134 PRACTICAL LESSONS IN SCIENCE. 



Cyanogen, formula, C 2 N 2 , is a colorless poisonous gas composed 

 of carbon and nitrogen, which is best known in combination with 

 the metals potassium and iron, in connection with which it will 

 be more fully discussed. 



No other two elements are capable of occurring in so many 

 different combinations as carbon and hydrogen. Compounds of 

 these elements are known as hydrocarbons, and include most of 

 the inflammable gases and a great many essential oils, naphthas 

 and other interesting substances. It is supposed that all these 

 compounds were originally derived from the vegetable kingdom, 

 and they may be considered under the head of organic chemistry. 

 Three members of this group of compounds may be mentioned 

 here for the purpose of affording a little insight into the mutual 

 relations of these important elements. When intensely heated, 

 as by the electric spark, carbon combines with hydrogen forming 

 a colorless gas called acetylene, formula C 2 H 2 . This gas is found 

 among the products of incomplete combustion, having a peculiar 

 odor something like that of geranium. It burns with a smoky 

 flame in the air, bursting spontaneously into flame when brought 

 into contact with chlorine. Olefiant gas, formula C 2 H 4 , is also a 

 product of the action of heat on coal, and is one of the illumi- 

 nating gases. This gas derives its name from its property of 

 uniting with chlorine and bromine, forming oily liquids. 



If the mud and decaying vegetable matter at the bottom of 

 stagnant pools is stirred up, bubbles of gas rise more or less 

 abundantly to the surface. This gas has received the name of 

 methane, or marsh gas. It is abundant in coal mines, as the fire 

 damp of the miners ; it is a constituent of petroleum oil and is 

 the initial member of an interesting series of hydrocarbons. It 

 is a colorless, odorless, inflammable gas, burning with a lumi- 

 nous flame. When petroleum is brought up to the air several 

 gases are given off. The simplest of these is methane, formula 

 CH 4 , the next Ethane, C 2 H 6 , the next Propane, C 3 H 8 , the next 

 Butane, C 4 H 10 , the next Pentane, C 5 H 12 , and so on, each member 

 differing from the preceding one by CH 2 , altogether forming what 

 is called an homologous series. Such series are distinguishing 



