1 66 THE WORLD'S LUMBER ROOM. 



heavy that it can be poured from one vessel into another ; 

 yet instead of resting upon the earth, as we might expect, it 

 mounts up and mingles with the other gases of the atmo- 

 sphere. 



By heating twelve grains of carbon* in thirty-two grains 

 of oxygen we produce forty-four grains of colourless gas, 

 which when frozen resembles snow. 



Ordinary air contains about ^-gVff of the gas, but there is 

 much more in the air of our rooms, owing to the presence of 

 fires and lights, as well as our own breathing. The air we 

 breathe out contains from three to six per cent', of carbonic 

 acid, and not enough to keep a candle alight. 



A man breathes out about a cubic foot f of the gas in an 

 hour and a half, and as one in a thousand is enough to make 

 the air unwholesome, his breath alone would make the air of 

 a room, measuring ten feet each way, unfit for respiration in 

 that time, supposing it to be air-tight. He would be able, 

 indeed, to go on breathing some time longer, but would 

 feel drowsy and heavy ; two per cent, of the gas would give 

 him severe headache (even one per cent, is not long to be 

 endured), and ten per cent, would stop his breathing 

 altogether. 



Fortunately our houses are not air-tight, but if we close 

 our doors and windows, cover all cracks and crevices with 

 list and sandbags, shut down the register, or put a chimney- 

 board before the fire-place, and then sleep in a room ten feet 

 square, the amount of fresh air which can come in is so small 

 that we must needs breathe what is unwholesome during the 



* The "black-lead " of our pencils is pure carbon, so also is the diamond, 

 f Nearly four and one-third gallons per hour. 



