CARBONIC ACID. 193 



the Grotto del Cane, or dog's grotto. A man walking 

 into it, is safe, but his dog, whose head is below the 

 surface of the gaseous lake, is immediately suffocated. 

 Baths of carbonic acid have recently been employed, 

 with advantage, in the treatment of rheumatism, and 

 other similar affections, and in cases of enfeebled 

 vision. 



475. How REMOVED FROM WELLS. Car- 

 bonicacid re- bonic acid often collects in the bottom of 



we ^ s ; an( l occasions danger, and some- 

 times death, to workmen employed in 

 cleaning them. A candle previously lowered into the 

 well will, indicate the danger, if it exist. The flame 

 will burn less brilliantly, or be entirely extinguished, 

 if much of the gas is present. By repeatedly lower- 

 ing pans of recently heated charcoal into the well, and 

 drawing them up again, the gas will be absorbed and 

 removed. The charcoal is first heated, to increase its 

 absorbing power. In this condition it absorbs thirty- 

 five times its own bulk of gas. 



476. CHARCOAL FIRES IN CLOSE ROOMS. 



now does 



burning char- Fatal accidents not unfrequently occur 

 To? accidents? from innalm g the fumes of charcoal, burned 

 in close unventilated rooms. These fumes 

 consist of mingled carbonic acid and carbonic oxide. 

 The latter gas will be hereafter described. 



477. SOLIDIFICATION OF CARBONIC ACID. 



How may car- r^ f > . . 



bonic acid be One oi the most interesting of all chemical 

 solidified? experiments, is the solidification of car- 

 bonic acid. By combined cold and pres'sure, this trans- 

 parent gas, which, under ordinary circumstances, is so 



9 



