CARBON AND CARBON DIOXIDE. 167 



greater part of the night, and it is no wonder if we awake 

 with a headache and feeling unrefreshed. The case is 

 worse still if we keep the gas burning, or even " just a bead," 

 as some are fond of doing. 



Those who live in towns are so accustomed to breathe 

 air which is not quite pure that they do not notice it unless it 

 be worse than usual, but an Arab, fresh from the pure air of 

 the desert, wears a frown of disgust when business obliges 

 him to enter a town, and usually goes about with cotton in 

 his nostrils or a handkerchief drawn over his nose, and if 

 obliged to pass a night within the walls, will at least not sleep 

 under a roof. 



It is computed that Manchester, with its large population 

 and numerous furnaces, engine-fires, &c., produces some 

 15,066 tons of carbonic acid gas every day; and the inhabi- 

 tants of London send up some 800 tons of carbon into the 

 air, in the same time. Yet all this great mass is quite in- 

 visible and has no share in producing fogs or darkness, for 

 the human furnace is happily so constructed as to consume 

 its own smoke. Each full-grown person contributes on an 

 average more than four ounces of solid carbon to the atmo- 

 sphere in twelve hours. 



But the gas is also generated by that slow burning of 

 animal and vegetable substances which we call decay, and 

 it is poured forth in very large quantities by volcanoes and 

 from cracks in the earth in volcanic districts. There are 

 more than a thousand carbonic acid springs in the Eifel and 

 Lake of Laach district alone. 



The various processes by which carbonic acid is pro- 

 duced go on chiefly in the northern hemisphere, since that 



