ELEMENTS 195 



224. Nitrogen. Like H and O, nitrogen is a colorless 

 and odorless gas. It occurs free in the air, nearly four 

 fifths of the air being N. In combination with O (i.e. NO 3 ) 

 it forms a part of those salts that are called nitrates, and 

 it is a factor in the proteids, which occur mostly in ani- 

 mal matter. N is not an active element, and it does not 

 support combustion. Owing to this last fact, N in the 

 air serves a very great use by checking fires; that is, 

 if a larger portion of the air were O, fires would burn 

 more fiercely and they could not be controlled so easily. 



225. Carbon. The element carbon is a solid. Several 

 substances are nearly pure C ; for example, charcoal, 

 coke, lampblack, boneblack, and gas carbon. Coal also 

 contains a large amount of carbon. Notice that each 

 of these substances is one that remains after some com- 

 pound has been broken up; for example, charcoal is 

 left when wood is burned imperfectly, lampblack when 

 oils are burned without a good supply of air, etc. This 

 shows that C occurs in compounds which may be broken 

 up by heat. The gases in the compounds are first driven 

 off, leaving the C. If plenty of air be supplied and the 

 heat be great enough, C will combine with O (i.e. will 

 burn) and pass off as a gas, CO 2 (carbon dioxide). 



Experiment 123. Burn a match (wooden) in air, allowing it 

 to burn completely. How much ash remains ? Now break up the 

 wood of a match or a splinter into bits, place these in a test tube, 

 cover with a little dry sand, and heat over a flame. Do you see 

 any evidence of decomposition? What remains in the tube? 

 Explain the difference between this result and that from the 

 burning in air. Similarly, heat some sugar in a test tube till it is 

 solid. Note and explain the result. 



