294 ELEMENTARY GENERAL SCIENCE < HAP. 



When it again meets sufficient oxygen at the top of the 

 fire, this monoxide burns to the dioxide. Carbon mon- 

 oxide may in fact be prepared by passing the dioxide over 

 heated charcoal instead of heated iron, but the temperature 

 required is higher than that necessary in the experiment 

 described. 



The gas so obtained is colourless and very poisonous. It 

 acts as a direct poison, since it replaces the oxygen from its 

 compound with the haemoglobin of the blood. For this reason 

 the ventilation of rooms heated by coke stoves, and the complete 

 removal of the products of combustion should be always well 

 attended to. 



Compounds of Carbon with Hydrogen. Carbon also forms 

 compounds with hydrogen, direct union occurring when the 

 electric arc is produced between carbon poles in an atmosphere 

 of hydrogen. The compound resulting is known as Acetylene, 

 and has the composition represented by C 2 H 2 . It burns with a 

 highly luminous flame, and is present to a small extent in the 

 products of the incomplete combustion of coal gas. It is now 

 prepared largely by the action of water on calcium carbide, and 

 its cleanliness and ease of preparation will probably lead to its 

 wider use as an illuminant. 



Ethylene, another compound of carbon and hydrogen, is a 

 product of the destructive distillation of coal, wood, &c. It is 

 hence present as a constituent of coal gas. It is readily obtained 

 by heating a mixture of alcohol and sulphuric acid. Like 

 acetylene, it burns with a bright, but less luminous flame. At 

 low temperatures it may be liquefied, and by allowing the liquid 

 ethylene to rapidly evaporate (page 155), the temperature is so 

 greatly reduced that air may be liquefied. The composition is 

 represented by C 2 H 4 , and the following equation may be given 

 for its preparation : 



C 2 H 5 OH-C 2 H 4 + H 2 0. 

 Alcohol Ethylene. 



In this only the final products are considered, intermediate com- 

 pounds are, however, really first formed between the alcohol and 

 the acid. 



Marsh Gas. Marsh gas, or methane, has the composition 

 CH 4 , and occurs naturally, being a product of the decomposition 

 of vegetable matter. It is found rising to the surface in 

 swamps, and thus obtains its name marsh gas. It is also fre- 



