94 



still more in badly ventilated rooms where gas is being burnt and 

 many people are collected the proportion is considerably higher. 

 The quantity is small relatively, but absolutely it is very large. 

 The late Sir Henry Roscoe calculated that the total amount of 

 carbon dioxide in the earth's atmosphere exceeds three billion tons 

 (3 X io 12 ), which, he says, is probably more than would be formed 

 by the combustion of all the animal and vegetable matters now 

 existing on the earth's surface. The waters of the ocean also 

 contain large quantities of carbonic acid, and interchange between 

 the air and water doubtless contributes to the maintenance of the 

 relatively constant proportion of the gas in the atmosphere in spite 

 of the many causes that tend to produce variation in this propor- 

 tion. By combustion, by the respiration of animals, by fermenta- 

 tion processes and by the slow decay of organic substances carbon, 

 in the form of its dioxide, is constantly passing into the air. Green 

 vegetables, on the other hand, in the presence and through the 

 agency of sunlight, are as constantly removing carbon from the 

 air. In this process solar energy is absorbed. This building, for 

 example, is warmed by hot water. The water is heated by fur- 

 naces in which coal, raw or previously coked, is burnt. The coal 

 is derived from the residue of ancient vegetation buried during long 

 ages in darkness and silence beneath the earth's surface. As it 

 burns the energy derived from the sun many ages ago is restored, 

 and we benefit by its outpouring. Carbon monoxide contains, to 

 a given weight of carbon, just half as much oxygen as the dioxide. 

 It is a lighter gas than the latter, almost insoluble in water, very 

 poisonous and burns with a blue flame. This very characteristic 

 flame is often seen in our domestic fires when the grate is 

 full of a glowing mass of coal. It is formed in this and other cases 

 when air or carbon dioxide comes into contact with heated carbon 

 or carbonaceous matter and can be prepared in a number of other 

 ways. It is an important constituent of water-gas, formed by the 

 action of steam on white-hot coke and used as a substitute for, or 

 mixed with, ordinary coal gas. Its reducing property, i.e., its 

 aptitude for removing oxygen from compounds containing it, 

 render it valuable in metallurgical operations. 



The third oxide of carbon (C' S Og) was discovered compara- 

 tively recently- Carbon suboxide is a gas of pungent odour con- 

 densible to a liquid boiling at 70C. It is obtained by distilling 

 malonic acid CH^(COOH)v) under reduced pressure, with ten 

 times its weight of phosphorus pentoxide. 



The hydrocarbons are the compounds of carbon and hydrogen. 

 These are exceedingly numerous, some hundreds being known. • 

 The simplest of all is methane or marsh gas (CH } ). It is evolved 

 during the decay of vegetable matter, hence its name. It is 

 present in coal gas and sometimes escapes from coal spontaneously, 

 being, in fact, the dreaded fire-damp of the coal mine. No other 

 hydrocarbon contains so large a proportion of hydrogen as does 

 methane. 



There are several other gaseous hydrocarbons, e.g., olefine 



