GRAINS, OILS, GUMS, AND SCENTS. 173 



its malic, and the nettle the formic acid which makes its 

 sting so painful. 



All the vegetables used as food contain from forty to 

 fifty per cent, of carbon, and foremost among these are the 

 cereals, the starch, sugar, gum, and oil of whose grain are 

 all hydro-carbons. One pound of flour contains on an 

 average seven ounces of carbon. 



The various pines and firs combine carbon with 

 hydrogen only, and produce turpentine; the lemon, berga- 

 mot, pear, lavender, pepper, camomile, clove, &c., do 

 the same, and even in the same proportions, and produce 

 the essential oils which help to give them their special 

 scents and flavours; while the laurel of China and Japan 

 adds an atom of oxygen, and produces the white crystal- 

 line gum known as camphor. 



Various tropical trees combine carbon with hydrogen, 

 and form such juices as indiarubber and guttapercha, 

 which, though white as they flow from the stem, turn black 

 and solid with exposure to the air ; and then other plants, 

 again, such as the rose, Tonquin bean, meadow-sweet, and 

 many others, convert the same two elements into the 

 sweetest perfumes by the addition of oxygen. 



But there is another gas of which the vegetable world 

 relieves us, and makes good use. This is ammonia, which, 

 like carbonic acid, is produced by the decay of animal and 

 vegetable substances, and is easily known by its strong, 

 pungent smell. It is a compound of hydrogen and nitrogen, 

 and is always present in the air, but does not on an average 

 amount to more than one part in fifty millions ; and though, 

 as it streams up from all decomposing organic matter, it is 



