid immediately. 



" Van Helmont says that the dunste or deadly vapour of 

 "burning charcoal is the same gas silvestre) but this was also 

 "a random conjecture. He does not even say that it ex- 

 u tinguishes flame ; yet this was known to the chemists of 

 "his day. I had now the certain means of deciding the 

 "question, since, if the same, it must be fixed air. I made 

 "several indistinct experiments as soon as the conjecture 

 " occurred to my thoughts ; but they were with little con- 

 " trivance or accuracy. In the evening of the same day that 

 " I discovered that it was fixed air that escaped from fer- 

 "menting liquors I made an experiment which satisfied me. 

 " Unfixing the muzzle of a pair of chamber bellows, I put a 

 "bit of charcoal, just red-hot, into the wide end of it, and then 

 " quickly putting it into its place again, I plunged the pipe to 

 " the bottom of a phial, and forced the air very slowly through 

 " the charcoal, so as to maintain its combustion, but not 

 "produce a heat too suddenly for the phial to bear. When 

 "I judged that the air of the phial was completely vitiated, 

 " I poured lime water into it, and had the pleasure of seeing it 

 " become milky in a moment. 



"I now admired van Helmont's sagacity, or his fortunate 

 " conjecture ; and, for some years, I took it for granted that 

 " all those vapours which extinguish flame, and are destructive 

 " of animal life, without irritating the lungs or giving warning 

 " by their converse nature are the gas silvestre of van Helmont 

 " or fixed air." 



It is thus evident that Black so early as 1757, the year 



