CONSTITUENTS OP PLANTS, ETC. 317 



bastion when the heat is applied; and the nitrogen, being 

 set free, expands with the other gases, and helps to give 

 force to the explosion. Sulphuric acid is commonly used 

 with the nitric acid in preparing gun-cotton. It is of use 

 only in taking the water from the cotton, by virtue of its 

 strong attraction for water ( 244), thus making more room 

 for nitric acid, and securing a larger combination of this 

 acid with the cotton than it could otherwise obtain. 



Collodion is a solution of gun-cotton in ether, making a 

 sirupy liquid. It is often used for court-plaster, and also* 

 for making small air-balloons. If it be put upon any thing 

 the exposure to the air causes an evaporation of the ether 

 at once, and the cotton is left in the form of a transparent 

 coating. 



434. Products from Wood by Heat When wood is con- 

 sumed with free access of air, it is decomposed, as you have 

 already learned, into its elements carbon, oxygen, and hy- 

 drogen ; and these, together with some oxygen from the air, 

 form carbonic anhydride and water in the condition of va- 

 por. When, however, there is imperfect combustion from 

 a restricted access of air, the products are different. They 

 are four in number: 1. Charcoal ; 2. Illuminating gas, which 

 is a mixture of several hydrocarbons with some carbonic ox- 

 ide and carbonic anhydride ; 3. Pyroligneous acid, or wood- 

 vinegar ; 4. Wood-tar. In the common burning of wood the 

 combustion is not perfect, and we have three of these prod- 

 ucts deposited in the form of soot, for this substance is 

 composed of particles of carbon which have passed off un- 

 burned in the current of smoke, having united with them 

 some of the pyroligneous acid and the wood-tar. In the 

 case of the air-tight stoves, so called, soot forms which con- 

 tains a much larger proportion of the acid and the tar than 

 the soot of an open fire, because the current up the chimney 

 is too sluggish to carry up much of the carbon. 



