172 THE WORLD'S LUMBER ROOM. 



the fleshy part decayed away, and only the bare bones 

 left; it is seen again in calico, linen, and paper, which, 

 white as they are, have only to be held near enough to the 

 fire to be scorched, and the carbon is revealed at once. 

 Cotton-wool, again, is almost pure cellulose. 



About one-half the substance of all wood-fibre is simply 

 carbon ; but it is not all used up in these ways, and 

 the immense variety of vegetable substances, composed 

 of the three elements, carbon, hydrogen, and oxygen, is 

 amazing. 



First there are the vegetable-oils, palm, olive, rape, lin- 

 seed, &c. All are hydro-carbons, as these compounds are 

 called, and the acids which give them their characteristic 

 flavours are hydro-carbons also. 



From the same materials, combined in different pro- 

 portions, the sugar-cane, beet-root, maple, and mallow, pro- 

 duce their cane-sugar, the vine and other fruit trees their 

 grape-sugar, and the acacias their gum-arabic. From the 

 same, again, the lemon produces its citric acid, the vine its 

 tartaric, the sorrel and lichens their oxalic, the rhubarb 



minute, are exceedingly important to health. According to Dr. Hartwig, 



the percentage of oxygen in the air of the 



Metropolitan Railway (Underground) is .. 20-70 



East End of London 20-857 



Pit of a theatre ... ... ... ... ... ... ... 20* 74 



Hyde Park ... 21* oo 



Coast of Scotland 20-999 



Suburb of Manchester, wet day 20*980 



Ditto ditto dry day 20-947 



Sitting-room ... 20- 89 



Mine 20- 14 



